Progress towards self-healing polymers for composite structural applications

Polymer 83 (2016) 260e282

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Polymer journal homepage: www.elsevier.com/locate/polymer

Feature article

Progress towards self-healing polymers for composite structural applications Margaret Scheiner, Tarik J. Dickens, Okenwa Okoli* High-Performance Materials Institute, Department of Industrial & Manufacturing Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St., Tallahassee, FL 32310, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 August 2015 Accepted 2 November 2015 Available online 9 December 2015

Repair in composite materials is tending towards autonomic healing systems. This is a technological departure from the mechanical repair currently practiced in industry. For reinforced polymer matrix composites, failure tends to occur in the matrix or matrix-reinforcement interface. The most common failure mode is the formation and propagation of microcracks that reduce the material's structural capabilities. Damage may be fixed through traditional bolted or bonded repair methods, but such repair requires temporary decommission of a part, collection of repair materials, and employee time and effort to enact the repair. This review describes methods of self-repair and healing for polymeric materials with a focus on structural applications of these self-healing materials. From intrinsically healing polymers to self-healing-enabled polymer composites with dispersed agents or vascular networks, this review examines the chemistries and mechanisms which enable self-healing. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Self-healing polymer composites Dispersed agents Vascular networks

Contents 1.

2.

3.

4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260 1.1. Traditional composite monitoring: NDI to SHM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 1.2. Composite repair practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 1.3. Self-repair: healing efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 Self-healing polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 2.1. Covalent bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 2.2. Supramolecular chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Self-healing composites: dispersed agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 3.1. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268 3.2. Remote self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 3.3. Shape memory assisted self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 Self-healing composites: vascular networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 4.1. Design considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 4.2. Scaling to bulk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 Knowledge assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

1. Introduction * Corresponding author. E-mail address: [email protected] (O. Okoli). http://dx.doi.org/10.1016/j.polymer.2015.11.008 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

Everything experiences wear and tear in everyday life. The

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difference between biological structures and mechanical structures is that biological beings automatically heal. The process undertaken by a structure to repair a damaged area without additional material is designated “self-healing”. Self-healing may involve the addition of energy (thermal, electrical, mechanical, etc.). This definition allows consideration of all healing processes while avoiding the problem of defining a ‘no thermal energy added’ state. Thus, healing can be categorized into two types: (1) that which requires external intervention (i.e. such as a temperature increase or application of ultraviolet radiation), and (2) that which does not require such intervention, typically referred to as ‘autonomous’ healing [1]. Biomimetic self-healing synthetic materials imitate the procedures from natural organic systems. Observation of the various biological methods used by living things to heal has led to the creation of synthetic materials capable of self-healing [2]. An example of biological self-healing is: after a child falls and scrapes the skin off his/her knee, blood wells up, clots form, and skin regrows. Mimicking the process as a whole is complex for there are clearly several disparate steps, each of which involves highly coordinated, complex activities on the cellular and even molecular level [3,4]. Rather than attempting to copy the entire process, engineers creating biomimetic systems can use the natural procedure to inspire and to guide material development [5], continuing the development of smart materials (which are responsive to external stimuli) [6]. Ideally, the self-healing process is repeatable: that is, the same sample can successfully heal after repeated incidents of damage. Self-healing parts should then have much longer lifetimes than those formed from non-healing materials [7]. With the SHM signals imitating the nervous system of a composite part and with the ability of the part to self-heal, concerns about composite part failure causing plane crashes should be mitigated. Fiber-reinforced polymer composites (FRPCs) are used in a vast variety of applications in diverse industries. For example, both military and civil aircraft include composite materials for their strength benefits and weight savings [8]. Boeing and Airbus have produced jetliners composed of 50% and 53% composite materials by weight for commercial flights [9]. FRPCs are relatively cheap, strong, and lightweight: weight savings turn into better gas mileage, meaning each flight the aircraft undertakes costs less. The biggest worry about heavy reliance on composite materials in commercial aircraft is part maintenance, repair, and overhaul [10]. For companies this translates to a trade-off of costs, but for the general consumer this translates to concern of part failure and aircraft crashes due to the use of these new composite materials. Any material may eventually fail even under normal loading conditions. The breadth of features which affect the structural health of composites makes prediction of their mechanical properties more difficult than for traditional materials [11e13], meaning physical inspection of parts is required to check for damage. FRPCs can suffer extreme internal damage from low-velocity impact and show little, if any, external indication that damage has occurred [14,15]. Thus, non-destructive inspection (NDI) beyond visible inspection is required to check for possible damage. If a mechanical structure could self-heal efficiently and reliably, the repair technologies discussed in the previous section would no longer be needed. The question which arises is: how can self-healing be enabled within mechanical systems? 1.1. Traditional composite monitoring: NDI to SHM Traditional NDI is costly and time-consuming, meaning that frequent inspection is often limited to small areas and critical damage can go unnoticed [15,16]. To prevent possible aircraft crashes and other catastrophic failures, much research has been

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devoted to improving NDI. Structural health monitoring (SHM) could be considered an extension of NDI since it involves damage detection, but in real-time rather than just at individual inspection times [17]. A SHM system incorporates sensors, data transmission devices, and external devices for data analysis or storage. Such a system enables continuous real-time updates on the integrity of the structure. A significant portion of early SHM systems characterized damage by analyzing vibrational changes, but progress has been made regarding the use of fiber optic sensors, wireless data acquisition, and microelectromechanical systems (MEMS) [18]. In the past decade, much more research has been done regarding SHM. The program for the 9th International Workshop on SHM [19] presents research both on the “traditional” types of SHM used in the first two Workshops and on the development of newer techniques, such as using flying [20] or climbing [21] robots to monitor civil engineering structures. The most common techniques currently used for SHM include acoustic emission and ultrasonic testing, imaging methods and radiography, and fiber optic methods [8,22]. 1.2. Composite repair practices After damage has been recognized, there remains the question of what to do about it. Repair practices are tailored to mend specific types of failure. FRPC materials have several failure modes [23]. Within a single lamina, the reinforcing fibers may break, the matrix may crack, or the interface between the two may fail potentially leading to fiber pullout. FRPC laminates may suffer failure within individual plies or between plies (delamination). Highlighting the progress from self-healing polymers to self-healing composites, this report focuses solely on matrix failure. While the shape memory composites and the vascular composites discussed in this report could be considered a type of functionalized reinforcement, it remains difficult to repair the typical glass or carbon fibers used in composites today [24]. Healing of interface failure has been investigated and can be researched elsewhere [25e28]. Following the theme of this article, the referenced repair practices are for addressing matrix failure rather than delamination, interface failure, or fiber breakage. A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged area [29]. Good patches are resistant to cyclic loading damage, have a high immunity to corrosion, and easily shape to fit the structure's geometry [30]. Material properties of the patch and the structural material should be well-matched. For example, if the thermal expansion coefficients are significantly different, temperature changes will cause stress planes between the part and the patch and increase the likelihood of patch failure [31]. The adhesive is as important as the patching material, for if the adhesive fails, the patch will de-bond and the damage will again be exposed [32]. Patches may be bolted or bonded to the damaged structure. Bolted repair is the current standard repair method for commercial composite aircraft [33e35]. Bonded repair is the method of choice in repairing damaged military composite aircraft [36]. Table 1 highlights some advantages and disadvantages of bolted and bonded repair, particularly as it pertains to composite aircraft. A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]. This technique can be used in metals [40] and composite materials [41]. This type of repair may be achieved using the same material as the matrix or a different adhesive. Ideally, the injected material should fill all voids within the matrix. Filling all voids prevents high stress concentrations which would lead to further crack growth [37,42]. Of course, a patch may be used in conjunction with injection, leading to significant recovery in tensile and bending strength [43]. This type of repair

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Table 1 Advantages and disadvantages of three repair types for composite aircraft. Repair Typical repair material method

Advantages

Disadvantages

Bolted

Permanent, damage tolerant [33], existing tools and skills [35], can made and disassembled in uncontrolled environment, effective repair of composite delamination [38]; More efficient for highly loaded structures [38] Permanent, damage tolerant, improved finish (aerodynamic; esthetic), long [33]; More efficient for lightly loaded structures [38]

Bolt holes lower load carrying capability and alter stress concentrations [38], protruding patches reduce aerodynamic properties while flush patches require a large number of fasteners [37] Requires controlled environment and strict cleaning preprocessing steps, high sensitivity to bond imperfections in thick structures, often over 24 h of part downtime necessary [38], processing steps are highly dependent on presence of moisture [37]

Aluminum or titanium [37]

Bonded Adhesive or resina; Appropriate composite patch, often multi-layer boron or carbon fiber prepreg a

Choice of an appropriate adhesive depends on many situational variables; resin may include chopped glass or carbon fibers [37].

may be used to heal surface or internal damage, but the damage location must first be known. 1.3. Self-repair: healing efficiency Healing efficiency of a material property Q is defined using Equation (1) [44].

