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Layer-by-layer assembled highly adhesive microgel films
Polymer 54 (2013) 4220e4226
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Layer-by-layer assembled highly adhesive microgel films Jianfu Zhang, Dongdong Chen, Yang Li, Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2013 Received in revised form 19 May 2013 Accepted 2 June 2013 Available online 10 June 2013
Water-based adhesives which have strong adhesion and can simplify the adhesion process, endow the adhesives with desired functions are important for various applications. In this work, water-based highly adhesive films with drug delivery ability are fabricated by layer-by-layer (LbL) assembly of chemically cross-linked poly(allylamine hydrochloride)-dextran (PAH-D) microgels and hyaluronic acid (HA). Strong adhesion as high as 6.95 0.92 MPa can be achieved when glass substrates deposited with LbL assembled PAH-D/HA films are slightly pressed together. Confocal laser scanning microscope (CLSM) measurements disclose that the strong adhesion originates from the intermixing of HA with PAH-D microgels at the interface of two contacted PAH-D/HA films. Free-standing PAH-D/HA films are released from substrate under assistance of a sacrificial layer for direct use as adhesives because PAH-D microgels have strong interactions with various surfaces. PAH-D/HA adhesive films can load negatively charged drugs such as ibuprofen based on electrostatic interaction between PAH-D microgels and ibuprofen molecules and release them in physiological conditions. Ibuprofen-loaded PAH-D/HA freestanding films can strongly glue periostea, promising their potential application as bioadhesives capable of accelerating the healing of damaged tissues or organs. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Layer-by-layer assembly Free-standing films Adhesive films
1. Introduction Adhesives which can bond materials of different shapes and properties to achieve connection and package are widely used in daily life [1]. Adhesives can be either pastes or films based on their physical forms. Compared with pastes, adhesive films have the advantages of convenience in storage and handling, and conformal adhesion due to their homogenous thickness. Traditional adhesive films composed of thermoplastic polymers usually require thermal treatments or organic solvents in the adhesive process, which make the adhesion process complicated and environmentally unfriendly [2,3]. Recently, various novel adhesive films have been produced, with their functions being much better than those of traditional adhesive films. For example, gecko-inspired synthetic adhesives composed of microsized non-sticky plastic pillars with high aspect ratios of heights to radii were fabricated by mimicking the nanotopography of gecko feet [4e7]. These gecko-inspired adhesives can achieve reversible adhesion on flat surface based on van der Waals and capillary forces. More importantly, they can act as tissue adhesives when biocompatible and biodegradable elastomer is used for producing micropillar arrays [7]. Tissue adhesives are important
* Corresponding author. Tel.: þ86 431 85168723; fax: þ86 431 85193421. E-mail address: [email protected] (J. Sun). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.06.002
in healing of tissues and organs where mechanical fastening using sutures might lead to unexpected damage [8]. The adhesive proteins secreted by marine mussels adhere strongly and rapidly to a variety of surfaces even in sea water. Inspired by a key composition in adhesive proteins, amino acid 3,4-dihydroxy-L-phenylalanine (DOPA) has been widely employed for the fabrication of adhesives suitable for use in aqueous environments [9e11]. As reported by Messersmith and co-workers, gecko-inspired adhesives lose most of their adhesion when immersed in water. However, when microsized pillars of gecko-inspired adhesives are coated with a DOPA-containing polymer layer, wet adhesion increases as much as 15-fold [12]. Meanwhile, substrates bearing oppositely charged polyelectrolytes, such as hydrogels [13] and polyelectrolyte brushes [14] can adhere strongly to each other based on electrostatic interaction between them. Adhesive films based on electrostatic interaction can be designed to bond/debond multiple times by an externally applied electric field [13]. With the demand to bond complex objects in a precise and controlled way, adhesive films with novel or unexplored functions are highly required. In particular, water-based adhesive films which can simplify the adhesive process, enable strong adhesion [15], and impart the adhesives with new functions are highly desirable but rarely reported. The layer-by-layer (LbL) assembly, which involves the alternate deposition of species with complementary interactions, is currently one of the most efficient methods to fabricate composite films with
