Polyethylene and polypropylene matrix composites for biomedical applications

CHAPTER

Polyethylene and polypropylene matrix composites for biomedical applications

6

Aravinthan Gopanna1,2, Krishna Prasad Rajan2,3, Selvin P. Thomas1,3 and Murthy Chavali4,5 1

Advanced Materials Laboratory, Yanbu Research Center, Royal Commission for YanbuColleges and Institutes, Yanbu Industrial City, Kingdom of Saudi Arabia 2School of Chemical Engineering, Vignan’s Foundation for Science, Technology and Research University (VFSTRU; Vignan’s University), Guntur, India 3Department of Chemical Engineering Technology, Yanbu Industrial College, Royal Commission Colleges & Institutes, Yanbu Industrial City, Kingdom of Saudi Arabia 4Shree Velagapudi Ramakrishna Memorial College, Acharya Nagarjuna University, Guntur, Andhra Pradesh, India 5MCETRC, Tenali, Guntur, Andhra Pradesh, India

6.1 INTRODUCTION Polyolefins are a class of commodity thermoplastics that are the most widely produced and consumed all over the world. Their low cost, easy availability, ease of processing, light weight, and easy recyclability are some of the reasons behind their wide popularity, mainly as packaging materials. Polyolefins can be categorized, on the basis of their monomeric repeating units and polymer chain structures, into ethylene-based polyolefins, propylene-based polyolefins, higher polyolefins, and polyolefin elastomers (Gahleitner, 2001). Ethylene-based polyolefins are predominantly of two types: mostly linear highdensity polyethylene (HDPE) usually manufactured under conditions of low pressure utilizing transition metal catalysts, and mostly branched low-density polyethylene (LDPE) prepared under conditions of high pressure using oxygen or peroxide initiators (Nwabunma and Kyu, 2008). Transition metal-based catalysts are used for the production of propylene-based polyolefins, which results in linear chain structures having a stereospecific arrangement of propylene units. Transition metal-based catalysts used for the preparation of higher polyolefins also result in linear and stereospecific chain structures. Metallic or single-site catalysts are also used for the production of polyolefin elastomers containing a mixture of ethylene and propylene. Dienes are normally included in these types of elastomers for the purpose of crosslinking. These types of polymers are predominantly amorphous and possess a heterogeneous type of phase structure and high molecular weights Materials for Biomedical Engineering: Thermoset and Thermoplastic Polymers. DOI: https://doi.org/10.1016/B978-0-12-816874-5.00006-2 © 2019 Elsevier Inc. All rights reserved.

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(Vasile, 2000). It can be summarized that a polyolefin can be considered as a homopolymer, copolymer, or terpolymer based on the type of monomers used in the polymerization process. The crystallinity or amorphous region in the polyolefin depends on the arrangement of the polymer chains, chain configuration, and also on the conditions applied during processing (Brydson, 1999). Research advancements in the area of single-site transition metal-based catalysts have helped in the development of novel polyolefin homopolymers, copolymers, and terpolymers with controlled molecular architecture, microstructure with a wide range of molecular weights, and molecular weight distributions. These advancements have helped expand the range of applications of polyolefins. At present, polyolefins and polyolefin-based polymeric materials are finding uses in various applications that are spread across many fields. These applications include packaging, consumer products (toys, household items, appliance body parts, etc.), transportation (automotive and aerospace components), biomedical, communication and electronics, cable and wire coatings, thermal, electrical, and acoustic insulation, building and construction products, etc., to name a few (AlMa’adeed and Krupa, 2015). Polyolefins can be easily processed into many shapes. They can be extruded into fibers or filaments, blown films and cast films, and pipes and profiles. They can easily be compression molded or injection molded into various shapes. They can also be foamed into cellular plastics with the help of various foaming additives and physical or chemical blowing agents. They can also be coated onto other materials. Polyolefin homopolymers, copolymers, and terpolymers can be produced through free radical or ionic polymerization of corresponding alkenes with the help of conventional free radical initiators, such as peroxides and organometallic-type catalysts (Ziegler Natta or metallocenes). Developments in the field of polyolefin polymerization technology and unique catalyst systems have helped produce polyolefins with a wide range of structures, configurations, morphologies, molecular weights, and molecular weight distributions. Subsets of polyolefin homopolymers are polyethylene (PE), polypropylene (PP), polybutylene (PB), poly-1 methylpentene (PMP), and higher polyolefins. Out of these polyolefin materials, PE and PP are the most produced polymers by all worldwide polyolefin manufacturers (White and Choi, 2005a,b). PE can again be subdivided based on its chain structures, crystallinity, and density, into HDPE, LDPE, linear low-density polyethylene (LLDPE), ultralow density polyethylene (ULDPE), high molecular weight polyethylene, and ultrahigh molecular weight polyethylene (UHMWPE). PP and the other higher polyolefins are commercially manufactured with three main stereospecific arrangements: isotactic, syndiotactic, and atactic (Nwabunma and Kyu, 2008).

6.2 POLYOLEFIN COMPOSITES Polyolefin composites can be considered as a subclass of polymer composites (Nwabunma and Kyu, 2008). Polyolefin-based composites are developed to

6.4 Biocompatibility Evaluation of Polyolefin-Based Biocomposites

address the demand for higher load-bearing materials that are required for engineering applications which cannot be satisfied by polyolefins alone. In addition to the base polyolefin matrix, polyolefin composites contain at least one nonpolymeric additive which acts as a reinforcement. The incorporation of this additive can lead to the development of polyolefin composites that show enhanced overall properties of the resulting composite structure. Nonpolymeric additives that are used for the fabrication of polyolefin composite materials can be of different sizes and shapes, natural or synthetic origins, particulate, fibrous (long, short, oriented), flakes, etc. Other than fillers, various other additives are also incorporated into polyolefin composites, such as stabilizers, plasticizers and processing aids, antioxidants, antiozonants, UV stabilizers, coupling agents, blowing agents, flame retardants, pigments, and fungicides, etc. (Tolinski, 2015). The various types of fillers used for the preparation of polyolefin composites include various natural fibers, glass fibers, minerals of clay types, carbon black, carbon fibers, single or multiwalled carbon nanotubes, graphite, titanium dioxide, magnesium hydroxide, calcium phosphate (hydroxyapatite, HA), aluminum trihydroxide, calcium carbonate, silica, etc. (Rothon and DeArmitt, 2017). The properties of the resulting polyolefin composites mainly depend on the properties of the matrix polymers, weight or volume fractions of the reinforcements, and also on the interactions between the matrix and the reinforcements.

6.3 BIOMEDICAL ENGINEERING The branch of engineering that deals with the application of engineering concepts and design methodologies in the field of medicine and health care is known as biomedical engineering. The areas under this field of engineering deal with various clinical sectors, such as diagnosis, monitoring, and treatment of diseases. The most popular applications in the field of biomedical engineering include scaffolds and tissue engineering, design and development of biocompatible implants, different types of therapeutic, diagnostic, or monitoring devices, medical imaging devices, drugs and pharmaceutical engineering, and drug administration techniques, etc., to name a few. Polyolefins as engineering materials find applications in most of the mentioned areas either as standalone materials or as composites in combination with fibrous or particulate fillers (Ramakrishna et al., 2001).

6.4 BIOCOMPATIBILITY EVALUATION OF POLYOLEFIN-BASED BIOCOMPOSITES Biocompatibility, or the evaluation of the biological responses of the prospective biomaterial, is an important step in the development of materials and devices for biomedical engineering related applications. The main challenge in the evaluation of biocompatibility of composites is due to the fact that composites contain more

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than one distinct phase. In composites, the matrix is the continuous phase and the reinforcement is the discontinuous phase. The matrix can be in macro- or microdimensions and the reinforcement usually exist on a micro- or nanolevel and the contributions of each of these phases toward tissue/material interactions should be carefully monitored. The biocompatibility of a material or device is defined as its ability to exhibit a favorable host response during its intended application. The surface of the device or material plays a vital role in controlling the host response. For conventional types of composite materials with one matrix at the tissue/device interface, the evaluation of biocompatibility is much easier than for composites with more than one matrix and many other additives as previously mentioned. There are two major challenges involved in the evaluation of biocompatibility of polyolefin composite materials. These are related to the evaluation of biocompatibility of the composite as a whole (including the individual contributions provided by the matrix and the reinforcement phase) and the surface characterization of the composite structure (Anderson and Voskerician, 2009). Developments in the field of nanotechnology, tissue engineering, and regenerative medicine pose more challenges to the evaluation of biocompatibility of polyolefin composite structures.