RðQ Þ ¼

Qhealed Qinitial

(1)

The subscripts refer to whether the material property is measured after healing (healed) or before damage occurs (initial). A perfectly healed material would have R(Q) ¼ 1. While reviewing the applicability of materials based on their healing efficiency, one may wish to keep in mind that skin scar tissue has a much lower toughness (K) than does uninjured skin, with R(K) z 0.2 [45,46]. In many cases, healing efficiency is defined in terms of the fracture toughness, R(K) [47e49], but some authors report healing efficiency in terms of the fracture stress or material strength, R(s) [50,51], elongation or extensibility [52e54], peak load [44,55], or various moduli (e.g. R(E0 ) [56,57]). Many reports do not attempt to define a healing efficiency, but only report that the material heals, often with the aid of optical images of damaged and healed samples. Table 2 summarizes the type of healing efficiency reported for an illustrative set of material systems. The choice of which tests to do and thus what healing efficiency to report changes between research groups, though fracture toughness is most often reported for epoxy systems. Healing efficiency varies widely within any material system. For example, neat poly(dimethyl siloxane) (PDMS) has R(K) ¼ 0.02, but R(K) ¼ 0.7e1.2 was reached by incorporating microcapsules with the relevant resin and initiator for the PDMS system [48]. Healing efficiency also varies widely between material properties. For

example, a poly(imide) system had a healing efficiency of 95% for elongation to break, but only 77% in terms of fracture toughness [60]. Healing efficiency is a good way to see how well a given material system recovers a given mechanical property, but it is not the entire story. A fracture strength healing efficiency of 100% was reported for a hollow fiber-reinforced epoxy composite. This value compares the healed composite to the pre-damage healing-enabled composite [61]. However, the added constituents affect the virgin (predamage) strength of the material [62], so the healing efficiency of 100% results in a material with only 87% of the strength of the unmodified laminate [61]. Fig. 1 shows the number of papers published per year containing

Fig. 1. Number of publications per year containing the phrase “self-healing polymer”, where 2015* contains number of publications for 2015 through July. Data from Ref. [63].

Table 2 Types of healing efficiencies reported in various material systems. Material property, Q

R(Q) [Ref.]

Matrix material

Cohesive recovery (1  Vt/Vt0) Extensibility

0e1a [58]

Epoxy

0.4e0.9 [52]; 1 [54]; 0.45 [53] 0.55e0.93 [59] 1.07e1.48a [44]; 0.09e0.24 [55] 0e0.95a [50]; 0e0.73a [51] 0.7e1.2 [48]; 0.84e0.97 [47], 0.3e0.9a [49] 0.94 [56]; 0e1.125 [57]

Poly(styrene); Poly(acrylamide stearyl methacrylate); Poly(n-butyl acrylate) þ poly(styrene) block copolymer Epoxy Epoxy; Poly(dimethyl siloxane) Poly(sulfide)s; Poly(vinyl alcohol) Poly(dimethyl siloxane); Epoxy Poly(n-butyl acrylate); Poly(urethane)

Flexural strength Fracture load Fracture stress (s) Fracture toughness (K) Tensile modulus (E) a

Estimated from figures.

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the phrase “self-healing polymer”, as found via Engineering Village [63]. Despite this interest, research is still needed to understand the virgin structural properties of self-healing materials: if they are to replace current structural materials, the toughness [64] and failure strength (among other properties) must be adequate. Healing efficiency only describes how well the material heals; it does not indicate how the healing-enabled material performs structurally compared to the original material. To determine which material system is the best for any given application, one must have a broad knowledge of all potentially relevant self-healing materials. This review outlines self-healing in polymeric composite materials with a biomimetic approach in mind. Engineered self-healing materials can be said to imitate various stages in the biological healing process of bleeding. The specific steps are (i) bleeding, (ii) clotting, and (iii) regrowth. The following sections discuss in detail three types of self-healing polymeric materials: self-healing polymers (regrowth); self-healing composites with dispersed agents (clotting); and self-healing composites with vascular networks (bleeding). 2. Self-healing polymers The final step in healing of a flesh wound is regrowth of the skin and underlying tissue. This level of healing involves fusion of the failure surfaces. Ideally, the healed area would be indistinguishable from undamaged areas. In a polymer system, regrowth is accomplished through mechanisms which reconnect the broken polymer chains. The presence of reactive groups, such as eC]C, eCOOH, eNH2, eOH, eSH, eSieO, eSeS, and eC]O (where C is carbon, O is oxygen, H is hydrogen, N is nitrogen, and S is sulfur) [65], free radicals, and cyclic structures enable self-healing. Types of fusion of failure surfaces within polymeric materials can be divided into two major groups: reactions involving molecular covalent bonds and those involving supramolecular chemistry [66]. 2.1. Covalent bonding Covalent bonds break and reform depending on the local environment. In terms of self-healing, this means bonds will reform after damage if given favorable conditions. Many polymeric materials exploit dynamic, reversible covalent bonding to enable selfhealing. Low molecular weight polymers tend to have high mobility and thus are often self-healing to some extent. However, not all low molecular weight polymers exhibit self-healing. For example, unmodified polystyrene has a relatively low molecular weight, but does not exhibit self-healing properties. However, simple modifications of polystyrene do enable self-repair [67]. Though the specifics depend on the exact material of interest, healing mechanisms based on covalent bonding can be grouped into three major categories: general chain exchange reactions, cycloaddition, and free radical reactions. Chain exchange reactions involve the reorganization of bonds (generally between chains, sometimes within a single chain). An example chain exchange reaction is the (re)formation of links between acylhydrazines grafted onto the ends of polyethylene oxide (PEO); photographs illustrating the healing properties of PEO by Deng et al. are shown in Fig. 2 [68]. Two PEO samples were created, colored (one with carbon black and the other with rhodamine), and broken. A carbon black half was placed in contact with a rhodamine half. After seven hours at room temperature, the two halves had fused into a single entity, with a strong enough bond to withstand being squeezed by tweezers. Healing in PEO is achieved at ambient conditions [69] via the room temperature formation of bonds between the acylhydrazine ends [70]. These networks self-heal at ambient conditions [69]. The bond-shuffling reactions of disulfide

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chains and silonate end groups are additional examples of chain exchange reactions [65]. Healing in these systems is quick, usually complete within 24 h even at room temperature [71]. Fig. 3 consists of time-delayed optical micrographs of a self-healing thiol-functionalized polymer [72]. A razor blade was used to create a 50 mm wide and 500 mm long cut in the >15 mm thick polymer film. Within the first minute, the ends of the cut began to close. The cut was barely visible after one hour of healing and it was fully healed within 24 h. Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]. Fig. 4 depicts a disulfide chain exchange [74]. Disulfide free radicals may be formed through heating [75], oxidation [76], or photolysis [77]. Bond cleavage resulting in ionic intermediates is known as ionic scission and may occur under other various conditions [78,79]. SeS bonds may also be broken through a reduction reaction where two thiol (SeH) groups are formed [65]. The SeS bonds will reform through an oxidation reaction. Disulfide bonds have been incorporated into low glass transition temperature (Tg) polymer networks (poly(ethylene glycol) [80]) and high Tg networks (poly(n-butyl acrylate) [72]). Amamoto et al. showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room temperature selfhealing under visible light [57]. Disulfide bonds also enable room-temperature self-healing in rubbers with near 100% healing efficiency of failure stress [50] and cohesive recovery [58]. A selfhealing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did successfully heal, but the fracture stress healing efficiency was only 50% [81]. Part of the reason for this low healing efficiency may be due to the concentration of reactive groups. Fig. 5 is a graph of recovery of strength as a function of disulfide group concentration [50]. Clearly, higher concentrations of the reactive group lead to higher strength recovery. While a given material system may not initially seem to have a high enough healing efficiency, one may not be analyzing the highest efficiencies possible for that material. However, the concentration of the active group cannot be increased indefinitely (up to the physical limit of 100%) without altering other material properties. Consider, for example, if Amamoto et al.'s polyurethane material was altered to contain 100% disulfide groups: it would no longer be polyurethane and one should not expect it to maintain polyurethane's properties. Some self-healing materials combine healing with sensing. That is, the material conveys the information that damage has occurred. A notable example of a self-healing polymer that also indicates damage has occurred is the covalently bonded poly(methyl methacrylate/n-butyl acrylate/1,3-dihydro-1,3,3-trimethylspiro[2Hindole-2,30 -[3H]-naphth [2,1-b] [1,4]-oxazine]-2-amino-2methylacrylate) or p(MMA/nBA/SNO) copolymer, shown in Fig. 6(a) [82]. When the material is scratched, the damaged area turns red, as shown in Fig. 6(b). Fig. 6(c) shows the reverse color change and healing of the wound after exposure to acidic vapors. Healing will also occur under sunlight or increased temperature. Bailey et al. have shown that self-healing polymers may have additional functionalities, such as electrical conductivity [83]. Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and form a ring. A common cycloaddition reaction is the DielseAlder reaction: reversible crosslinking via a [4 þ 2] cycloaddition. The bracket notation indicates the number of electrons each molecule contributes. In the case of a DielseAlder reaction, one molecular contributes four electrons while the other contributes two. The DielseAlder reaction has been harnessed to enable self-healing in a number of materials including epoxies, polyacrylates, and polyamides [84]. In these materials, cracking or elevating the temperature of the material breaks the

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Fig. 2. Optical images of self-healing covalent PEO gels: (a) broken gel containing carbon black; (b) broken gel containing rhodamine; (c) bicolor gel; (d) healed gel; (e) squeezed healed gel [68].