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precisely controlled film structure and compositions [16e24]. By deliberately tailoring deposition parameters, the LbL assembly enables controlling the interdiffusion of polyelectrolytes within polyelectrolyte multilayer films [25,26]. Taking advantage of the interdiffusion of polyelectrolytes, we successfully fabricated polyelectrolyte films capable of healing damage such as deep cuts and scratches [27]. Adhesion of two physically contacted solid substrates covered with LbL assembled polyelectrolyte films such as poly(diallyldimethylammonium chloride) (PDDA)/poly(sodium styrenesulfonate) (PSS) or poly(allylamine hydrochloride) (PAH)/ poly(acrylic acid) (PAA) was conveniently realized in the presence of water and pressure, thanks to the interdiffusion of polyelectrolytes [28,29]. These previous studies show that LbL assembled polyelectrolyte films are promising as adhesives. However, considering the wealth of functions of LbL assembled polyelectrolyte films, their functions as adhesive films have not been fully explored because no other function has been integrated into the LbL assembled adhesive films. Moreover, the objects to be bonded must be pre-covered with polyelectrolyte adhesive films, making the adhesion process largely inconvenient. It will be convenient if free-standing polyelectrolyte films can be directly used for adhesion purpose. Here, a new type of highly adhesive microgel films are fabricated by LbL assembly of chemically crosslinked poly(allylamine hydrochloride)-dextran microgels (named PAH-D) with hyaluronic acid (HA). The PAH-D/HA adhesive films deposited on solid substrates can adhere strongly when they are slightly pressed with their surfaces getting wet with water. More importantly, free-standing PAH-D/HA films can be directly used as adhesives. The abundance of amine groups in PAH-D microgels allows the incorporation of drugs in PAH-D/HA adhesive films, imparting the adhesive films with drug delivery ability. Ibuprofenloaded PAH-D/HA free-standing films can firmly bond periostea, promising their application as bioadhesives with drug delivery ability.
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of water and finally dried by N2 flow. Caution: Piranha solution reacts violently with organic material and should be handled carefully. The newly cleaned glass or silicon substrate was first immersed in PAH-D aqueous solution (w1 mg mL1, pH 7.4) for 15 min to obtain a layer of PAH-D, followed by a rinsing and drying step using water and N2 flow. The substrate was then transferred to an aqueous HA solution (1.0 mg mL1, pH 7.4) for 15 min, followed by another water rinsing and N2 drying step. (PAH-D/HA)*n multilayer films can be deposited on the substrate by repetition of the above deposition processes in a cyclic fashion until a desirable thickness is achieved (n refers to the number of film deposition cycles). Freestanding PAH-D/HA adhesive films were produced by employing a sacrificial cellulose acetate (CA) layer [31]. Acetone solution of CA (30 mg mL1) was spin-coated on a newly cleaned glass substrate at 1000 rpm to obtain a homogeneous CA layer (w1 mm). The CA layer was further treated with oxygen plasma for 3 min to obtain a negatively charged hydrophilic surface, which can facilitate the deposition of PAH-D layer. The deposition process of the PAH-D/HA film on the oxygen plasma-treated CA layer was the same as that on the bare glass substrate. Large-area free-standing PAH-D/HA film was exfoliated from the substrate by immersing the resultant film in acetone solution to dissolve the sacrificial CA layer. 2.3. Loading and release of ibuprofen
2. Experimental section
Ibuprofen was loaded into PAH-D/HA films by immersing the films into an aqueous solution of ibuprofen sodium (15 mM, pH 7.4) for 3 h [32]. The release of ibuprofen from PAH-D/HA film was conducted by immersing the film into a vial containing 3 mL of phosphate buffered saline (PBS) at 37 C, which was replaced by a fresh one after an appropriate time to ensure constant release conditions. The releasing profile of ibuprofen was obtained by monitoring the absorbance of ibuprofen at 220 nm in PBS. As the absorbance of ibuprofen at 220 nm in PBS obeys LamberteBeer law, the amount of released ibuprofen can be determined using the calibration curve for ibuprofen in PBS.