6.4.1 TESTS FOR BIOCOMPATIBILITY The tests for evaluation of biocompatibility are broadly divided into three types. These are primary (level I), secondary (level II), and preclinical (level III) tests (de Moraes Porto, 2012). Level I tests are generally in vitro cytotoxicity studies. These cell culture tests are considered as primary screening tests for the material and involve the interaction of the biomaterial with a selected cell line outside of the biological environment. There are three primary cell culture assays for the evaluation of biocompatibility. These are direct contact, agar diffusion, and elution (extract dilution) (Ratner et al., 2004). The cell lines used for cytotoxicity studies include mouse fibroblasts, lymphocytes, human lymphocytes, polymorphonuclear leukocytes and mixed leukocytes, mouse macrophages, mouse embryo cells, etc. (de Moraes Porto, 2012). The selection of a cell line for testing a material intended for a specific application should be based on the type of assay, measurement endpoints, such as viability, enzymatic activity, species receptors, etc., and also on the practical experience of the investigator (Morais et al., 2010). Level II or in vivo tests include tests for irritation, intracutaneous reactivity, systemic toxicity, subchronic toxicity, genotoxicity, implantation, hemocompatibility, chronic toxicity, carcinogenicity, reproductive and developmental toxicity, biodegradation, and immune responses (Anderson, 2004). The results from in vivo tests are based on the assessment of tissues from test animals that have received the implants. The selection of these test methods depends on the type of application of the material/device and the nature or duration of body contact as described by ISO 10993. A generalized description of various tests as per ISO and FDA is given in Table 6.1.

Table 6.1 Generalized Description of Various Tests as per ISO and FDA

External communicating devices

Indirect blood path Tissue/bone/ dentin Circulating blood

Implant devices

Bone/tissue

Blood



O Evaluation required by ISO and FDA. Additional evaluation required by FDA.

O O O O O O O O O O O O O O O O O O O O O O O

O

O











O



O O O



O

O



O O

O O

O O



O O O

O

O



O O O O O



O

O O

O O

O O

O O O

O O O O O

O O

O O O O O

O O O

O

O

O

O

O

O



O O O O O

O O

Biodegradation

O



Reproductive/ Developmental Toxicity

 

Carcinogenicity





Chronic Toxicity



Hemocompatibility

Implantation

Irritation

O O O O O O O O O O O O O O O O O O O O O O O O

Genotoxicity

Breached surfaces

O O O O O O O O O O O O O O O O O O O O O O O O

Subchronic Toxicity

Mucosal membrane

A B C A B C A B C A B C A B C A B C A B C A B C

Systemic Toxicity

Skin

Contact Duration A: ,24 h; B: 24 h for 30 days; C: .30 days.

Sensitization

Body Contact Surface contacting devices

Supplementary Evaluation

Initial Evaluation

Cytotoxicity

Device Categories

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6.5 FABRICATION TECHNIQUES FOR POLYOLEFIN BIOMEDICAL COMPOSITES Polyolefin matrix composites have achieved commercial accomplishment in biomedical applications. A vital step in the preparation of biomedical polyolefin matrix composites is fabrication. Fabrication is defined as the technology of transferring raw polymer into substances in a preferred shape and size. An important step in the thermoplastic processing method is to produce flowability. In the heating stage of the processing method, the polymer molecules slip past each other and produce flow. In the cooling stage of the processing method, the polymer molecules solidify and are shaped. The phases of flowing, shaping, and hardening of thermoplastic materials can be carried out in the fabrication machine. Composites consist of reinforcement materials held in place by a matrix system. Fabricating a composite entails the process of incorporating reinforcement material in the polymer matrix with a definite orientation to provide specified characteristics for the finished product and, thereby, execute its design function. The growth of polyolefin-based biomedical composites both in a number of applications and in volume is related to the ease of their processability. While high processing temperatures, high melting viscosity, lack of drape, etc., cause complications during the polyolefin composite fabrication process. Polyolefin composites are stronger than thermosets composites with the contribution of their amorphous and semicrystalline structures. The amorphous portion has a number of entangled chains and provides enhanced toughness to the composites. The semicrystalline portion contributes to the plasticizing consequence of the materials. Polyolefin composites offer a longer shelf life and a shorter processing time. Polyolefin high molecular weight characteristics result in high processing temperatures and high melting viscosity during processing. The shear thinning flow behavior of polyolefin materials prevents easy fiber wet out and causes complexity during fabrication. Conventional processing techniques are modified to the fabrication of thermoplastic biomedical composites. Polyolefinbased composites are shaped into various biomedical devices through processing methods such as injection molding, compression molding, extrusion, electrospinning, etc. Parameters like softening temperature, size, and shape are considered when choosing a processing method. In general, biomedical composites are fabricated in a clean room to diminish the incorporation of inappropriate substances into the products (Olabisi and Adewale, 2016; White and Choi, 2005a,b; Biron, 2012; Nwabunma and Kyu, 2008; Ambrosio, 2009). A schematic representation of processing techniques for polyolefin-based composites is shown in Fig. 6.1. This section will explore modified techniques used in the manufacturing of polyolefin composites. The basic processing steps as well as the advantages and disadvantages of each processing method are discussed.

6.5 Fabrication Techniques for Polyolefin Biomedical Composites

FIGURE 6.1 Flowchart of processing techniques for polyolefin-based composites.

6.5.1 MOLDING A mold is a hollow structure that imparts its shape to finished products. The term “molding” includes injection, compression, blow, and rotational molding processes. Injection molding is the most commonly used processing method used for the manufacturing of polyolefin biomedical composites. Injection molding is a rapid technique to produce products by injecting polymer materials into a mold. In the injection molding process, thermoplastic filled materials are melted in a chamber to a temperature that leads the materials to flow and they are then pumped into a closed mold through a rotating screw. The polymer melt solidifies on cooling and the mold is opened for the removal of the finished product. The injection molding technique provides excellent product consistency, little postproduction scrap, good dimensional control, high production rate, etc. The disadvantages of the injection molding process include the requirement of expensive equipment investment and product design restrictions, etc. (Bryce, 1996; Olmsted and Davis, 2001; Rosato and Rosato, 2012). Short-fiber polyolefin composites are injection molded, however, fiber orientation and distribution are difficult to control. Haneef Mohammed et al. prepared HDPE composite materials for use in orthopedic applications with hybrid reinforcement materials, such as titanium dioxide and alumina particles using an injection molding machine (Haneef et al., 2013). In the compression molding process, compounded pellets are placed between stationary and movable molds and then pressure and heat are applied to achieve a consistently shaped composite. The compression molding technique offers the least expensive tooling, large part production, little material wastes, etc. The disadvantages of the compression molding process include its slower process

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FIGURE 6.2 Flowchart of molding process.

time, its inappropriateness for complex product design, high labor cost, etc. Juhasz et al. developed biomedical composites for use as implant materials that consist of glass ceramic apatite wollastonite particulate matter reinforced with HDPE using a compression molding technique (Juhasz et al., 2004). All ceramic particle-reinforced polyethylene composites without chemical coupling are manufactured through compression molding (Wang et al., 1998a). Fig. 6.2 shows a flowchart of the molding process.

6.5.2 EXTRUSION Extrusion is a versatile process used for manufacturing products with a uniform cross-section, like hoses, films, sheets, etc., by forcing materials through a die under controlled conditions. The five main components of the extrusion process are a drive system, a feed system, a barrel system, die assembly, and a control system. The polymer extrusion process includes single-screw and twin-screw extruders. Melting, compression, and metering are the main steps of the extrusion technique. In the melting section, pellets are transferred from a hopper and converted into molten polymer. In the compression section, the molten polymer is compressed and mixed with the different additives. The metering section is to produce the desired product through a shape-forming die. The final products are shaped and cooled (Carley, 1989; Nakajima, 1997; Rauwendaal, 2014). Films are formed through film blowing thin-walled tubes or drawing cast films. The extrusion technique is also used for the purpose of compounding polyolefin-based biomedical composites, for example, hydroxyapatite particulate reinforced UHMWPE nanocomposite with superior toughness for orthopedic applications is compounded using twin-screw extrusion (Fang et al., 2006). Ram extrusion consists of a hopper that permits material to go into a heated cavity, a reciprocating ram, and a die unit. UHMWPE-based biomedical composite products can be manufacture using a ram extrusion process (Kurtz, 2004). In hydrostatic extrusion, the workpiece is fully enclosed by pressurized liquid. When the ram moves forward, the delivered force pressurizes the liquid and applies pressure to all surfaces of

6.5 Fabrication Techniques for Polyolefin Biomedical Composites

FIGURE 6.3 Schematic representation of hydrostatic extrusion process.

the workpiece that pushes the work through the die. Fig. 6.3 shows a schematic representation of the hydrostatic extrusion process. Hydrostatic extrusion is useful for enhancing the mechanical properties of hydroxyapatite/HDPE composites for load bearing implant applications. Polyethylene chains can align in the extrusion direction to produce high strength and high modulus parts (Wang et al., 1998b; Ladizesky et al., 1997).