Fig. 3. Optical micrographs of thiol-functionalized polymer under ambient conditions [72].

Fig. 4. Disulfide chain exchange; figure modified from Ref. [74].

Fig. 5. Recovery of strength as a function of disulfide group concentrations. Figure modified from Ref. [50].

bond between diene and dienophile [85]. Lowering the temperature after damage causes the covalent bonds to reform, healing the crack [86,87]. In-depth analysis of a furan thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of crosslinking groups increases ability to self-heal [88], similar to the healingeconcentration relationship in disulfides [50]. Changing the reactive groups present in methacrylate polymers alters healing behavior, with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]. It has even been shown that nanoparticles may be used to introduce this type of healing capability into other polymers [90,91]. In addition to the DielseAlder reaction, other cycloaddition reactions may be utilized to form self-healing polymers. The [2 þ 2] cycloaddition of 1,1,1-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms cyclobutane [92]. When the CeC bond in the cyclobutane ring breaks, there are only separate cinnamoyl groups. Under UV exposure (>280 nm), [2 þ 2] cycloaddition heals the bond, reforming the cyclobutane ring. This reaction is illustrated in Fig. 7 [92]. A similar [2 þ 2] cycloaddition can be observed in coumarin, [93]. Perfluorocyclobutane polymers break under stress, forming trifluorovinyl ether monomers [94]. Further stress causes a [2 þ 2] cycloaddition to reform the polymer network, indicating that stress-induced crosslinking may be a useful mechanism for self-healing. Anthracene derivatives polymerize under UV radiation via a [4 þ 4] cycloaddition reaction [95] and could also be incorporated to synthesized self-healing polymers.

Fig. 6. Optical images of p(MMA/nBA/SNO) copolymer: (a) pre-scratch; (b) post-scratch; (c) repaired [82].

M. Scheiner et al. / Polymer 83 (2016) 260e282

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Fig. 7. Self-healing via [2 þ 2] cycloaddition within cinnamoyl groups [92].

While light-induced self-healing shows much promise for creating self-healing structural materials, the radiation requirement may cause problems [65]. First, obviously, a light source is required, preferably of monochromatic radiation. Secondly, the radiation may have unintended effects: side-reactions may occur. For example, radiation may increase the local temperature, which could negatively affect the overall healing process. A number of chain exchange reactions involve free radical intermediates. As already discussed, the cleavage and restructuring of disulfide bonds may or may not involve free radicals, depending on how the bonds break. For most self-healing polymers, such as polyurethane [96], the healing process requires free radical intermediates. Free radicals are very reactive in liquid or gaseous phases, but their mobility (and thus reactivity) drop within solid networks. For healing to occur, cleaved chain ends with reactive groups must move to meet each other and react-all before other reactions intercept the free radicals. For efficient self-healing, it is imperative to avoid radicaleoxygen interactions [97]. If the free radicals interact with oxygen, they cannot interact with other chain ends and thus the material will not self-heal [1]. Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (CeON) to form dynamic, reversible crosslinks [98]. Fig. 8 shows the disassociation of the alkoxyamine group and subsequent free radical formation [74]. This material, along with many others, will only heal if damage causes free radicals to form. Damage which severs the CeC backbone does not result in reactive groups on the chain ends and thus does not allow for self-repair. Environmental conditions are quite important for free radical stability. Temperature has a major effect on free radical stability [99,100], but there are other considerations. In polycarbonate chains, the presence of sodium carbonate (Na2CO3) facilitates chain end interactions [101]. Better interactions between chain ends

means more chain reconnections and thus better network repair. The pH of a system may also be important. The maximum strength of 3,4-dihydroxphenylalanine-functionalized poly(ethylene glycol) (DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the polymer's acid dissociation constant [102]. The DOPA-functionalized PEG can easily be edited to modify the dissociation constant, allowing precise design of a pHcontrolled material. Trithiocarbonates (TTCs)-compounds containing CS3-enable bond reshuffling via free radical intermediates [65]. Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate) (PMMA) and polystyrene [103]. The CeS bonds in TTC rupture and reform when stimulated by UV radiation of the appropriate wavelength [56]. Reversible addition-fragmentation chain-transfer (RAFT) polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via highly mobile free radicals [56]. The poly(BA) material reliably selfheals under UV radiation, even after repeated damage. Fig. 9 shows photographs of poly(BA) (a) after damage and (b) after healing under 330 nm radiation for 24 h [51]. Four-membered rings form particularly stable free radicals [65]. Four-membered rings also tend to have low ring-opening activation energy: oxetanes, for example, require just 10e40 kJ [104], roughly equal to the amount of energy released by burning a single gram of coal. Ghosh et al. developed a self-healing heterogeneous network comprised of polyurethane, oxetane (OXE), and chitosan (CHI) [96]. The OXE provides a four-membered ring and the CHI provides UVsensitivity [105]. The same research group went on to develop an oxolane (OXO)-CHI-polyurethane network [106]. OXO was chosen for its structural similarities to OXE and its much lower activation energy [107]. Both the OXE-CHI and the OXO-CHI polyurethane networks self-healed under UV light, but the OXO-containing network repaired more slowly [106]. The difference in repair times was attributed to a difference in ring strain. Materials which do not require external stimuli to initiate the healing process are of particular interest for commercial applications. Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in several types of polymers [108]. Chosen for its easily obtained state of thermodynamic equilibrium [109], cleaved DABBF forms stable free radicals with high oxygen tolerance [110]. Polymers incorporating DABBF were found to self-repair at room temperature without any external stimuli with fracture stress healing efficiency over 95% [108].

2.2. Supramolecular chemistry

Fig. 8. Chain exchange facilitated by alkoxyamine free radical [74].

Supramolecular chemistry has been a focus of research efforts for over 50 years [111,112]. Several self-healing mechanisms depend on the structure of the overall network, rather than the organization of individual molecules. Supramolecular interactions allow

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Fig. 9. Photographs of BA polymer (a) after damage and (b) after healing [56].

faster networks remodeling than do covalent bonds. Though both covalent bonds and supramolecular interactions are directional, supramolecular interactions tend to be more sensitive [113]. Unfortunately, supramolecular polymers networks tend to have a lower Tg, meaning the polymers are relatively soft and may not be useful in structural applications. Supramolecular chemistry of interest in self-healing materials can be categorized as: hydrogen bonding, pep stacking interactions, and ionomer healing. Even though hydrogen bonds are generally weaker than covalent bonds, significant strength can be obtained due to the hydrogen bonding within certain materials [114,115]. Alignment of multiple hydrogen bonds in a row allows control over many material properties including viscosity and chain length [116]. Furthermore, units with four hydrogen bonds tend to be more stable than those with just two or three and may have increased strength [117,118]. Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant, which aids in constructing polymers with high degrees of polymerization [119]. UPy is very stable partially due to its quadruple hydrogen bonds [120]. An example of UPy's hydrogen bonding is shown in Fig. 10 [121]. An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk properties, including melt viscosity, are highly temperature dependent [122]. The temperature dependent properties of UPy can be combined with a thermally responsive polymer matrix to develop materials with thermo-regulated self-healing behavior [123]. UPy has also been used within poly(ethylene-co-

Fig. 10. Chemical structure of a hydrogen bonded UPy-dimer [121].

butylene) reinforced with cellulose nanocrystals [124], resulting in a UV-sensitive self-healing composite. A number of other self-healing materials have been created using the properties of hydrogen bonding, such as poly(isobutylene) (PIB). PIB exhibits extensive hydrogen bonding [125,126]. Switching out the hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus control over its self-healing [127]. Coumarin-functionalized PIB heals under sunlight and has been successfully used to create a selfhealing coat for photovoltaic devices [128]. Incorporation of dangling polar side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly self-healing hydrogels [129]. Poly(vinyl alcohol) (PVA) hydrogels autonomously selfheal with ~72% fracture stress healing efficiency [51]. The selfhealing behavior of PVA gel can be seen in the photographs in Fig. 11 [51]. Similar to the covalently healing PEO gels in Fig. 2 [68], two separate PVA blocks were formed and one was colored with rhodamine B [51]. The blocks were cut and one half of each was placed to form a bicolored gel. After 12 h at ambient conditions, the bicolored gel healed into a single unit. The healed gel can be stretched up to 100% extension. Fig. 12 shows the fracture stress of PVA samples healed under identical conditions after different amounts of separation time [51]. Longer separation time results in less fracture stress recovery. The lower healing efficiency may be due to a decrease in concentration of reactive groups over time. As demonstrated in other systems (see Fig. 5) [50], healing efficiency greatly depends on reactive group concentration. As time passes, these groups react. If the void volume is too large, reactions may occur on a single side of the damaged area, leading to a partially healed state. Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room temperature [130]. The process for creating these rubbers is simple, with just three steps required. Slight variations produce a wide variety of solid and viscoelastic rubbers [131]. The healing in these rubbers is activated by the damage event, a promising characteristic for autonomy [132]. Unfortunately, exposure to raised temperatures or moisture significantly decreases healing ability. Above 110  C, irreversible cross-linking prevents healing [133]. Heterogeneous systems are particularly interesting for the design of self-healing materials. Clever combination of a “hard” backbone (high Tg materials like polystyrene) with “soft” brushes (low Tg materials like poly(n-butyl acrylate)) yields a self-healing polymer [53]. The backbone provides strength while the brushes facilitate hydrogen bonding. Polystyrene (backbone) and polyacrylate amide (brushes) form a spontaneously self-healing multiphase polymer [52]. Similar hydrophobic/hydrophilic

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Fig. 11. Optical images of PVA gel: (a) two separate blocks; (b) two halves of each original hydrogel; (c) bicolored gel; (d) bent healed hydrogel; (e) stretched healed hydrogel [51].