2.1. Materials
2.4. Characterization
Poly(allylamine hydrochloride) (PAH, Mw ca. 56 000), hyaluronic acid sodium (HA, Mw ca. 1 630 000) were purchased from SigmaeAldrich. Lucifer yellow cadaverine (LYC) was purchased from Biotium. Dextran (Mw ca. 40 000) was purchased from Tokyo Chemical Industry Co., Ltd. Ibuprofen sodium was purchased from Fluka. All chemicals were used without further purification. PAH-D microgels were synthesized by chemical cross-linking of PAH and dextran according to our previously reported method [30]. The cross-linking was conducted by first partially oxidizing hydroxyl groups of dextran to produce aldehyde groups, which then reacted with amine groups of PAH via the formation of imine (eNeCe) bonds. Finally, the eNeCe bond in the cross-linked PAH and dextran microgel was reduced into eNeCe. The synthesized PAH-D microgels contain PAH and dextran with a monomer molar ratio of 1.5:1. LYC-labeled HA (HA-LYC) was synthesized according to a literature method and was briefly described in our previous work [27]. The molar ratio of LYC to carboxylate groups of HA was chosen at 1:50. The unreacted LYC was removed from the reaction solution by dialyzing against deionized water for one week. Periostea were torn away from fresh bovine ribs.
Scanning electron microscopy (SEM) images were recorded on a XL30 ESEM FEG scanning electron microscope. Ultraviolet-visible (UVevis) absorption spectra were collected on a Shimadzu UV2550 spectrophotometer. The cross-section of glued (PAH/HA)*30 films deposited on two silicon wafers was viewed by an Olympus FV1000 confocal laser scanning microscope (CLSM). The thickness of the films deposited on glass substrates was determined by a Veeco Dektak 150 surface profiler. Adhesion strength measurements were taken with an autograph universal testing machine (Shimadzu AG-I 1 kN) at ambient environment. The stretching velocity was set as 1 mm min1. A PDC-002 plasma cleaner (Harrick Plasma company, US) was used for oxygen plasma treatment of CA films. The stressestrain curves of PAH-D/HA free-standing films were measured with a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN).
2.2. Preparation of PAH-D/HA adhesive films Glasses and silicon wafers were first immersed in piranha solution (1:3 mixtures of 30% H2O2 and 98% H2SO4) and heated until no bubbles released. Then they were rinsed with copious amounts
3. Results and discussion 3.1. Preparation of PAH-D/HA adhesive films on solid substrates Glass and silicon wafers are selected as substrates to examine the adhesive ability of LbL assembled PAH-D/HA films. Aminecontaining PAH-D microgels and glucuronic acid-containing HA are positively and negatively charged in aqueous solution with pH of 7.4, respectively. Therefore, PAH-D and HA can be LbL assembled on glass or silicon substrates based on electrostatic interaction
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between them [32]. The thicknesses of the PAH-D/HA films with different deposition cycles deposited on glass substrates, determined from their corresponding cross-sectional images measured by a surface profiler, are shown in Fig. 1. The as-prepared PAH-D/HA films exhibit a typical exponential deposition behavior in the initial 10 deposition cycles and thereafter a rapid linear growth with an increment of approximately w190 nm per deposition cycle. A (PAHD/HA)*30 film has a thickness of w4.2 mm. The “in-and-out” diffusion of at least one kind of polyelectrolytes during the LbL deposition process is usually required for achieving exponential growth of LbL assembled polyelectrolyte films [25,26]. CLSM image of a (PAH-D/HA)*30 film with the top layer of HA being conjugated with LYC shows that HA-LYC can diffuse deeply into the (PAH-D/HA) *30 film (Fig. 3c). Therefore, the exponential growth of PAH-D/HA films is ascribed to the “in-and-out” diffusion of HA during the film deposition process. The adhesion strength of PAH-D/HA films with different deposition cycles deposited on glass substrates was investigated by lap shear tests [13,14,33]. As schematically shown in Fig. 2a and b, glass substrates with a size of 5 10 mm2 are deposited with PAH-D/HA films and used for adhesion tests. Two glass substrates are glued together with a contact area of 5 5 mm2 in the