6.5.3 MELT ELECTROSPINNING Electrospinning is a technique used to produce fibers with a diameter less than 100 nm. Electrospinning includes a high voltage power supply, grounded collector, and positively charged capillary packed with polymer fluid. In this process, a high voltage electric field is applied to form fibers from polymer fluid that is conveyed through a capillary. A liquid polymer jet is formed and elongated under the action of electrostatic repulsion and finally deposited on a grounded collector which serves as an electrode. Electrospun fibers have great prospective in biomedical applications, such as in scaffolds, drug delivery, wound dressing, implants, etc. Melt electrospinning is an interesting alternative technique to conventional solvent electrospinning (Ramakrishna, 2005). In the melt electrospinning process, the polymer melts, which is normally more viscous when polymer solution is used. The molten electrified jet requires cooling for solidifying and results in micron diameter fibers. Melt electrospinning is suitable for nonsoluble polymers, like polyolefin, and thereby ensures that no solvent is present in the final fiber. Organic solvents that lead to cellular cytotoxicity are not used in this process and benefit from the

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FIGURE 6.4 Schematic representation of melt electrospinning setup.

applications in biomedical sectors (Ferrari et al., 2007; Agarwal et al., 2008). Melt electrospun fibers show excellent properties, like large surface area, outstanding mechanical performance, high length/diameter ratio, etc. Fig. 6.4 shows a schematic representation of a melt electrospinning setup.

6.5.4 FILAMENT WINDING Filament winding is a process that involves winding strands under tension over a male mandrel, which is suitable for producing open or closed-end structures. The filament winding technique is used to produce parts with excellent repeatability, high performance, quality internal surface, high fiber-to-resin ratios, minimum labor involvement, etc. The disadvantages of this process include the requirement for a high initial investment and the inability to generate reverse curvature, etc. In the thermoplastic filament winding process, fiber strands unwind and continuously pass a thermoplastic resin tank. In the resin tank, the fiber strands are completely impregnated with thermoplastic resin. These resin impregnated strands are wound around the mandrel in a controlled manner and in a definite fiber orientation (Hoa, 2009; Strong, 2008). Kazanci et al. developed a butane ethylene copolymer reinforced with UHMWPE fibers using a filament winding technique, and these composites were potentially intended for ligament or tendon prostheses (Kazanci et al., 2002a,b). Fig. 6.5 shows a representation of fiber wound on a mandrel in a filament winding method.

6.5 Fabrication Techniques for Polyolefin Biomedical Composites

FIGURE 6.5 Representation of fiber wound on a mandrel in filament winding method.

FIGURE 6.6 Flow diagram represents the steps in the thermoplastic pultrusion process.

6.5.5 THERMOPLASTIC PULTRUSION Pultrusion is a continuous and cost-effective processing technique used for the production of composites with close-dimensional cross-sections. Pultrusion is an ideal process for the manufacturing of either solid or hollow profile-like flat bars, channels, pipes, tubing, rods, etc. (Hoa, 2009; Strong, 2008). In the thermoplastic pultrusion method, preheated continuous fiber strands are pulled into the impregnated apparatus in order to wet out the fibers. Then the melt impregnated reinforcements are passed through a cooling die, which controls the shape, size, and finish of the finished products. A puller is used to control the speed of the process and generate a dragging force on the products. Finally, a pelletizing system is used to cut the final products (Tao et al., 2015). The pultrusion technique is wellsuited for parts that need good dimensional tolerance, high fiber volume fractions, excellent reinforcement alignment, precise control of resin and fiber, low scrap rates, etc. Some of the limitations of this process are the requirement for a high initial investment and for skilled labor, etc. Fig. 6.6 shows the steps involved in the thermoplastic pultrusion process.

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6.6 POLYETHYLENE MATRIX Polyethylene is produced from ethylene monomers having a molecular weight of 28. The chemical formula of polyethylene is (C2H4)n , where n is the number of repeat units. A representation of the chemical structures of ethylene and polyethylene is shown in Fig. 6.7. Polyethylene is available in several forms, such as LDPE, LLDPE, HDPE, and UHMWPE, which are produced with different molecular weights and chain structural designs (Peacock, 2000). Schematics of the molecule alignment in different forms of polyethylene are shown in Fig. 6.8.

FIGURE 6.7 Representation of the chemical structures of ethylene and polyethylene.

FIGURE 6.8 Schematics of molecule alignment in different forms of polyethylene.

6.6 Polyethylene Matrix

LDPE has a branched chain structural design and LLDPE has a linear chain structural design with a molecular weight of less than 50,000 g mol21. HDPE has linear chain architecture with a molecular weight of 200,000 g mol21 and a crystallinity of 60% 80%. HDPE resin is naturally translucent and offers good low-temperature toughness, creep resistance, moisture barrier, impact resistance, chemicals resistance, etc. The molecular weight of UHMWPE cannot be calculated by conventional techniques, however, it is inferred by its intrinsic viscosity. UHMWPE generally has a viscosity average molecular weight of 6 million g mol21 with a crystallinity of 50% 60%. UHMWPE is an odorless, tasteless, and nontoxic material made up of extremely long chains of polyethylene that all align in the same direction. UHMWPE resin provides high ultimate strength, good impact strength, excellent wear resistance, resistance to corrosive chemicals, extremely low moisture absorption, low coefficient of friction, etc. (Kurtz, 2004; Brydson, 1999). The significant mechanical properties of HDPE and UHMWPE compared to other polyethylenes contribute to the development of their composites as matrices or reinforcements for various biomedical applications, such as knee/hip/shoulder joints. Typical average properties of different forms of polyethylenes are given in Table 6.2.

6.6.1 HDPE-BASED BIOMEDICAL COMPOSITES HDPE material is chosen for various biomedical and technical applications due to its excellent chemical and creep resistance characteristics. Bones and joints made up of natural composite materials are often fractured due to diseases, impact stress, traumatic situations, etc., and thereby need to be temporarily or permanently restored. The search for a bone replacement material that combines the mechanical and biological necessities has been extensively carried out in ceramics, metals, and polymers (Ambrosio, 2009). Composites of HDPE reinforced with HA have been considered as materials for bone replacement, middle ear prostheses, and orbital floor implants without any inflammatory response (Hule and Pochan, 2007; Zhang et al., 2007). The concept of fabricating bioactive composites for bone replacements was developed in the early 1980s by William Bonfield and coworkers with bioactive HA-reinforced HDPE composites (Bonfield et al., 1981; Wang and Porter, 1994). HDPE reinforced with 40 vol.% of hydroxyapatite particles was commercialized with the trade name HAPEX. Hydroxyapatite particles within composite materials were observed to perform as microanchors, providing encouraging cell attachment sites (Huang et al., 1997). An HA content of 40 vol.% concentration in HDPE with controlled surface topology provides greater bioactivity with enhanced osteoblast proliferation, skeletal tissue restoration, and durability of implants (Di Silvio et al., 1998, 2002; Dalby et al., 2000, 2002). Zhang et al. carried out in vitro biocompatibility of 30% of HA/HDPE composites with human osteoblast cells from femoral heads and skull bones and observed normal growth cycles on the composites. Rough surface HDPE/HA composite implants offer superior cellular response compared to

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Table 6.2 Typical Average Properties of Different Forms of Polyethylene Property

LDPE

LLDPE

HDPE

UHMWPE

0.910 0.925 ,0.01

0.92 0.94 ,0.01

0.941 0.965 ,0.01

0.928 0.94 ,0.01

16 90 800 0.20 0.40 0.08 0.60 41

30 500 0.30 0.70 0.60 1 45

38 20 1000 0.60 1.40 1 2 31

41 200 500 0.7 0.8 1 1.7 No break

81 97 Shore A

44 70 Shore D

33 66 Rockwell R

66 Shore D

105 118 -

122 124 -110

126 135 -110

130 135 -110

32

35

54

48

5.6 12.2

8

6.1 7.2

7.8

460 700

600

450 500

900

2.25 2.35 0.0002 10 15

2.36 0.0002 10 15

2.30 2.35 0.0003 10 15

2.30 2.35 0.0002 10 18

135 160

200 230

200 250

250 350

1.51 4 50

68 92

1.54 10 50

-

Physical Density (g cm23) Water absorption, 24 h, 1/8 in. thick (%) Mechanical Tensile strength (MPa) Elongation at break (%) Tensile modulus (GPa) Flexural modulus (GPa) Izod impact, notched (kJ m22) Hardness Thermal Melting temperature (Tm) ( C) Glass transition temperature (Tg) ( C) Deflection temperature at 1.8 MPa ( C) Coefficient of linear thermal expansion ( 3 1025 in. (in.  F)21 Electrical Dielectric strength (V mil21) short time, 1/8 in. thick Dielectric constant at 1 kHz Dissipation factor at 1 kHz Volume resistivity (ohm-cm) at 73 F, 50% RH Arc resistance (sec.) Optical Refractive index Transmission, visible (%)

smooth-surfaced implants, and their adequate impact characteristics make them a likely contender for skull implants. (Zhang et al., 2007). Wang et al. investigated the effects of HA particle size on composite properties and found that HA particle-reinforced composites with smaller sized HA particles had higher tensile strength and tensile modulus and lower strain to failure (Wang et al., 1998b). The