Fig. 12. Fracture stress of various samples healed under identical conditions 0, 1, or 24 h after damage [51].

interactions are utilized in certain self-healing hydrogels. The copolymer of acrylamide and stearyl methacrylate (C18) self-heals via reversible crosslinking zones [54]. Healing in the C18acrylamide gels seems to be driven by free, non-associated C18 blocks near the failure surface. Another type of supramolecular interaction which has been investigated is the stacking of p electron orbitals, such as that found between pyrenyl derivatives and diimide residue in certain polymers [134]. A blend of folding “tweezer-type” polyimide and linear polysiloxane as a backbone has been found to self-heal at 100  C [135]. Similarly, a polyimideepolyamide network heals with 100% tensile modulus healing efficiency at 50  C [136]. These polyimide polymers are able to heal due to careful positioning of phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the polydiimide [137]. pep stacking can be used in conjunction with hydrogen bonding in hybrid polymers. Polyimide with pyrenemethylureafunctionalized polybutadiene has a toughness healing efficiency of 77% [60]. Similarly, bis-pyrenyl-functionalized polyamide selfheals at 140  C with 100% tensile modulus healing efficiency [138]. A drawback of these supramolecular polymeric networks is

that they are, necessarily, rather weaker than chemically bonded networks. To develop a gel with a higher mechanical strength Xu et al. synthesized a number of self-healing nitrobenzoxadiazoleappended cholesterol derivatives [139]. With an appropriate gelator concentration, the yield strength of such gels reaches 23 kPa, an improvement over other low-molecular mass gelators, but on par with the yield strengths reports in Ref. [129] (35 kPa) and Ref. [51] (200 kPa). The healing efficiency of Xu et al.'s gels was not reported. An additional self-healing reaction which does not fit well into the above categories is that of the ionomer poly(ethylene-comethacrylic acid). In this material, the healing of puncture wounds is significantly different from the healing of sawing or cutting damage [140]. This type of healing has been termed an ionic interaction [65], but it has actually been determined that ionic components are unnecessary for healing to occur [141]. This type of self-healing occurs in two steps. In the first step, the projectile impact disrupts the ionomeric network and friction between the projectile and the material generates heat. The heat is transferred to the polymer surrounding the damage area, causing localized melting. In the second step, the molten surfaces fuse together as would polymer chains with high mobility [142]. Ionic concentration may help the process along, but too high a concentration actually reduces the healing efficiency [141]. 3. Self-healing composites: dispersed agents Before skin can regrow over a flesh wound, the wound must close. Many engineered materials mimic this clotting step. The healing agents may by liquid or solid. In the previous section, the healing agent was simply the solid polymer matrix. However, many of the materials described in the preceding section have low Tg, toughness, and/or strength, making them undesirable as structural materials. This section discusses the development of self-healing composite materials capable of holding the loads required of structural components. The method of activating the healing agents is a major factor in the range of achieved healing efficiencies. As shown in Fig. 13, the different components for certain material systems may be (a) manually injected into the crack, (b) incorporated within the material matrix, or (c) a combination of incorporation and injection [16]. For an epoxy material system, toughness healing efficiency ranged from 38% to 99% depending on the method of incorporation [16] Up to 119% healing efficiency has been

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Fig. 13. Three methods of inclusion for the healing agents and catalyst in a composite: (a) injection; (b) incorporation); (c) a combination of injection & incorporation. Figure modified from Ref. [16].

reached by altering the epoxy chemistry [143,144]. For true selfhealing, injection is not a valid incorporation method. Healing efficiencies reported in the following sections are for self-healing specimens. The healing agents in a self-healing composite are often liquids which must be encapsulated to separate the healing agent from the matrix material, as discussed in the next subsection. The subsequent subsections describe remote self-healing and shape memory assisted self-healing techniques, either of which could be used in conjunction with an appropriate encapsulation technique for that material system. 3.1. Encapsulation The idea of capturing crosslinking reactants and/or catalysts within microcapsules was first presented for use in the textile industry [145]. The exploitation of encapsulation quickly expanded to include the materials needed for polymer coatings [146]. An encapsulated system which specified that the microspheres rupture under light pressure was proposed in 1978, again for use in textiles [147]. The idea of enclosing reactants and implanting the capsules within another material was developed for use in biological wound healing [148] and later in composite materials and coatings to enable self-healing and protection [149]. Encapsulation may be accomplished using hollow fibers [150], nanotubes [151], or microspheres [44]. Following the discussion of these types of encapsulation is a discussion on the various materials which may be used in these systems, specifically the catalyst and healing agents as well as the concentration and dispersion of these materials. Dry proposed a self-repairing composite material based on incorporated hollow fibers [150]. The size, shape, and composition of the fibers can be altered as a particular application dictated. The hollow fibers are filled with a healing agent. The invention was proposed for use in both cementitious and fiber-reinforced polymer composites. Hollow fiber encapsulation is often grouped with vascular systems, more fully discussed in the following section. The key difference is that vascular systems are accessible from outside the bulk material: additional liquid healing agent can be added to the system at will. Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a reinforced polymer material [152]. Motuku later showed that other materials could be used as the hollow fiber or capillary [153]. Copper and aluminum capillaries were found to be less useful for self-healing than glass capillaries, since glass' brittleness means it breaks easily and allows the encapsulated resin to flow out into the crack. In both Dry's and Motuku's experiments, the flow of the resin into the crack was visually observed: healing efficiency was not determined. Many features factor into the efficiency of these self-healing systems. The viscosity of the healing agent and the diameter of the hollow fibers determine how well the resin flows out into the crack. Fig. 14 shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area, due to a poor viscosity-

Fig. 14. Fractured resin-filled hollow glass fiber [154].

diameter match [154]. Related to viscosity is, of course, the temperature of the system and the time allowed for healing. Additional factors to consider are related to the method of incorporation for the resin-infused fibers [155]. Fiber spacing and length, the fraction of filled fibers versus simple fibers for reinforcement, weave, and lamination pattern may all have an effect on both the healing efficiency of the system and the virgin mechanical properties. Nanotubes may also be used to encapsulate materials necessary for healing. Carbon nanotubes (CNTs) [156] are being widely embraced as reinforcement materials for polymer composites for their impressive mechanical properties and potential for additional functionalities [157], such as electrical resistance-based sensing [158]. The question then arises: since composites are already being fabricated containing CNTs, can the CNTs be further functionalized to enable self-healing? From a molecular dynamics point of view, Lanzara et al. proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials [159]. Of course, such a system will only be possible if the healing agent can be injected inside the CNTs and only be effective if the CNTs actually rupture to release the encapsulated materials. The research on failure of CNTs is extensive [160] and complex [161], but as of yet they have not been utilized as nanoreservoirs despite being used as reinforcement [162]. The major issue is getting the healing agent to release upon damage since CNTs are very strong and thus may not rupture. Concerns about the small diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes [151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within anti-corrosion coatings, proving that nanoreservoirs are viable. Nearly three decades after the initial encapsulation patent, White et al. presented a polymer composite incorporating catalyst and a healing agent encapsulated within microspheres [44], such as the hollow glass bubbles shown in Fig. 15 [164]. The key behind

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Fig. 15. SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164].

White's self-healing polymer is ring-opening metathesis polymerization (ROMP). Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride (Grubbs' catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature. To create a self-healing composite, the catalyst is dispersed throughout the resin matrix and DCPD is encapsulated in-situ. Insitu encapsulation is accomplished with urea-formaldehyde (UF) shells [165]. Damage to the composite causes the microcapsules break, releasing the DCPD into the matrix where it reacts with the catalyst. Fig. 16 illustrates the damage-to-healing process [44]. Before any damage occurs, there are microcapsules and smaller catalyst particles dispersed throughout the matrix. The microcapsules contain liquid healing agent. In Fig. 16(a), crack initiation

Fig. 16. Diagram of healing process in a microencapsulated system: (a) crack initiation; (b) healing agent release; (c) curing [44].