presence of 5 mL deionized water, which provides a water rich condition for polyelectrolyte diffusion. To enhance the adhesion, a weight of 500 g was placed on top of the glued substrates for 2 h at room temperature, which produces a pressure of w200 kPa. Two pieces of iron substrates with a hole in the end were glued on both ends of the sample by commercially available cyanoacrylate glue to enable fixation of the sample with clamps of the tension apparatus. A uniformly increasing force was loaded until debonding between the film and the glass substrates occurred. Fig. 2c shows the lap shear strengths as a function of film thickness, which is controlled by the number of film deposition cycles. Adhesion between (PAHD/HA)*10 films is observable, but the lap shear strength is too weak to be measured. The lap shear strength increases with increasing number of film deposition cycles and reaches 6.95 0.92 MPa for two glued (PAH-D/HA)*30 films. In other words, the glued (PAH-D/ HA)*30 films with an area of 5 5 mm2 can hold a weight of approximate 17 kg. Such a high adhesive strength of PAH-D/HA films is important for their practical applications. Although the adhesive strength of (PAH-D/HA)*30 films is smaller than commercially available cyanoacrylate glue (10e30 MPa), but it is stronger than adhesives of marine mussel extracts (2.97 MPa) [34],
electrophoretically adhered hydrogels (60.1 7.3 kPa) [13] and polyelectrolyte brushers (1.52 0.43 MPa) [14]. SEM image of the debonded substrate in Fig. 2d indicates that the adhesive failure is not due to the separation of two glued films, but originates from the detachment of the glued PAH-D/HA films from the surface of the substrates. Amine and hydroxyl groups in PAH-D layer ensure strong electrostatic and hydrogen bonding interactions of the PAHD/HA film with silanol groups on a glass surface [30]. However, these interactions can be broken under an externally applied force. This result confirms that the adhesion between the glued films is stronger than that between the film and the substrate surface. Fig. 2d also shows clearly that a thicker PAH-D/HA film has a higher adhesive strength because thick films are difficult to rip and then tear away from the substrate surface. The adhesion of two PAH-D/HA films deposited on glass was further confirmed by SEM measurements. Fig. 3a is the crosssectional SEM image of a (PAH-D/HA)*30 film on a glass substrate, which has a constant thickness of w4.5 mm. This value is consistent with the thickness measured by a surface profiler in Fig. 1. When two (PAH-D/HA)*30 films are adhered together, they get a thickness of w9 mm (Fig. 3b). The adhered films shows no visible interface or separation at the points where they adhere, confirming that these two (PAH-D/HA)*30 films are well connected together. To understand the adhesion mechanism of two PAH-D/HA films in the presence of water and pressure, the outmost layer of one (PAH-D/HA)*30 film was deposited with LYC-labeled HA. Crosssectional CLSM image in Fig. 3c shows that HA-LYC can diffuse deeply into the (PAH-D/HA)*30 film. The film thickness determined from the cross-sectional CLSM image is w5.2 mm, which is larger than w4.5 mm determined from cross-sectional SEM image. The overestimated thickness by CLSM is due to “bleeding effect” which occurs for thin films [28]. A (PAH-D/HA)*30 film with the outmost layer being HA-LYC was glued with the same (PAH-D/HA)*30 film without fluorescent labels and their cross-section was examined by CLSM. As shown in Fig. 3d, CLSM image shows that the luminescent region in the glued film increased to be w6.9 mm. This result discloses that diffusion of HA-LYC into the glued (PAH-D/HA)*30 film occurs, where the presence of water and pressure can facilitate the diffusion process. The diffusion of polyelectrolytes enables the intermixing of HA with PAH-D at the interface of two contacted PAH-D/HA films, which reforms electrostatic interaction between HA and PAH-D and glues these two PAH-D/HA films together. Water is critically important to enable the tight adhesion of two pieces of PAH-D/HA films. 3.2. Free-standing PAH-D/HA films for loading and release of ibuprofen
Fig. 1. Thicknesses of (PAH-D/HA)*n films increase as a function of the number of deposition cycles.