6.6 Polyethylene Matrix

modulus of HDPE/HA composites closely matches to bone, thus, providing a practical solution for the bone resorption problem (Wang and Bonfield, 2001). Zhang and Tanner studied the impact properties of HDPE/HA composites and found that the impact strength of HDPE/HA composites decreased with increasing HA content due to weak filler-matrix interfaces resulting in creation of voids and the propagation of cracks (Zhang and Tanner, 2003). The uniaxial and biaxial fatigue behaviors of 40 vol.% HA-reinforced HDPE composites were investigated by That et al. (That et al., 2000a,b). The uniaxial fatigue test results found that the ultimate strengths were uppermost in compression and torsion. The composites were ductile in torsion, but more brittle in tension. The weaker filler/matrix resin interface was liable for the failure in torsional fatigue. The biaxial fatigue was dominated by a shear mechanism and the test results revealed that out-of-phase loading was less destructive than in-phase loading, while failure was not observed at 25% of ultimate tensile strength and ultimate shear stress. The incorporation of a compatibilizing agent and a surface treatment of HA favor the dispersion, distribution, and interfacial interaction of the filler particles in a polymer, which enhances the mechanical properties of HDPE/HA composites. Wang et al. reported the enhancement of HDPE/HA composite properties through mechanical interlocking at the matrix-reinforcement interface by the incorporation of acrylic acid grafted HDPE as a compatibilizer and through surface treatment of HA with silane coupling agents (Wang et al., 2000; Wang and Bonfield, 2001). Balakrishnan et al. explored the use of maleated high-density polyethylene (mHDPE) as a compatiblizing agent for HDPE/HA composites for bone replacement applications. The addition of mHDPE as a compatibilizer enhanced the interfacial adhesion between HDPE and HA particles through the formation of HDPE fibrils and this fibril network was interconnected with the HA particles. Albano et al. investigated composites based on HDPE reinforced with surface treated HA (Albano et al., 2009). The surface treatment was carried out with an ethylene acrylic acid copolymer and acrylic acid. An in vitro study of the composites with the ethylene acrylic acid copolymer treated HA showed improved cell adhesion. The results of the surface treated HA composites were credited to the interactions between the ethylene acrylic acid copolymer and the HA. Joseph et al. studied the effect of an HDPE matrix on the rheological behavior of HA filled composites (Joseph et al., 2001a,b, 2002). The incorporation of HA powder into the HDPE matrix increased the plateau value and reduced the melt compressibility. Carmen et al. found that a zirconate coupling agent was better executed than a titanate coupling agent in HDPE/HA composites based on its mechanical properties. Zirconate as a coupling agent has a richer electronic density than a titanate coupling agent, which means the elevated electronic richness of the metal-oxygen bonds contributed to the enhanced interactions between the polymer and the reinforcement (Carmen et al., 2007). The formation of an apatite layer, a “seaweed-like” structure, was observed with increasing stimulated body fluid (SBF) immersion time on the outside of HDPE/HA composites with mHDPE as a compatibilizer. The enhanced growth of

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apatite layers indicates the excellent biocompatibility properties of the composites and highlight its potential for use in bone implantation (Balakrishnan et al., 2013). Apatite layer formation on the outside of HDPE/HA composites in SBF exhibits in vitro bioactivity behavior and are capable of tying to living bone face in the human body (Kokubo and Takadama, 2006; Rea et al., 2004). The development of an apatite layer is an effect from the ion exchange process between ions (Ca21 and HPO422) from SBF and HA during immersion (Cao and Hench, 1996). The growth of apatite layers is induced by HA and is associated with the size of the HA particles. The smaller particle size of HA offers greater surface area to the SBF solution and induces the increased growth of apatite layers (LeGeros, 2008; Fang et al., 2006; Espigares et al., 2002). HA/HDPE composite material is used in orbital floor replacement devices and has accomplished the necessary requirements, like the ability to attach well to the orbital floor and exhibiting an unchanged volume of implant (Downes et al., 1991; Tanner et al., 1994). In the first design, the HA/HDPE composite material was compression molded as a disc and used to seal the bottom of the eye socket following rupture of the orbital floor, thus, avoiding the extrusion of the soft tissues into the sinus spaces. In the second design, for patients who had lost an eye, HA/HDPE composite material can be used as a space-filling implant (Ambrosio, 2009). An HA/HDPE composite was used to replace UHMWPE in middle ear implants as shafts which were cut into the necessary lengths to fit on staples that convey and strengthen sound vibrations from the outer ear to the inner ear. HAPEX shafts increase the long-term stability of implants, make it easier to trim intraoperatively due to the presence of the HA particles, and increase the sound transfer through the increased density of the composites (Dornhoffer, 1998; Goldenberg, 1994; Goldenberg and Driver, 2000). HA powder reinforced HDPE composites were compression molded into preferred plates and irradiated with different doses by Smolko and Romero (Smolko and Romero, 2007). The incorporation of HA powder increased the young’s modulus and tensile strength at break, whereas it decreased the yield strength and elongation. However, the increase in dose of radiation enhanced the tensile and yield strength and reduced the elongation from 800% to 5%. HDPE/nanoparticle hydroxyapatite composites with 10 30 wt.% of fillers were prepared by Fouad et al. (2013). It was observed that increasing the content of reinforcement decreases the melting temperature and the crystallinity of composites due to the restriction of the mobility of molecules. However, with ageing, HDPE-based nanohydroxyapatite composites showed a decrease in melting temperature and crystallinity increase due to chain scission and oxidation. The addition of HA nanoparticles improved the hardness and wear resistance of the composites. Li and Tjong prepared HDPE/HA nanorod composites, fabricated by melt compounding with 20 wt.% of reinforcement (Li and Tjong, 2011). The effective incorporation of hydroxyapatite nanorods (HANRs) enhanced hardness, young’s modulus, yield strength, thermal stability, and wear resistance compared to pure HDPE significantly. HA and alumina restricted to a total of 40 vol.%

6.6 Polyethylene Matrix

were added into an HDPE matrix by Nath et al. (2009). It was found that an elevated elastic modulus, superior hardness, and higher wear resistance were obtained in a HDPE/20 vol.% HA/20 vol.% alumina composite. In vitro cell culture studies confirmed the constructive cell adhesion properties in the prepared hybrid composites. The wear resistance of HDPE/GNP composites was examined using pin-on-disc wear testing equipment under various sliding velocities by Liu et al. (2014). A wear resistance enhancement of about 97% under 1.3 m s21 sliding velocity was realized in silanized GNP-reinforced HDPE composites. Haneef et al. developed hybrid polymer matrix composites using HDPE as a matrix material with titanium oxide and alumina particles as fillers. It was found that the hybrid polymer matrix composite offered superior mechanical and tribological characteristics, which are required for bone substitution materials. HDPE with 10 wt.% titanium oxide and with various proportions of alumina hybrid composites (5%, 10%, 15%, and 20%) were prepared and subjected to mechanical and tribological characterization The study reported that overall better mechanical and tribological properties were attained with a 10% titanium oxide and 20% alumina reinforced HDPE hybrid composite (Haneef et al., 2013). Spinal diseases and loads experienced by the spine through daily activities are common problems that affect the intervertebral discs (IVDs) and other spinal parts (Martz et al., 1997). Many manmade devices are developed to restore spinal stability and function. IVDs offer spine flexibility and provide an extensive variety of postures to the body. Each IVD has soft “nucleus pulposus” surrounded by “annulus fibrosus.” IVD degeneration causes back pain due to dehydration of the nucleus associated with tiny tears in the annulus (Hukins, 2005; Tsantrizos et al., 2005). Various IVD prostheses has been developed with materials like metals, polymers, and ceramics considering their properties, such as biocompatibility, resistance to compressive creep, and endurance (Bao and Yuan, 2000; De Santis et al., 2000; Ramakrishna et al., 2001; Traynelis, 2002; Gloria et al., 2008; Shikinami et al., 2004). Poly(2-hydroxyethylmethacrylate) (PHEMA) hydrogels were used in IVD prosthesis, however, the mechanical properties of PHEMA in hydrated conditions are not appropriate for biomedical applications with a necessity for high mechanical strength (Netti et al., 1993; Ambrosio et al., 1998; Peppas et al., 2000; Hoffman, 2012). The incorporation of hydrophobic components, like poly(caprolactone) (PCL) and polymeric fibers improved the mechanical properties of polymer hydrogels (Davis et al., 1992; Ambrosio et al., 1998; De Santis et al., 2004). Ambrosio et al. found that PHEMA/poly(methylmethacrylate) (PMMA) semi-interpenetrating polymer networks with poly(ethylene terephthalate) (PET) fibers show considerable potential for use in IVD replacements with fabrication by filament winding and molding processes (Ambrosio et al., 1998; Ambrosio, 2009). The stiff and hard metal endplates that cause bone resorption are replaced with HA reinforced polyethylene composites, which help to fasten the device to the vertebral bodies (De Santis et al., 2004; Ambrosio et al., 2007; Gloria et al., 2007). The addition of bioactive materials, like HA and/or calcium phosphate, is beneficial and provides the stiffness required for the endplates (Giordano et al., 2006).

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FIGURE 6.9 IVD prosthesis consisting of PHEMA/PMMA semi-interpenetrating polymer networks hydrogel reinforced with PET fibers and two hydroxyapatite reinforced polyethylene composite endplates (Ambrosio, 2009).