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occurs and a crack starts propagating through the matrix. In Fig. 16(b), the crack continues to grow and ruptures two microcapsules, releasing healing agent into the damaged area. In Fig. 16(c), the healing agent reacts with catalyst particles in the damaged area. The healing agent cures, repairing the damage. The encapsulation process has been well documented [166] and proves to be useful in many industries including electronics, packaging, automotive [167] and even sports [168]. A numerical model describing the crack retardation and closure in this type of composite has been developed [169]. Either or both of the catalyst and healing agent may be encapsulated [170]. White et al. [44] paved the way for encapsulation-based selfhealing [171]. A phenomenological cure kinetics model shows DCPD should heal at 80  C with nearly twice the efficiency it would have at room temperature [172]. A number of experiments have been done to investigate the effect on material strength and healing of different types of microcapsules. Inclusion of 180 mm diameter UF shells has been found to increase the virgin toughness up to 127% that of neat resin [173]. Smaller diameters tend to lower the failure load [174]. A variety of materials may be used for the microcapsules: initial microspheres were made of UF, but silica [173] and melamineeureaeformaldehyde [175] have also been used. Special interest has been given to employment of the catalyst. Several models have been developed to describe the curing behavior based on catalyst concentration [172]. More recently it has been determined that Grubbs' catalyst exists in at least three polymorphs, each with its own distinct crystal shape, dissolution kinetics, and thermal stability [176]. 2nd generation Grubbs' catalyst was considered for use in self-healing composites, particularly for its thermal stability [177]. Later the two forms were revisited and 1st generation Grubbs' catalyst was favored since it was found to catalyze faster as well as have a tendency to be more homogeneously distributed through the matrix [178]. To avoid using the ruthenium-based Grubbs' catalyst, tungsten(VI) chloride (WCl6) was identified as a potential catalyst [179]. WCl6 is cheaper, is widely available, and has a significantly higher melting point (275  C) than does Grubbs' catalyst (153  C). In an epoxy matrix, a toughness healing efficiency of 20% when both DCPD and WCl6 were embedded, but an efficiency of 107% was reached when the WCl6 was embedded and DCPD was injected into the crack [180]. More recently, scandium(III) triflate has been suggested as a solid phase alternative catalyst showing up to 86% healing efficiency when paired with (diglycidyl ether bisphenol A)-(ethyl phenylacetate) as a healing agent [181]. The other healing agents involved in the healing reaction are of equal importance. DCPD has two stereoisomers, with the form highly affecting healing mechanics [182]. The exo-isomer is over an order of magnitude more reactive than the endo-isomer [183], but has a lower healing efficiency because the fast polymerization blocks the full release of the healing agent [182]. The endo-isomer has the added benefits of being commercially available, having a long shelf life, and having a low viscosity [182]. Blending DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction, requiring less catalyst [184]. CuBr2-(2methylimidazole)4 is a stable alternative to DCPD with higher adhesion strength than the typical epoxy healing agent [185]. A healing efficiency of 104% has been reported for a system using epoxy with mercaptan as the hardener [74]. DCPD can also be replaced with a liquid phase diisocyanate which, being reactive with water, removes the need for any catalyst [186]. Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and may find a use in bulk self-healing materials [187,188]. Mixing a low-viscosity healing agent with a diisocyanate may further improve healing ability [189]. It is, of course, important to match the matrix material, the healing agent,

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and whatever hardener or catalyst is required. What healing agent is best in one matrix may not be ideal in a different matrix. Fig. 17 highlights this difference with the healing efficiencies of three different epoxy matrices using three different healing agent mixtures [177]. M1 is EPON 828 cured with diethylenetriamine (DETA). M2 is EPON 828 containing Heloxy 71 as a flexibilizer and cured with Ancamine K53. M3 is EPON 862 cured with EPICURE 3274. Healing agents were DCPD either alone, mixed with 5-norbornene2-carboxylic acid (NCA), or mixed with 5-ethylidene-2-norbornene (ENB), with the norbornene compounds included as adhesion promoters. Furthermore, self-healing composite systems do not require an epoxy matrix. For example, poly(dimethyl siloxane) (PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing material [55]. This material holds the notable benefit of stability in humid or wet environments, though the fracture stress healing efficiency is rather low, under 25%. The PDMS/PDES material has been proposed for a self-healing coating for structural materials [170]. Other matrix materials may be chosen by careful consideration of polymers capable of selfhealing, like PDMS [190]. Concentration and size of both the healing agent and the catalyst need to be considered [191]. As seen in Fig. 18, a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may have an average toughness healing efficiency anywhere between 2% and 115% based on the concentrations of the resin capsules and the initiator capsules [48]. The samples in Fig. 18(a) were formed with 5 wt% initiator microcapsule concentration. The samples in Fig. 18(b) were formed with 10 wt% resin microcapsule concentration. The effect of microcapsule concentration on healing efficiency is additionally linked to the size of the microcapsules. Fig. 19(a) shows the toughness healing efficiency in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule concentration and size [173]. Part of the jump in healing efficiency, however, is the effect of microcapsules on the virgin toughness of a specimen. Fig. 19(b) shows the difference between virgin and healed fracture toughness for the same material system as in Fig. 19(a), with 180 mm diameter capsules [47]. Though the healing efficiency with 5 wt% capsule concentration is greater than that with 15 wt% capsule concentration, the actual fracture toughness for the healed sample is (slightly)

Fig. 17. Healed peak fracture load for samples using three different epoxy matrices and three different encapsulated healing agents [177].

higher at 15 wt%. Tagliavia et al. showed that the capsule wall thickness does not affect flexural strength of the composite [192]. Additionally, dispersion and method of incorporation of the additives must be considered. Unlike continuous fibers, which can be woven into the reinforcing structure, microcapsules must be dispersed somehow during the resin infusion process. Uniform distribution is difficult to obtain [193]. Dispersion is especially important in the case of nanocapsules to avoid clumping. 70 nm silica (SiO2) particles coated with poly(ethylene imine)/poly(styrene sulfonate) show promise for use as protective coatings, but will form clumps if improper processing conditions are used [194]. SiO2 has the added advantage that the nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]. SiO2-polymer hybrid nanotubes allow pH-, temperature- or redox-dependent release depending on the polymer graft [151]. Finally, the environmental conditions of the system during the healing process must be stated by the material developer before use. The healing efficiency of many systems depends on temperature allowed during healing. Fig. 20 illustrates the temperature dependence of an epoxy system [74]. Note the time dependency follows a t1/4 relationship, as expected for self-healing polymers [142]. Similar dependencies are to be expected in pH- or redoxdependent systems. 3.2. Remote self-healing Dispersed agents need not be encapsulated healing materials. Remote self-healing via polymer flow induced by localized melting has been realized using superparamagnetic g-Fe2O3 nanoparticles embedded within a thermoplastic film [196]. Applying an oscillating magnetic field excites the magnetic moment of the nanoparticles, increasing the nanoparticleepolymer interface temperature. The increased temperature causes localized melting of the thermoplastic which then flows into the crack, as seen in Fig. 21 [65,196]. This material heals with up to 98% efficiency in terms of the Young's modulus and strain at break and can be healed multiple times. For some material systems, healing may be achieved through a variety of stimuli. For example, graphene layers cause localized heating upon the application of infrared light, electricity, or electromagnetic waves. Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength healing efficiency of 98% [197]. As may be observed in Fig. 22, the healing efficiency in this system varies consistently with weight fraction above a certain threshold level [197]. Interestingly, this required threshold changes based on which healing method is employed. Results by Huang et al. show the threshold is (a) 1 wt% graphene for infrared healing; (b) 5 wt% for electrical healing; and (c) ~1 wt% for electromagnetic wave healing. This system also heals reliably for multiple damageehealing cycles, with 98% healing efficiency even after 20 cycles. A drawback of this method is that localized temperature increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets, limiting options for structural materials. Elsewhere [198e201], light-responsive protective coatings have been implemented by combining the ideas of remote self-healing and encapsulation. Light-sensitive porous TiO2 coated in benzotriazole (a corrosion inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearly instantaneously with a UV trigger. UV irradiation causes photocatalytic processes at the TiO2 surface, effecting a localized pH change. The pH change then causes the polyelectrolyte shell to open, releasing the inhibitor. SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]. The requisite wavelength for the healing stimulus changes based on the

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Fig. 18. Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration and (b) as a function of initiator capsule concentration [48].

Fig. 19. (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule diameter [173]; (b) Fracture toughness of virgin and healed samples with 180 mm diameter capsules [47].

3.3. Shape memory assisted self-healing

Fig. 20. Healing efficiency of an epoxy/mercaptan system as a function of time at different temperatures [74].

nanoparticle substance. Noble metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for use in self-healing composites. For example, a blend of gold nanoparticles and zinc phthalocyanines heals under laser pulse irradiation and could be incorporated to enable healing in a composite [198].

A key aspect of healing is network remodeling: the sides of the crack must close to accomplish healing. The dispersed agents composites discussed earlier in this section heal when extra parent material is available to fill the crack and react so the area regains its mechanical properties. Higher healing efficiencies are reached when the healing agent fills the entire crack [202]. A shape memory material (SMM) has a ‘set’ starting shape; after the proper stimulus is applied, it ‘resets’ to the original shape [203]. Metallic SMM wires incorporated within composite materials reduce crack size once activated [204], permitting higher healing efficiencies with minimal healing agent [205]. A schematic of this process is shown in Fig. 23 [206]. SMMs respond to a wide variety of stimuli including temperature [207], magnetic [208] or electrical [209] fields, water [210] or other chemicals [211], and light [212]. With so many options to work with, development of SMMs is a growing field and their unique properties may give materials many additional functionalities [213]. Composite SMMs are also being created, such as an electroactive carbon nanotube-reinforced polyurethane composite [214]. Composite sandwich panels of carbon nanotube reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of repeated impact damage [215]. With added components, systems get more complex, but good models explain how the many constituents affect a composite's material properties. A model of the thermomechanical properties of self-healing SMM functionalized syntactic foam has been

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Fig. 21. Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles oscillate in a magnetic field. Image from Ref. [65], based on Ref. [196].