Fig. 6.9 shows composite biomimetic total IVD prosthesis made up of PHEMA/ PMMA semi-interpenetrating polymer network hydrogels reinforced with PET fibers and two HA reinforced polyethylene composite endplates. The mechanical and tribological properties of HDPE/MWCNT (multiwall carbon nanotube) composites were investigated by Kanagaraj et al. (2007). The composites showed improvements in mechanical properties with an increase of carbon nanotube (CNT) content as well as good load transfer effect and interface link between reinforcement and polymer. Theoretical calculation of volumetric wear rate was estimated by Ratner’s correlation, Wang’s model, and reciprocal toughness. It was observed that the volumetric wear rate of the composites decreased with the incorporation of CNTs. The reciprocal toughness and volumetric wear rate showed a linear relationship that maintains the theoretical calculation of the microscopic wear model. Composites with bioactive reinforcement and ductile polymer matrices were popular for implant applications due to their superior mechanical and biological properties. Glass-ceramic apatite wollastonite (A W) is a bioactive ceramic material that helps bone regeneration and offers strong interfacial linking between the implant and host tissue in biomedical applications (Yamamuro, 1993, 1995). Juhasz et al. studied the effect of filler content and particle size on the mechanical properties of glass-ceramic A W particulate reinforced HDPE composites. The manufacturing process of these composites involved blending and compounding through a twin-screw extruder followed by centrifugal milling and compression molding. HDPE-based composites consisting of glass-ceramic A W with volume fractions varying from 10% to 50% with average particle sizes of 4.4 and 6.7 mm were prepared and compared with HAPEX, a commercially available composite of HA and HDPE, with 40 vol.% filler content. It was observed that raising the glass-ceramic A W content

6.6 Polyethylene Matrix

increases the young’s modulus, yield strength, bending strength, and decreases the strain to failure behavior, but the young’s modulus, yield strength, and bending strengths were found to be slightly reduced with increasing filler particle size. In general, implant materials should preferably exhibit the characteristic of ductile behavior with a high strain to failure in order to evade any disastrous failure in the body. The results showed that glass-ceramic A W particulate reinforced HDPE composites with 50 vol.% have the potential for implant applications (Juhasz et al., 2004).

6.6.2 UHMWPE-BASED BIOMEDICAL COMPOSITES UHMWPE shows a low friction coefficient against steel, superior wear resistance, and good impact strength, which are essential for biomedical applications. It’s excellent load bearing characteristics make it useful in joint endoprostheses in combination with metal or ceramic fillers. A UHMWPE composite reinforced with carbon fibers was developed and used in orthopedic implants. UHMWPEbased biomedical composites reinforced with chopped carbon fibers with random orientation were manufactured through a compression molding technique (Kurtz, 2004). Ainsworth et al. found that carbon fiber-reinforced biomedical composites have lower wear rates, greater stiffness, flexural yield strength, and elastic modulus with the capability to endure higher compressive loads (Ainsworth et al., 1977). However, the poor fiber-matrix adhesions in carbon fiber-reinforced UHMWPE composites contributed to short-term clinical failures and they were ultimately neglected for use in joint replacement. Interestingly, studies at Drexel University revealed that long-term implanted carbon fiber-reinforced UHMWPE composites showed well-consolidated fibers in the matrix (Kurtz, 2004). The observed results from the long-term clinical test create an impulse to reexamine carbon fiber-reinforced UHMWPE biomedical composites for hip and knee joint replacement. UHMWPE homocomposites used in joint replacement applications were manufactured by sintering oriented UHMWPE fibers together or through the reinforcement of polymer matrices with UHMWPE fibers. Self-reinforced UHMWPE materials offer elevated tensile strength, tensile modulus, and abrasion resistance with respect to UHMWPE bulk materials (Price et al., 1997). The arrival of crosslinked UHMWPE and challenges encountered during the processing of the UHMWPE homocomposites pose difficulties for the commercialization of self-reinforced UHMWPE for orthopedic applications (Chang et al., 2000). UHMWPE matrix composites reinforced with nanoparticles, nanotubes, and nanofibers are used in orthopedic bearing applications. UHMWPE material filled with Al Cu Fe powders and chemically crosslinked UHMWPE reinforced with quartz particles of micron size were developed and are at present in the experimental analysis phase for orthopedic applications (Anderson et al., 2002; Xie et al., 2003; Liu et al., 1999). The fibrous reinforcement in composite materials improved the yield strength and elastic modulus of the surface treatment of the filler materials by plasma or chemical etching, usually, prevent filler

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aggregation issue and thereby improve reinforcement-matrix adhesion (Hofste et al., 1998). Anderson et al. observed that Al Cu Fe/UHMWPE composites have better wear resistance to volume loss as compared to pure UHMWPE and alumina/ UHMWPE composites. The volume loss of pure UHMWPE was due to the removal of the polymer materials during wear. Al Cu Fe/UHMWPE composites showed a 35% decrease in volume loss as compared to pure UHMWPE. The enhanced wear resistance of Al Cu Fe/UHMWPE composites has been recognized in the high young’s modulus, high hardness, and low coefficient of friction of the composites (Anderson et al., 2002). UHMWPE-based hybrid composites reinforced with bioactive HA, bioinert aluminum oxide, and CNTs were fabricated using compression molding by Gupta et al. The hybrid composites exhibited higher elastic modulus and toughness compared to that of pure UHMWPE (Gupta et al., 2013). The abrasive wear performance of UHMWPE reinforced with quartz powder was studied (Liu et al., 1999). The quartz powder offered increased surface hardness and enhanced ploughing and cutting resistance to the composite. The incorporation of quartz filler in UHMWPE reduced deep furrow formation and improved wear resistance by about a maximum of four times compared to unfilled UHMWPE. Larger filler particles are superior to smaller particles for the improvement of wear resistance. UHMWPE/quartz composites with vinyl triethyloxyl silane were prepared (Xie et al., 2003). Vinyl tri-ethyloxyl silane acts as a crosslinking agent for the UHMWPE matrix. The crosslinking of UHMWPE leads to an enhancement of wear resistance in the composites. The mechanical and wear resistance properties of UHMWPE/quartz composites reached a maximum at 0.5 phr of vinyl tri-ethyloxyl silane. Disease conditions, like osteoarthritis and degenerative joint disease, that cause the breakdown of cartilage in the joints are the main reason for having a hip or knee replacement. Total joint substitution helps to treat these debilitating diseases and improve the quality of life pf patients (Ambrosio, 2009). Two important components of prosthesis for total hip substitution are the femoral and acetabular parts. Femoral parts are made up of Co-Cr alloy, Ti alloy, aluminum, or zirconium. Typically, UMHWPE material is used to fabricate the acetabular part. When compared with the hip joint, the knee joint is considered as more complicated in geometry as well as movement mechanics. In general, the prosthesis of total knee joint substitution has femoral, tibial, and/or patellar components. The tibial and patella surfaces are made up of UHMWPE. The femur end commonly uses metal implants (Ambrosio, 2009; Davim, 2012). Fig. 6.10 shows the prosthesis for total hip/knee joint replacement. UHMWPE fibers embedded in an ethylene butene copolymer matrix composite material was found as a potential candidate for tendon and ligament prostheses (Kazanci et al., 2001; Kazanci et al., 2002a,b; Ratner et al., 2005). Kazanci et al. studied the fatigue test under cyclic loading for a filament wound flat strip of this composite material. Fatigue tests were carried out at room temperature under tension tension sinusoidal load at R 5 0.1 and a frequency of 1 Hz. In the fatigue

6.6 Polyethylene Matrix

FIGURE 6.10 Prosthesis for total hip/knee joint replacement (Davim, 2012).

test, three different copolymer compositions and two different winding angles were used to study the effects of branching density in the matrix and reinforcement angle on the fatigue response of the composites. The short-term fatigue test at high-stress levels exhibited improved fatigue resistance for a copolymer with a lower branching density and smaller reinforcement angle. However, the long-term fatigue test at moderate stress levels was managed by the fatigue rate of degradation, which reduced with branching density and winding angle (Kazanci et al., 2002b). Tribological characterization of biomedical composites plays a vital role in the existence of orthopedic implants in total joint replacements. Kalin et al. studied the tribological characterization of a UHMWPE-based composite with 5%, 10%, and 30% HA reinforcement under different loading conditions. It was revealed that the wear resistance of the UHMWPE/HA composites enhanced with increasing reinforcement content (Kalin et al., 2002; Ambrosio, 2009). Fang et al. manufactured UHMWPE/HA composites by mechanically mixing nanosized HA with UHMWPE in a ball mill and then compression molded the composite into solid blocks. The prepared blocks were allowed to swollen in paraffin oil to improve the UHMWPE chain mobility and reinforcement/matrix interface adhesion before ending hot-press. It was found that ball milling and swelling treatment enhanced the mechanical properties of the HA/UHMWPE composites compared to that of pure UHMWPE (Fang et al., 2005). Mirsalehi et al. studied the effect of the addition of 10 50 wt.% HA nanoparticles on the mechanical properties of UHMWPE, which was synthesized via a sol gel method. It was observed that composites with 50 wt.% exhibited higher young’s modulus and yield strength than that of pure UHMWPE (Mirsalehi et al., 2016). Bovine bone hydroxyapatite (BHA) reinforced UHMWPE composites were developed using a heat pressing formation technique. The incorporation of BHA filler particles enhanced the hardness and creep modulus of the composites as well as the biotribological properties