Fig. 22. Tensile strength healing efficiency of the few grapheneepolyurethane system showing clear thresholds required for healing incited by (a) infrared light; (b) electrical signals; (c) electromagnetic waves [197].

Fig. 23. Illustration of SMM wires acting to close a crack [206].

developed and verified against uniaxial experiments [216]. The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing agents. However, there are some major concerns with the design of SMM-enabled self-healing materials. For example, improper alignment of the SMM within the composite may not result in crack shrinkage and may even increase the crack size [65]. Additionally, incorporation of supplementary materials can be expected to affect mechanical properties, either beneficially or detrimentally, depending on the overall structure [204]. Li and Zhang showed that healing efficiency increases as SMM fiber length increases, but non-linearly, so careful study of these materials is necessary before their behaviors can be fully understood [217].

Finally, some SMMs may not be useful in certain industries: thermally activated SMMs, for example, could not be used in an application where they are regularly exposed to temperature cycles including their ‘shape setting’ and ‘shape resetting’ temperatures. Several shape memory-assisted self-healing composites have been fabricated which consist of only thermoset and thermoplastic polymers and do not require any encapsulated healing agents. 6% thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65% of the peak bending load when healed at 150  C for just 20 min [218]. Unfortunately, healing efficiency in this system decreases significantly as cycles of damage and healing occur, with a sharp decline after the 4th healing cycle seen in terms of peak bending load in Fig. 24 [218]. Thermoplastic linear poly(3caprolactone) (l-PCL) embedded in thermoset end-functionalized

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Fig. 24. Decreasing trend in peak bending load as a function of healing cycle [218]. Fig. 25. Flexural strength of (A) undamaged samples and (BeE) samples stored for various amounts of time before damage and healing [59].

poly(3-caprolactone) (n-PCL) has a peak load healing efficiency of 95% [219]. Building off the l-/n-PCL network, Luo et al. developed a composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]. Upon heating, the epoxy matrix works to regain its original shape while the fibers simultaneously work to fill in the crack. 4. Self-healing composites: vascular networks In order for clotting to occur, the required materials must gather at the damaged location. A major component of healing in biological systems is the flow of materials to the area of damage. The human circulatory system transports necessary oxygen, nutrients, and blood to every single cell in the body. This idea of distributed transport was presented as a method to enable self-healing in cement [220] and has since been embraced in the development of self-healing polymer systems [153]. The major identifying characteristic of a vascular system is an interconnected hollow network which either can be refilled manually or is connected to a reservoir of healing agents. Pang et al. investigated the effect of storage time on healing efficiency [59]. Identical samples were prepared, then stored for various amounts of time before damage. The same methods for damaging, healing, and testing were then used for each sample. Fig. 25 shows the flexural strength of these samples. The overall trend indicates that a healing agent that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength of the structure. After 9 weeks, no healing is seen as the flexural strength is actually worse than that of the damaged sample. Connecting a vascular network to an external reservoir allows easy maintenance of the healing agents, so expired material can be switched out with new material. Healing efficiencies as high as 95% have been reported in 60 mm hollow glass fiber-reinforced epoxy healed at room temperature for 24 h [221]. The use of UV fluorescent dyes included in the healing agent allow easier visual analysis and very obviously highlights surface damage, decreasing the time needed for part inspection [59]. Many vascular networks are created by embedding hollow tubing within some matrix material [59,153,221]. However, a vascular network without tubing may be created by using a sacrificial material to form the network. After the part is created, the sacrificial material is removed, leaving a hollow network throughout the part. Such a tube-free microvascular network can be created in a part through layer-by-layer techniques. Direct-write

assembly [222] has been used to create networks of fugitive ink within a ductile matrix [223]. The ink is readily removed with moderate heat under light vacuum. The vascular network is filled with a liquid healing agent. A (a) schematic and (b) optical image of this set up is shown in Fig. 26 [223]. Fig. 26(b) shows bubbles in the coating caused by released healing agent. Using the same chemistry as [44], toughness healing efficiencies, R(K), up to 70% were initially reported, though efficiencies drop to around 40% after repeated damage [223]. Additional research has led to an increase in R(K) to average values over 80% with a healing efficiency over 60% reported after 16 healing cycles for an epoxy system using Epicure 3046 [224]. For certain material systems, healing efficiencies may remain

Fig. 26. (a) Schematic of microvascular substrate; (b) optical image of actual microvascular system after damage [223].

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above 50% even after 25 damageehealing cycles, as shown in Fig. 27 [225]. 4.1. Design considerations The pressure within the vascular system needs to be high enough that healing agents are distributed throughout the network [226]. In animals, the heart muscle pumps blood throughout arteries and veins. In very dense networks, pumps may not be necessary, as capillary forces serve to ensure flow [49]. In such a system, healing agents mix within the crack through diffusion. External pumps can be used to direct the flow of the healing material to damaged areas [227], but such a system requires a computer or human intervention to activate the pumping routine. Such a highly pressurized flow may improve mixing and thereby increase healing efficiencies. External pumps have also been used in various pumping routines in a sparse vascular network, with different pumping routines resulting in different average healing efficiencies, as seen in Fig. 28 [228]. Increased toughness healing efficiencies are found for pressurized networks versus systems at static pressure, at least for the first eight healing cycles. The organization and architecture of the vascular network is important for mechanical properties, flow dynamics, and crack propagation [226]. It is well known that additives affect the mechanical properties of composite materials, it is, after all, the entire reason for including reinforcement materials. It is harder to establish what the exact effect is, especially as the effect depends on the additive's material, morphological properties, and distribution as well as the matrix material and the properties of the interface between them. It has been shown that the volume fraction of microcapsules affects crack patterns and propagation. Fig. 29 shows how crack propagation in (a) neat resin differs from that in (b) resin with incorporated microspheres [229]. Embedded capillaries are expected to show similar crack propagation patterns, particularly since resin pockets tend to form around vascules, as seen in Fig. 30 [230]. Zainuddin et al. have shown that sharp cracks form near the hollow glass fibers incorporated into composites [231]. It has not yet been determined if the effect on crack propagation within these composites is detrimental. Indeed, it may even be beneficial: biologically, crack redirection within cortical bone increases the bone's toughness [232]. Several network architectures have been proposed, from a simple structure of uniplanar parallel hollow fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in lungs [234]. Fig. 31(a) shows a diagram of a straight vascular system [233]. Fig. 31(b) is a schematic of a more

Fig. 28. Average healing efficiency versus healing cycle number for self-healing samples with identical vascular networks using two different pumping routines or only static pressure [228].

complex branching network [234]. Such uniplanar architectures are not effective for healing delamination. To avoid this issue, threedimensional vascular networks may be included in a composite via vaporization of sacrificial fibers [235], similar to the direct-write assembly technique discussed earlier [223,224]. In these networks, a fiber is woven through the composite layup. In Esser-Kahn's work, the sacrificial fibers were made of polylactide (PLA) [235]. After the composite was cured, the PLA was vaporized by heating the sample above 200  C. Fig. 32 shows (a) a schematic (b) and an optical image of a straight weave three-dimensional network [235]. Healing efficiencies of a herringbone three-dimensional network are 80e125% whereas a parallel network using the same materials reports healing efficiencies of 35e80%, as shown in Fig. 33 [236]. Interestingly, the highest efficiencies in this system were found after the second and third self-heal cycles rather than the first healing cycle. More work is needed to identify the major advantages and disadvantages of various architectures, paying particular attention to benefits versus complexity [237]. 4.2. Scaling to bulk Scaling the vascular system for bulk materials rather than just coatings involves several potential complications that are present

Fig. 27. Average healing efficiency of microcapsule (blue), single vascular network (red), and dual vascular network (black) systems [225].

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Fig. 29. SEM micrographs of fractures surfaces for (a) neat resin and (b) resin with 0.15 volume fraction microspheres [229].

a duplicated network. Some complications are dependent on the local environment. For example, the liquid agent must have a low enough viscosity to easily flow through and out of the vascular network, but viscosity will change depending on temperature. However, as vacuum-assisted resin transfer molding has been successfully used to create vascular composites [239], the outlook for this type of self-healing composite is promising. 5. Knowledge assessment

Fig. 30. Optical micrograph of circular vascule (diameter of 200 mm) and the resin-rich pocket forming around it within a fiber laminate [230].

A number of self-healing materials have been termed “autonomic” e that is, they heal automatically as soon as damage occurs, with no external energy added to the system. Table 3 summarizes potentially autonomic and non-autonomic self-healing material systems. Materials in the “(Potentially) Autonomic” column have been proven to heal autonomously at room temperature. Materials listed in the “Non-Autonomic” column with temperature as the activation either did not heal at room temperature or did not have

Fig. 31. (a) Diagram of a straight vascular system, modified from Ref. [233]; (b) Schematic of multi-branched vascular network [234].

but less essential in coatings [238]. Adequate fluid flow is dependent on sufficient pressure within the network, possibly requiring use of a pump. Fluid supply cannot be interrupted: extremities of the vascular system may break to release healing agents, but for continued healing ability, there must be an uninterrupted connection between the local network and the reservoir for the healing material. If the fluid circulates through the network, there must be an uninterrupted path in two directions, possibly requiring

room temperature healing data reported. For many applications, the material will not be in a 20  C environment. Work is needed to characterize the effect of temperature (and temperature cycles) on healing efficiency for the variety of mechanisms. Future work could include further developing fiber optic damage monitoring methods, which have the major advantage of providing in-situ and distributed sensing [240]. To illustrate the property deficiency of current self-healing

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Fig. 32. (a) Schematic and (b) optical image of a straight-weave three-dimensional network. Sacrificial fibers (pink) are woven throughout a glass fiber mat [235].

be healed. For additional analysis of healing efficiencies, variables in an experimental design could include healing temperature and time. Furthermore, while the healing of pure polymer systems has been described with the reptation model [142], models for selfhealing composite systems are sorely lacking. 6. Concluding remarks

Fig. 33. Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular network [236].

epoxy-based composites, one may compare the healing efficiency of those materials to a relevant structural property, such as virgin fracture toughness, as seen in Fig. 34. Fig. 34 indicates achieved healing efficiencies versus virgin fracture toughness for self-healing epoxy-based composites (data from Refs. [16,47,74,173,180,182,204,205,228]). One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture toughness under 3 MPa m1/2, roughly 10% that of typical carbon fiber-reinforced epoxy composites (25e40 MPa m1/2 [241]). At present, self-healing epoxies are not useful for commercial structural applications. The incorporation of microcapsules within a matrix is known to have an effect on the structural properties of the material [174]. However, characterization of the effect of microcapsule size on failure strength or failure toughness has yet to be performed. Capsule diameter, wall thickness, and material are obvious variables of interest. Such characterization could combine analysis of the effect on structural properties with analysis of the effect on healing efficiencies, since samples must be broken before they can

Though fiber-reinforced polymer composites are widely used in many industries, failure prediction in these materials is still being developed. Without accurate and precise failure prediction, parts and structures must be physically inspected to check for damage. As composite materials can suffer internal damage without showing any external sign, non-destructive inspection can be costly and time-consuming. This expense of inspection led to the idea to create self-healing structures, structures formed of materials which are able to repair damage without additional material. To quantify the healing ability of these engineered materials, “healing efficiency” for a given material property is defined as the ratio of healed and virgin quantities. Healing efficiency may be reported in terms of any measurable material property including, but not limited to, fracture toughness, fracture stress, extensibility, or various moduli. The major drawback of using healing efficiency as a metric of “goodness” of composite material systems is that it does not take into account the effect of enabling self-repair: specifically that added constituents may weaken composite structures. A material with 100% healing efficiency may sound like a perfect option for a building material, but it should not be used if its strength, toughness, or moduli are not high enough for the given application. For clever design of stronger, tougher, or stiffer materials, one first needs to understand existing materials. With this aim in mind, this paper summarized self-healing materials into three major sections and discussed several examples. Self-healing within bulk polymers may occur by a number of mechanisms. Covalent bonds may break upon damage and reform (heal) under favorable conditions. Polyethylene oxide (PEO), for example, heals via a chain exchange reaction at room temperature [68e70]. Disulfide bonds are particularly adept at undergoing chain exchange reactions and have been used to enable healing in a number of materials [50,57,58,72,81]. Cyclic groups may also enable healing and have been incorporated within several self-healing materials [86,88,92e94]. Cycloaddition occurs under materialspecific conditions. Damaged perfluorocyclobutane polymers, for example, undergo cycloaddition and heal under stress [94] while other materials require radiation to heal [92,93,95]. Drawbacks of light-induced self-healing include (i) a light source is necessary and (ii) radiation may have unintended side-effects. Self-healing may also be accomplished via free radical interactions [98,102,103,108].

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Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems. Type

Polymer

(Potentially) Autonomic

Non-autonomic

Material [Ref.]

Material

Activation [Ref.]

thiol-functionalized poly(n-butyl acrylate) [72]

polyethylene oxide (PEO)

ploy(ethylene-co-methacrylic acid) [140,141] cyanoacrylate/epoxy [152] diarylbibenzofuranone-functionalized polymers [108] poly(isobutylene) [127] poly(vinyl alcohol) [51] thermoreversible rubbers [130,131] styrene-(n-butyl acrylate) copolymer [52,53]

poly(dimethyl siloxane) (PDMS) thirum disulfide-functionalized polyurethane tris-(cinnamoyloxymethyl) ethane coumarin-functionalized polyurethane perfluorocyclobutane polymers anthracene derivatives methyl methacrylate/n-butyl acrylate/ spironapthoxazine) copolymer trithiocarbonate-functionalized n-butyl acrylate trithiocarbonate-functionalized poly(methyl methacrylate) oxtane-chitosan oxolane-chitosan

pH [68]; pH > 100  C [69] 90  C [190] visible light [57] >280 nm radiation [92] 254e350 nm radiation [93] 180  C [94] 366 nm radiation [95] acidic vapors, sunlight, or increased temperature [82] 220e390 nm radiation [56] submerged in anisole under nitrogen atmosphere [103] 120 nm radiation [96] acidic solution [105] 302 nm radiation [106] 320e390 nm radiation [124] 140 deg. C [138] 100  C [60] 80  C [136] 115  C [135] low temperature or humid environment [123] 50  C [55,170] pH, redox reaction, temperature [151]

acrylamide-(stearyl methacrylate) copolymer [54]

UPy-functionalized poly(ethylene-co-butylene) bis-pyrenyl-functionalized polyamide polyimideepolybutadiene polyimideepolyamide polydiimideepolysiloxane ueridopyrimidone (UPy) Polymer epoxy þ dicyclopentadiene (DCPD) þ Grubbs' catalyst [44] composite 5-ethylidene-2-norbornene (as healing agent for a polymer matrix) [175] epoxy þ DCPD þ tungsten chloride [180] epoxy þ (diglycidyl ether bisphenol A)-(ethyl phenylacetate) þ scandium triflate [181] epoxy þ DCPD þ 5-ethylidene-2-norbornene [184] epoxy þ CuBr2(2-methylimidazole)4 [185] epoxy þ mercaptan [74] poly(dimethyl siloxane) resin & initiator [48] polymer þ isophorone diisocyanate þ water [186]

PDMS-poly(dimethyl siloxane) (PDES) SiO2-polymer nanotubes e.g. containing benzotriazole (for anti-corrosion) thermoplastic film þ superparamagnetic nanoparticles polyurethane þ graphene layers

oscillating magnetic field [196]

infrared light, electricity, electromagnetic waves [197] shape memory epoxy þ poly(3-caprolactone) fibers 80  C [206] linear/network poly(3-caprolactone) 80  C [219] shape memory polystyrene þ copolyester 150  C [218]

Fig. 34. Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems. Data from Refs. [16,47,74,173,180,182,204,205,228].

A major limitation of free radical healing is the reactivity of the free radicals: they may react with contaminants such as oxygen before reacting with each other and thus not heal. Supramolecular chemistry may also be harnessed to enable self-healing, including hydrogen bonding [51e54,123,124,127,132,133], pep stacking

interactions [60,135,136,138,139], and ionomeric healing [140,141]. Some limitations of these materials are that healing efficiency depends on reactive group concentration, size of damaged area, and time between the damage event and initialization of healing [51]. Furthermore, cross-linking at higher temperatures reduces the healing ability of certain materials including self-healing rubbers [133]. Self-healing may be enabled via dispersed agents within polymeric materials including structural composites like fiberreinforced epoxy. Self-healing may be enabled by various dispersed agents including encapsulation, remote self-healing, and shape memory assisted self-healing. Encapsulation may be accomplished using hollow fibers [150], nanotubes [151], or microspheres [44]. The encapsulating material may be glass [152], metal [153], or polymer [44,165]. The viscosity of the healing agent must be matched to the diameter of the capsule to obtain good flow [154]. More work is needed to characterize the effect on mechanical properties and healing efficiency of the capsules' size, concentration, and dispersion. Significant research may also be done on the healing materials: different liquid healing agents, hardeners, and catalysts may yield better healing properties. In certain matrices, the dispersed agents need not be healing agents, but rather materials which can be excited to induce localized melting [196,197]. Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for several different stimuli [197]. Unfortunately, localized heating will only cause melt in thermoplastic polymers and not thermosets, so the choice of matrix materials is limited. Dispersed shape memory materials (SMMs) can be used to assist healing by reducing crack size and thereby