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for biomedical applications (Wang et al., 2009). UHMWPE/nanohydroxyapatite composites were prepared by vacuum hot-pressing and the composites were irradiated with gamma rays in a vacuum and then molten heat treated in a vacuum immediately after irradiation. The friction coefficient and wear rate in deionized water lubrication were diminished by the addition of nanohydroxyapatite (nHA). Whereas, the friction coefficient was improved and wear rate was decreased by gamma irradiation. The results showed that UHMWPE/nHA composites irradiated at a dose of 150 kGy could be used as bearing materials in artificial joints due to its superior wear resistance over that of pure UHMWPE (Xiong et al., 2009). The enhanced tribological properties in polyamide-6(PA-6)/UHMWPE composite can be achieved by incorporation of tiny UHMWPE particles, which play the role of lubricating the agent as well as PA-6 prevent the UHMWPE particles being transferred into the counterpart. Liu et al. studied the effect of contact pressure, sliding distance, and sliding speed on the wear properties of a polyamide-6 (PA-6)/UHMWPE composite. Contact pressure was found to be the main significant parameter in the wear rate of the PA-6/UHMWPE composite, followed by sliding distance and sliding speed (Liu et al., 2001). Liu et al. investigated the effect of contact pressure on the wear rate of PA-6, UHMWPE, and PA-6/ UHMWPE under dry and lubricated sliding conditions. The contact pressure as well as lubricating condition, have strong influences on the wear loss of the materials. The materials showed greater wear loss under elevated contact pressure under the dry sliding condition compared to that of the lubricated condition. The study observed that UHMWPE established the maximum dry sliding wear rate, under both 1 and 2.5 MPa of contact pressure, followed by PA-6/UHMWPE and PA-6, but PA-6 showed the uppermost wear rate in the lubricated sliding condition, followed by UHMWPE and PA-6/UHMWPE (Liu et al., 2006). Dangsheng studied the influences of carbon fiber content on the tribological characterization of UHMWPE in different concentrations, which is useful as an artificial joint acetabular material. The hardness of the carbon fiber-reinforced UHMWPE composites increased with reinforcement content. The composites wear volume loss was reduced with carbon fiber content under dry and distilled water lubricating conditions. The friction coefficients of the carbon fiberreinforced UHMWPE composites were greater than that of pure UHMWPE under the dry sliding condition and the friction coefficients were lower than that of pure UHMWPE under the distilled water lubrication condition (Dangsheng, 2005). In a study of the tribological characterization of UHMWPE/MWCNT composites, Kanagaraj et al. concluded that the wear resistance of UHMWPE was enhanced by the incorporation of CNTs. The addition of CNTs decreased the wear volume and wear coefficient of the composites due to the improved interfacial strength between the CNTs and the UHMWPE and superior load transfer effect to the CNTs from the UHMWPE. The wear coefficient of the composites decreased with an enhancement of the sliding distance in a linear model (Kanagaraj et al., 2010). A composite consisting of 80% UHMWPE and 20% HDPE reinforced with

6.6 Polyethylene Matrix

MWCNTs varying between 0.2 and 2 wt.% was studied by Xue et al. (2006). The composites showed a significant decrease in wear rate with an increase of both pretreated CNTs in boiling nitric acid and untreated CNT content. The incorporation of 0.5 wt.% of CNTs into a UHMWPE/HDPE blend caused a 50% reduction in the wear rate. CNT-reinforced UHMWPE/HDPE blends offered excellent wear properties compared to UHMWPE/HDPE blends without fillers and UHMWPE alone. The composites with untreated CNTs showed better wear performance compared to that of composites with pretreated CNTS due to the higher creep resistance characteristics of composites with untreated CNTs. The influences of applied pressure on the wear volume of PP, UHMWPE, and PP/UHMWPE blends were studied by Hashmi et al. (2001). Maximum wear in PP and minimum wear in UHMWPE were noticed. The wear rate of PP was more susceptible to pressure than UHMWPE. The frictional heat amplified temperature of the contact surface with sliding distance and makes PP softer, and thereby distorts and may loose its structural integrity. The incorporation of a small weight fraction of UHMWPE in PP enhanced the wear resistance of the PP to a considerable amount and controlled the rise in temperature at the interfacial region by reducing the frictional heat. Liu et al. studied the antiwear characterization and wear mechanisms of UHMWPE and UHMWPE/PP. It was observed that the antiwear properties of UHMWPE were improved by the addition of PP. The friction coefficient and wear rate of the UHMWPE/PP blend was much inferior compared to that of the pure UHMWPE during sliding. The rod-shaped debris from the UHMWPE/PP composites presented between the two contact surfaces acted as a lubricant and helped to decrease the friction coefficient and wear to a significantly lower level (Liu et al., 2004). Self-reinforced polymer composites, also referred to as single polymer composites, are used in a wide range of commercial applications (Alcock and Peijs, 2011; Gao et al., 2012). A homocomposite of an UHMWPE matrix and an UHMWPE reinforcing phase was manufactured and studied by Mosleh et al. (1998). In this process, UHMWPE powder was filled with short chopped UHMWPE fibers with fiber volume fractions between 25% and 75% or continuous UHMWPE fabric portion with fiber volume fraction of 60%, in a layered structure and then heated under pressure to consolidate the shape. Deng and Shalaby observed that the mechanical properties of self-reinforced UHMWPE composites were better compared to pure UHMWPE, however, the wear properties of these composites were found to be the same as pure UHMWPE (Deng and Shalaby, 1997). Chang et al. studied the wear performance of bulk-oriented and fiber-reinforced UHMWPE and reported that the failure of consolidated fiber oriented composites was due to poor interaction between the UHMWPE fibers and the bulk matrix and suggested that superior fiber-matrix interaction would improve the wear behavior of self-reinforced composites (Chang et al., 2000). Self-reinforced UHMWPE composites have been suggested for load-bearing biomedical applications. The utilization of UHMWPE homocomposites, along with

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their tribological performance in an articulation surface for knee joint prosthesis were investigated (Suh et al., 1998). UHMWPE fibers with high mechanical properties produced by gel spinning reinforced with HDPE and LDPE matrices were widely studied (Alcock and Peijs, 2011). Composite materials made up of UHMWPE fibers embedded in an ethylene butene copolymer matrix through filament winding were produced and characterized for elastic, viscoelastic, and fatigue behavior. Composites with UHMWPE fiber volume fractions of 65% have the potential to be used in biomedical applications (Kazanci et al., 2001, 2002b). Creep and wear performance evaluations of ethylene butene copolymers reinforced by UHMWPE fibers were carried out by Jacobs et al. by testing in a ball-on-prism tribometer against steel balls (Jacobs et al., 2002). It was found that the creep resistance of the pure matrix was considerably reduced with increasing branches of the copolymer. The UHMWPE fiber-reinforced copolymers revealed the same wear rate and creep resistance as the solid polymer matrix. HDPE/tricalcium phosphate/UHMWPE nanocomposites were prepared and characterized for their mechanical and biological capability as a substance for bone tissue substitution. The tensile properties of the HDPE/UHMWPE blends were affected by the incorporation of nanosized tricalcium phosphate with enhancments in yield strength, young’s modulus, and a reduction in elongation at break. The addition of tricalcium phosphate nanopowder enhanced osteoblast activity, osteoinduction, and osteoconduction processes. Biological tests showed that the composites were biocompatible and had no toxicity (Abadi et al., 2010). Corona and silane surface treatments of UHMWPE fibers enhanced the mechanical properties of dental fiber-reinforced composites due to good interfacial bond formation between the reinforcement and matrix resin. The fiber surface roughness, hardness, and elastic modulus were enhanced with 5s corona discharge, but were reduced with an additional increase in exposure time (Bahramian et al., 2015). UHMWPE-based composites with graphene oxide were prepared by liquid-phase ultrasonication dispersion and then by hot-pressing. The incorporation of graphene oxide sheets of up to 1 wt.% into UHMWPE enhanced the mechanical and biocompatibility properties, making these composites a potential candidate for artificial joints in the human body (Chen et al., 2012). Wang et al. studied the biotribological behavior of UHMWPE composites containing Ti in a hip joint simulator (Wang et al., 2007). The incorporation of titanium particles into the UHMWPE matrix showed advantages, such as enhanced wear resistance of UHMWPE under simulated body fluid (SBF-9) lubrication. UHMWPE/titanium particle composite cups showed decreased wear rates against 316L steel ball heads. An utmost drop in wear rates of 50% with 20 wt.% content of titanium particles was observed. Abrasive wear and fatigue wear were the major wear mechanisms of UHMWPE/titanium particle composites in the hip joint simulator. Titanium particles over 12 wt.% in the composites led to high wear debris sizes.

6.7 Polypropylene Matrix

6.7 POLYPROPYLENE MATRIX Polypropylene is manufactured by addition polymerization of propylene monomers. Chemically, propylene can be described as 2-methyl ethylene and has an additional CH3 group compared to ethylene. The CH3 group is important as it can be arranged in different spatial conformations in the macromolecules and thereby result in products with differing properties. Broadly, the resulting polypropylene products can be classified as isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. In the first case, the CH3 groups are arranged on the same side of the main chain of the polymer. When the CH3 groups are symmetrically arranged on the two sides of the main chain it is termed syndiotactic. If the CH3 groups are randomly distributed in a spatial relationship to the main chain it is termed atactic. Among the three categories, atactic polypropylene has little value due to its amorphous nature even though it has a slight rubbery nature. Isotactic polypropylene has a high melting point due to its high crystallinity as well is as it being stiff. Most commercial polymers are made up of isotactic polymers about 90% 95%. The crystallinity will be fairly high, which contributes to improved softening point, stiffness, tensile strength, modulus, and hardness. Polypropylenes have higher values of Mw/Mn 5 5.6 11.9 compared to polyethylenes and typical molecular weights are depicted as Mn 5 38,000 60,000 and Mw 5 220,000 700,000. A breakthrough invention for producing polypropylene was made by Natta in 1954 (Natta and Corradini, 1967). He used a modified Ziegler process for producing high molecular weight polypropylene. The material was later commercialized under the trade name Moplen by Montecatini in 1957. Several other processes were introduced thereafter, such as the Spheripol process (1983); the Valtec process (1988); and the Himont process (1990) (Maier and Calafut, 1998). The molecular weight and molecular weight distribution of polypropylene affects its properties, especially the rheological and mechanical properties. Compared to polyethylene, polypropylene deviates sharply from Newtonian to non-Newtonian behavior as depicted by rheological investigations. The physical properties of polypropylene are also different to those of polyethylene even though they have similar structures. For example, the density of polypropylene is around 0.90 g cm23, which is lower than that of polyethylene, however, it has a higher Tg and Tm. The most important property of polypropylene enabling its use as a biomedical material is its high melting point as it allows for autoclave sterilization. Another added advantage of PP is its almost similar chemical resistance to PE, which is essential for any biomedical application. However, PP is more susceptible to oxidation, chemical degradation, and crosslinking (irradiation, violet light, and other physical means) than polyethylene. Other properties, such as better creep resistance and higher environmental stress cracking resistance compared to polyethylene are useful in biomedical applications. The typical structure of polypropylene is given in Fig. 6.11. The inherent properties of polypropylene are given in Table 6.3.

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Polypropylene (PP)

[CH2–CH(CH3)]

H C

Isotactic Syndiotactic atactic

H H C H H C H

H C CH3 H C CH3 H C CH3

H C H H C H H C H

H C CH3 CH3 C H CH3 C H

H C H H C H H C CH3

FIGURE 6.11 Typical structure of polypropylene.

Polypropylene can be processed by injection molding, extrusion, blow molding, compression molding, and thermoforming techniques. Therefore, it can be converted into different shapes, such as blown film, flat film, sheets, tubes, packaging films, tapes, etc., which is highly advantageous for biomedical applications. Another advantage of PP processing is that no predrying is necessary except for hygroscopic additives. However, stabilizers and antioxidants are needed for specific purposes. It has an exceptionally high flex rate, excellent wear resistance, high temperature resistance, and low cost. Fiber applications, such as suture, braided ligament, skin and abdominal patches, and sewing rings are worth mentioning.

6.7.1 FINGER JOINT IMPLANTS The incorporation of PP into silicone rubber in order to improve its properties for use in finger joint replacements has been reported by Ziraki et al. (2016). Silicone rubber has unique properties; however, it often fails due to its poor mechanical properties. In order to improve the mechanical properties, silica nanoparticles and PP fibers were incorporated. The tensile properties showed an improvement from 5.6 to 6.21 MPa in comparison with a 2 wt.% silica/silicone composite. A drop in strength in SBF was lower when incorporated with PP fibers rather than with nanoparticles alone. Voids and fiber degradation were noted after the composites were soaked in SBF (Fig. 6.12).

6.7 Polypropylene Matrix

Table 6.3 Properties of Polypropylene Unit

Value

g cm23 %

0.90 0.915 0.01 0.035 1.47 1.51 16.3

Physical Properties Density Water absorption Refractive index, nD20 Solubility parameter

MPa1/2

Mechanical Properties Bulk modulus Tensile strength Elongation at break Young's modulus Fracture toughness Hardness Compressive strength Poisson's ratio Shear modulus

GPa MPa % GPa MPam1/2 MPa MPa GPa

1.6 2.5 21 40 100 300 1 1.6 1.7 2.1 60 100 30 45 0.4 0.45 0.4 0.6

Thermal Properties Melting temperature (Tm) Glass transition temperature (Tg) Service temperature in air without mechanical loading (short term) Service temperature in air without mechanical loading (long term)



C C  C

160 180 -30 to -3 140



100



C

6.7.2 BONE CEMENT Bone grafts or synthetic materials are used to treat bone defects. The technique called autograft is cumbersome as the bone has to be transferred from another site within the patient’s body, thus leading to limitations such as site morbidity and availability. Instead allograft tissue can be employed; however, it also has some disadvantages. The use of bone from another person might transfer diseases associated with the selected bone and it will also be difficult to reshape the bone into the desired shape to fit in exactly as the defective bone. Therefore, the use of polymeric materials such as bone cement is gaining attention. Polypropylenebased materials have been utilized as bone cement. An injectable paste composed of a polypropylene fumarate and calcium phosphate composite were characterized (Peter et al., 1999). An injectable polypropylene fumarate calcium phosphate paste was prepared with appreciable properties, such as compressive strength, compressive modulus, gel point, and curing

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FIGURE 6.12 SEM micrographs of silicone rubber composites; (A) 2 wt.% silica and (B) 2 wt.% PP after being soaked in SBF. Reprinted with permission from Elsevier.

6.7 Polypropylene Matrix

behavior. These properties are suitable for clinical orthopedic applications and the mechanical properties of the cured composites are suitable for trabecular bone replacement. Bacakova et al. prepared materials useful for bone tissue engineering (Bacakova et al., 2007). Among them a terpolymer of polytetrafluoroethylene, polyvinyl difluoride, and polypropylene mixed with 4 wt.% of single- or multiwalled carbon nanotubes was reported. Several biomedical studies have conducted on such materials, for instance, seeding was performed with human osteoblastlike MG 63 cells and it was found that CNT-containing materials showed well spreading of the cells and contained distinct beta-actin filament bundles, whereas the cells on the pure terpolymer were rounded and clustered into aggregates. The single-walled carbon nanotube-filled terpolymer showed a large concentration of the components of focal adhesion plaques such as vinculin and talin. This was exhibited by an enzyme-linked immunosorbent assay on the cells by about 56% and 36%, respectively, compared to the pure terpolymer. Osteogenic differentiation was performed by measuring the concentration of osteocalcin. It was found that the concentration of osteocalcin was lower in cells on the terpolymer containing multiwalled nanotubes. The reason is probably due to the more active proliferation of these cells (on day 7, they reached a 4.5 times higher population density than cells on the unmodified terpolymer). The concentration of ICAM-1, a marker of immune activation, in MG 63 cells showed that the addition of both single- and multiwalled nanotubes into the terpolymer had no effect at all. The polypropylene-containing terpolymer material with CNTs showed exemplary results, such as good support for the adhesion and growth of bone-derived cells. These materials were proposed to be considered for the fabrication of bone implants as well as for applications in bone tissue engineering. Chan et al. fabricated binary and hybrid composites based on PP with hexagonal boron nitride (hBN) and nHA (Chan et al., 2015). The prepared composites were tested in order to utilize them for human bone replacements. The composites were prepared through a melt mixing technique and the specimens were prepared by an injection molding technique. The nHA translated its biocompatibility properties to the composites along with improvements in the mechanical properties by virtue of boron nitride particle incorporation. The mechanical properties, such as elastic modulus, showed improvements with respect to hBN content. Cytotoxicity studies, cell cultivation, and MTT assay results showed the attachment and proliferation of osteoblasts on binary and ternary composites. Tjong et al. studied the design, fabrication, and characterization of the microstructure, physicochemical properties, and biocompatibility of PP reinforced with carbon nanofiber (CNF) and HANR fillers (Tjong et al., 2014). The composites were evaluated toward the development of good mechanical behavior, thermal stability, and improved biocompatibility in order to use the materials as craniofacial implants in orthopedics. The composites were fabricated by varying the loading of CNF by up to 2% and making hybrids with 20% loading of HANR using an extrusion technique. The elastic modulus and tensile strength of PP showed

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FIGURE 6.13 Scanning electron micrographs of cultured osteoblasts of PP/2% CNF 20% HANR hybrid composite for 2 days (A) and 4 days (B). (C) High magnified SEM image showing long filopodias. (Abbreviations: CNF, carbon nanofiber; HANR, hydroxyapatite nanorod; KV, kilo volt; PP, polypropylene; WD, working distance). Reprinted with permission from DOVE medical press.

improvement with the addition of CNF, while tensile ductility and impact toughness were not diminished. The addition of a HANR filler clearly showed improvement in tensile properties indicating enhanced filler-matrix interaction. Among the composites, the hybrid with a 2% loading of CNF and 20% HANR showed the maximum mechanical properties, such as tensile strength and stiffness. The thermal stability of the composites also showed manifold improvement due to the incorporation of both the fillers. It was shown that CNFs act as effective nucleating agents as evidenced by DSC measurements. Biocompatibility studies show that CNF nanofillers enhance the cell adhesion and viability of osteoblasts on PP. Among the composites, the PP/2% CNF/20% HANR composite showed good biocompatibility, which was further established by MTT assay results as shown in Fig. 6.13.