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increase healing efficiency [204e206,218,219]. An intriguing aspect of shape memory assisted healing is that SMMs respond to a variety of stimuli. Major limitations of using SMMs within self-healing materials are (i) improper alignment of the SMM within the composite may increase crack size [65], (ii) the inclusion of SMM will affect mechanical properties [204], and (iii) applications will be limited by the SMM's mode of activation. A third type of self-healing may be accomplished via vascular networks incorporated into a composite. These networks may be formed by embedding hollow tubing with a matrix [59,153,220,221] or by incorporating a sacrificial material which is then removed [223e225]. These material systems are capable of repeatedly self-healing even after 25 damage cycles [225], but only so long as the incorporated healing agents are relatively new [59]. To avoid issues with shelf-life of healing agents within the vascular system, the network may be connected to an external reservoir and a pump system to allow for fresh healing materials to be flushed through the system as needed. External pumps can be used to improve mixing and healing efficiencies, but utilizing pumps requires pumping routines to be developed for the specific vascular network architecture being used [227,228]. The network architecture will affect composite microstructure [230] and may increase difficulty of manufacturing. Network architecture is also expected to affect mechanical properties, flow dynamics, and crack propagation as well as failure modes of these composites [226,233,236,237]. In addition to these considerations, addition potential complications must be addressed before vascular systems can be used in bulk structural materials: adequate fluid healing agent flow must be maintained, necessitating pressure control within the network as well as uninterrupted fluid supply. While a number of self-healing materials have been presented, few are capable of autonomous healing and those that have been identified as potentially autonomous are typically only characterized at ambient conditions (i.e. 20  C). Work is needed to characterize the effect on healing efficiency varying temperature and cyclic temperature may have. Furthermore, most of the self-healing materials presented herein are not structurally capable. A comparison of self-healing epoxy-based composites and typical epoxy composites highlights this property deficiency: fiber-reinforced self-healing epoxy composites have virgin fracture toughness roughly 10% that of typical carbon fiber-reinforced epoxy composites [16,205,241]. Perhaps the greatest limitation on commercialization of self-healing materials is that lack of characterization of effect on mechanical properties of healing-enabling constituents such as microcapsules or vascular networks. References [1] M.W. Urban, Dynamic materials: the chemistry of self-healing, Nat. Chem. 4 (2) (2012) 80e82. [2] R.S. Trask, H.R. Williams, I.P. Bond, Self-healing polymer composites: mimicking nature to enhance performance, Bioinspiration Biomimetics 2 (1) (2007) 1e9. [3] R.F. Diegelmann, M.S. Evans, Wound healing: an overview of acute, fibrotic and delayed healing, Front. Biosci. 9 (2004) 283e289. [4] T. Velnar, T. Bailey, V. Smrkolj, The wound healing process: an overview of the cellular and molecular mechanisms, J. Int. Med. Res. 37 (5) (2009) 1528e1542. [5] Y. Bar-Cohen, Biomimeticsdusing nature to inspire human innovation, Bioinspiration Biomimetics 1 (1) (2006) P1eP12. [6] P. Theato, et al., Stimuli Responsive Materials, 42(17), Chemical Society, 2013, pp. 7055e7056. [7] E.N. Brown, S.R. White, N.R. Sottos, Retardation and repair of fatigue cracks in a microcapsule toughened epoxy compositedPart II: in situ self-healing, Compos. Sci. Technol. 65 (15e16) (2005) 2474e2480. [8] D.O. Olawale, et al., Progress in triboluminescence-based smart optical sensor system, J. Luminescence 131 (7) (2011) 1407e1418. [9] D. Michaels, Their New Materials, in: The Wall Street Journal, Dow Jones & Company, New York City, New York, U.S.A, 2013. [10] T. Chady, Airbus versus Boeing e Composite Materials: the Sky's the Limit,

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Products Related to the Project 1 T.J. Dickens, C. Armbrister, D. Olawale, O. Okoli, Characterization of triboluminescent enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture assessment, J. Lumin., http://dx.doi.org/10.1016/j.jlumin. 2015.02.030. 2 M. Roy, K. Joshi, T. Ndebele, K. Williams, D. Olawale, T. Dickens, Preliminary investigation: additive manufacturing of soluble mold tooling for embedded devices in composite structures, in: Society for the Advancement of Material and Process Engineering (SAMPE) (CAMX), Orlando, Florida, October 18e22. 3 O. Okoli, B. Wang, T.J. Dickens, Systems, Methods, and Apparatus for Structural Health Monitoring, Florida State University. Tallahassee, FL., 22nd November, 2012, U.S. Patent and Trademark Office, No. 12/691.537. 4 T.J. Dickens, J. Breaux, D.O. Olawale, W.G. Sullivan, O.I. Okoli, Effects of ZnS:Mn concentrated vinyl ester matrices under flexural loading on the triboluminescent yield, J. Lumin. 132 (7) 1714e1719, http://dx.doi.org/10.1016/j.jlumin.2012.01.056. 5 T.J. Dickens, O.I. Okoli, Enabling damage detection: manufacturing composite laminates doped with dispersed triboluminescent materials, J. Rein. Plast. Comp. 30 (22) (2011) 1869e1876, http://dx.doi.org/10.1177/0731684411413490.

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6 T.J. Dickens, O.I. Okoli, Z. Liang, Harnessing triboluminescence for structural health monitoring of composite structures, in: Society for the Advancement of Material and Process Engineering (SAMPE) Annual Conference, Long Beach, California, May 18e22, SAMPE, Long Beach, CA, 2008.

Other Significant Products 1 J. Yan, M.J. Uddin, T.J. Dickens, D.E. Daramola, O.I. Okoli, 3D wire-shaped dyesensitized solar cells in solid state using carbon nanotube yarns with hybrid photovoltaic structure, Adv. Mater. Interfaces 1 (6) (2014) 7, http://dx.doi.org/ 10.1002/admi.201400075. 2 O. Okoli, J. Yan, T.J. Dickens, M.J. Uddin, Dye-Sensitized Solar Cells Including Carbon Nanotube Yarns, Florida State University. Tallahassee, FL, 22nd July, 2014, U.S. Patent and Trademark Office, No. 62/027, 608. 3 M.J. Uddin, D.E. Daramola, E. Velasquez, T.J. Dickens, J. Yan, E. Hammel, F. Cesano, O.I. Okoli, A high efficiency 3D photovoltaic microwire with carbon nanotubes (CNT)-quantum dot (QD) hybrid interface, Phys. Status Solidi RRL 8 (11) (2014) 898e903, http://dx.doi.org/10.1002/pssr.201409392. 4 D.O. Olawale, K. Kliewer, A. Okoye, T.J. Dickens, M.J. Uddin, O.I. Okoli, Getting light through cementitious composites with in-situ triboluminescence damage sensor, Struct. Health Monit. 13 (2) (2014) 177e189, http://dx.doi.org/10.1177/ 1475921713513976. 5 D.O. Olawale, K. Kliewer, A. Okoye, T.J. Dickens, M.J. Uddin, O.I. Okoli, Real time failure detection in unreinforced cementitious composites with triboluminescent sensor, J. Lumin. 147 (2014) 235e241, http://dx.doi.org/10.1016/j.jlumin.2013. 6 M. Scheiner, M. McCrary-Dennis, D. Olawale, O. Okoli, NSF-Retaining Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT), in: 121st ASEE Annual Conference & Exposition Proceedings, Indianapolis, Indiana, United States, June 15e18, 2014, 2014. 7 X. Xin, M. Scheiner, M. Ye, Z. Lin, Surface-treated TiO2 nanoparticles for dyesensitized solar cells with remarkably enhanced performance, ACS Langmuir 27 (23) (2011) 14594e14598.

Ms. Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at Florida State University with a BS in Materials Science & Engineering from Cornell University. She has contributed to research on dyesensitized solar cells, synthesis of highly triboluminescent crystals, and pulsed laser deposition of non-stoichiometric thin films. Her current research aims to create a selfhealing composite with integrated structural health monitoring capabilities. She is a teaching assistant for the Industrial Engineering program's Senior Design Project course, is a coordinator of the summer internship

programs (NSF-REU and AFRL-DREAM), and has extensive STEM outreach experience through DreamOn as well as local chapters of the Society of Women Engineers, the Society for the Advancement of Material and Process Engineering, Golden Key International Honour Society, and Phi Kappa Phi.

Dr. Tarik Dickens' research interest includes focus on cradle-to-grave production of additively manufactured composite structures/tooling and systems integration for AM performance technologies. With development of nanostructured hybrid materials for mechanical toughening, energy conversion/storage and integrated-Structural Health Monitoring with over 20þ publications. He has 2 US patent applications (awarded and pending) in the areas of advanced composites and sensory-scaled composite manufacturing, and ubiquitous real-time structural health monitoring. In addition, he runs the Industrial Composite Engineering (ICE) lab involving sensing techniques & non-destructive testing of advanced materials at the High Performance Materials Institute (HPMI) for failure analysis. He has outreach experience in organizing and supervising programs involved with STEM initiatives (NSFREU and AFRL-DREAM summer programs).

Dr. Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the Florida A&M University e Florida State University (FAMU-FSU) College of Engineering. His research group has provided extensive insight into the development of functional and affordable composite manufacturing technologies, for which he has received several awards. Dr. Okoli's research efforts include the development of integrated structural health sensing within concrete and within advanced composite structures. He also focuses on the development of photovoltaic sensors, innovative 3D energy conversion systems, and scalable processes to allow the manufacture of customizable multifunctional composite structures. He has 7 US patent applications (awarded and pending) in the areas of advanced composites and multiscale composites manufacturing, structural ceramics, and ubiquitous realtime structural health monitoring. He is a chartered engineer and a chartered scientist.