6.7.3 SCAFFOLDS In tissue engineering scaffolds are extremely important. They should have appropriate surface chemistry and properties to support cell attachment, proliferation, migration, and growth. The scaffolds should be biocompatible and should act as a

6.7 Polypropylene Matrix

template for cell growth. Additionally, they should act as an aid in the segregation of cells and support the production, organization, and maintenance of any extra cellular matrix. In order to facilitate cell mitigation and nutrient distribution, highly interconnected macro and micro porous networks should be present in the scaffolds. Cell migration depends on the physical aspects of the scaffolds. These physical aspects depend on physical structure and chemical and biological agents in order to help the cell differentiation and adhesion with the surface. Polypropylene as such has difficulty showing all these aspects, and therefore, the modification of PP through blending or fiber formation has been reported. Park et al. reported the preparation of polypropylene carbonates/poly(lactic acid) (PLA) composite nanofibers by sol gel electrospinning and studied their surface morphology, mechanical properties, and cell viability with cultured myoblasts (Park et al., 2016). The mechanical properties showed drastic improvement after a heat treatment and the nanofibers were nontoxic to the cells. Therefore, the composite nanofibers were deemed applicable for biomedical devices. Shi et al. fabricated porous ultra-short CNT nanocomposite scaffolds for bone tissue engineering (Shi et al., 2007). They utilized polypropylene fumarate as the polymer, ultra-short CNTs and modified CNTs for preparing the composites. The porosities were precisely controlled by a technique called thermal crosslinking particulate leaching and porosities of 75, 80, 85, and 90 vol.% were achieved. The porous scaffolds were characterized by different techniques, such as microCT, mercury intrusion porosimetry, and SEM, in order to establish the pore structures. Fig. 6.14 shows SEM images of the scaffolds. All the scaffolds showed 100% interconnectivity at the order of 20 mm and had specific porosities. When the scaffold porosity increased the pore connections also become bigger. This is why the mean pore size of 80 90 vol.% is significantly higher than 75 vol.% scaffolds. It has an adverse effect on the compressive mechanical properties and it got decreased in highly porous scaffolds. Therefore, the advantages of high porosity have to be compromised for many applications. The modification of short CNTs, in fact, reinforced the scaffolds, however, it did not reflect in the mechanical properties. It may be due to the sample variations and preparation techniques. The osteoconductivity of the scaffolds showed excellent results under static culture conditions. Therefore, the highly porous nanocomposite scaffolds prepared from polypropylene fumarate and ultra-short CNTs show potential for development of peculiar bone tissue engineering scaffolds.

6.7.4 ANTIMICROBIAL APPLICATIONS Healthcare devices offer good health to patients and research to develop materials that reduce microbial attack of the devices is always interesting. A large number of healthcare-acquired infections (HAIs) occur through surface contact by hands or other body parts or by devices, such as catheters, surgical incisions, or intravenous lines. A study by Curtis showed that 2% 5% of patients who had undergone

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FIGURE 6.14 SEM images of scaffolds made of: (A1 4) polypropylene fumarate; (B1 4) ultra-short tube nanocomposite; and (C1 4) modified ultra-short tube nanocomposite with increasing porogen fractions of 75, 80, 85, and 90 vol.% (from top to bottom). Scale bar represents 500 mm. Reprinted with permission from Elsevier.

surgery has an infection at the wound site (Curtis, 2008). Also, 80% 95% of HAIs of the urinary tract happens to be from urinary catheters (Dohmen, 2006). Antimicrobial materials need to be developed in order to reduce the number of HAIs. In this regard, polypropylene-based antimicrobial devices are noteworthy (Delgado et al., 2011; Essa and Khallaf, 2016). Palza et al. systematically studied the effect of copper nanoparticles on the antimicrobial properties of polypropylene (Palza et al., 2015). They tested the filler agglomeration and copper ion release from the composite. In order to improve the dispersion, several methods were employed. The compatibilization technique reduced the agglomeration to a large

6.7 Polypropylene Matrix

extent and improved the copper ion release to 40% more than the original matrix material, thus, paving the way for designing materials with tailored antimicrobial properties. Abbas et al. prepared superhydrophobic polypropylene coating suitable for biomedical applications with self-cleaning properties (Abbas et al., 2014). In order to prepare the surfaces several parameters were changed, such as the concentration of PP, solvent evaporation rate, loading of TiO2 nanoparticles and the heating rate for PP dissolution, to name a few. The static contact angle of the superhydrophobic surface was 165 degrees which is noteworthy. To establish the antibacterial properties cytotoxicity studies and bacterial anti-sticking effects were employed. The surfaces showed encouragingly positive results for antisticking effect against Staphylococcus aureus bacterial suspension. Zhao et al. fabricated a polypropylene-based nonwoven fabric membrane (PPNWF) with a switchable surface from antibacterial property to hemocompatibility (Zhao et al., 2013). The fabrication started with the synthesis of a cationic carboxy betaine ester monomer, [(2-(methacryboxy) ethyl)]-N,N-dimethylaminoethyl ammonium bromide, and methyl ester (CABA-1-ester), followed by the introduction of the same molecule via plasma pretreatment and UV-induced graft polymerization on the PPNWF surface. Membranes with different grafting densities were fabricated. Several experimental techniques were adopted to measure the properties and ATR-FTIR and gravimetric methods clearly showed that the cationic modified surface could be transformed to a zwitterionic modified surface under mild hydrolysis conditions. Effective antibacterial property against S. aureus was detected through biological tests for the cationic modified surface while the zwitterionic modified surface showed resistance to protein adsorption, platelet adhesion, and activation. The latter also exhibited enhanced clotting time. Thus, the fabricated PP-based membrane is recommended for dual functional biomaterial applications. Another interesting approach to decrease the growth of bacteria on the surface was reported by Aumsuwan et al. (Aumsuwan et al., 2009). PP surfaces were treated with microwave plasma reactions in the presence of maleic anhydride, which ultimately produced acidic groups. The prepared surfaces were again modified with two molecular groups; polyethylene glycol followed by penicillin V and diglycidyl polyethylene glycol followed by gentamicin. The former one was developed to counter the growth of S. aureus and the latter for creating antimicrobial surfaces so that Pseudomonas putida growth could be curtailed. The antimicrobial strength of the surface was measured simultaneously using gram positive and gram negative bacteria by varying the molecular rations on the surface. Spectroscopic and biological tests indicated appreciable results in the reduction of bacterial growth and the polypropylene surfaces were recommended for the formation of tunable antimicrobial devices.

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6.7.5 SUTURES Surgical sutures are extremely important medical devices employed to knit together the body after a surgery or injury from any cause. Different materials are used to manufacture sutures, such as nylon, polypropylene, biodegradable polymers, etc. There are two types of sutures available currently; absorbable and nonabsorbable. PP-based sutures are nonabsorbable due to their nonbiodegradable nature. Prolene is the trademark of the PP suture available on the market. However, there are many reports of improving the properties of PP sutures by grafting smaller molecules or drugs into it. There are several reports on the development of PP-based sutures available in the literature (Chatzimavroudis et al., 2017; Lo´pez-Saucedo et al., 2017; Tummalapalli et al., 2016).

6.8 CONCLUSIONS Polymer application in biomedical devices and instruments is noteworthy and several discoveries or inventions in the field have raised the standard of human living. Among the polymer polyolefins—polyethylene and polypropylene—were systematically reviewed here. The initial part of the chapter provided a general introduction about the properties of polyolefins and their applications. The different fabrication processes for biomedical devices were touched upon. A brief account of the biocompatibility of polyolefins was also provided. Because of the ubiquitous properties of polyethylene and polypropylene, they were considered for fabricating biocompatible devices. They are cheap, mechanically strong, and biocompatible for many applications. They were used for making devices such as scaffolds, bone cement, antimicrobial applications, hip prostates, and sutures, to name a few. The surface modification of these polyolefins by various techniques has led to the creation of a large number of biocompatible matrices. A detailed account of the application of such surface modification is included in the chapter, which covered drug delivery devices, tissue adhesives, bone substituents, etc. Compared to engineering plastics, the suitability of commodity plastics, such as polyethylene and polypropylene, for biomedical applications is exemplary in the long run. The coming years will witness research and development in the exploration of the potential of polyolefins-based biomedical devices accessible to the common man. Automation and robotics play a big role in developing ultra-pure, sterilizable, and designable materials for the development of medical devices. As explained in the chapter, they offer many advantages, such as low cost and high-performance biomedical products and are sustainable for large scale production due to automation. However, they do have some disadvantages, such as the removal of the degradable products from polyethylene and polypropylene from the human body or body fluids and the commercialization of the currently available products due to the performance inferiority compared to biodegradable and speciality polymers.

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