The abundance of protonated amine groups in PAH-D microgels allow the incorporation of negatively charged drugs into PAHD/HA films [32]. Herein, ibuprofen is selected as a model drug because it is widely used as non-steroidal anti-inflammatory drug and is easily detected by spectroscopic means. Ibuprofen is a weak acid and is hardly soluble in water. Herein ibuprofen sodium is used for incorporation into PAH-D/HA films. Ibuprofen sodium in aqueous solution contains carboxylate and carboxylic acid groups and can have electrostatic and hydrogen bonding interactions with amine groups of PAH-D microgels. Fig. 4a shows that after immersing an as-prepared (PAH-D/HA)*30 film in aqueous ibuprofen solution for 3 h, a characteristic absorption peak of ibuprofen at 220 nm appears, confirming the successful loading of ibuprofen into the film. The pH of aqueous ibuprofen sodium solution for drug loading is 7.4, which is the same with the pH of PAH-D and HA dipping solutions. Therefore, dissociation of PAH-D/ HA films is largely avoided during the loading of ibuprofen.
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Fig. 2. (a, b) Schematic illustration of the adhesion process (a) and the following lap shear tests (b) of glass substrates coated with PAH-D/HA films. (c) Lap shear strengths for glass substrates coated with PAH-D/HA films of different deposition cycles. (d) SEM image of a glued glass after being separated.
Moreover, the incorporation of ibuprofen does not produce an influence on the stability of PAH-D/HA films, meaning that the dissociation can be neglected. Large-area free-standing PAH-D/HA films can be released from substrate by dissolving the sacrificial CA layer in acetone as PAH-D/HA films are stable in acetone. The stressestrain curve in Fig. 4b shows that the (PAH-D/HA)*30 freestanding films are mechanically robust, which satisfies their applications as adhesive films. The calculated ultimate tensile strength and Young’s modulus of (PAH-D/HA)*30 free-standing films are 144.1 19.5 MPa and 3.4 0.4 GPa, respectively. The ultimate tensile strength of (PAH-D/HA)*30 free-standing films
Fig. 3. (a, b) Cross-sectional SEM images of a (PAH-D/HA)*30 film deposited on glass substrate (a) and two glued (PAH-D/HA)*30 films (b). (c, d) Confocal laser scanning microscope characterization of LYC-labeled HA diffusion in PAH-D/HA films. (c) A (PAH-D/HA)*30 film with the outmost layer of HA conjugated with LYC. (d) The (PAHD/HA)*30 film in (c) glued with a (PAH-D/HA)*30 film without luminescent labels.
exceeds the value for some daily used industrial plastics, which usually have a value of 20e66 MPa [35,36]. Fig. 4c shows the digital image of a 3 3 cm2 free-standing (PAH-D/HA)*30 film loaded with ibuprofen, which was obtained by first loading of ibuprofen and then released it from substrate. The free-standing (PAH-D/HA)*30 film with ibuprofen is transparent and defectfree. A w20% thickness increase was found after ibuprofen loading in (PAH-D/HA)*30 free-standing film, implying that a large amount of ibuprofen molecules were loaded into the film. When an ibuprofen-loaded (PAH-D/HA)*30 free-standing film was immersed in PBS solution, ibuprofen molecules were released from the film due to the dissociation of electrostatic and hydrogen bonding interactions of ibuprofen with PAH-D component. Fig. 4d indicates the time-dependent release profile of ibuprofen molecules from a (PAH-D/HA)*30 free-standing film in PBS. The PBS solution was replaced by a fresh one after each measurement was performed. Approximately 67% and 84% of the loaded ibuprofen was released from the film within 1 h and 24 h, suggesting a rapid release of ibuprofen in the beginning. The rest of ibuprofen was slowly released in a sustainable way. The rapid release of ibuprofen in the beginning is related to its high concentration in the film. The rapid release of drugs is practically useful in case that healing of the wounds or killing of bacteria requires high dosage of drugs. By summing up the amount of released ibuprofen in PBS solutions, the total amount of ibuprofen loaded in the (PAH-D/HA) *30 film was calculated to be w106 mg cm2. Previous study showed that ibuprofen loaded in layer-by-layer assembled polyelectrolyte multilayer capsules released more rapidly in an aqueous solution of higher pH value [37]. Therefore, the release of ibuprofen is pH dependent and can be slowed down in an acidic medium.
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Fig. 4. (a) UVevis absorption spectra of a (PAH-D/HA)*30 film before (black line) and after (red line) loading of ibuprofen. (b) Typical stressestrain curve for a (PAH-D/HA)*30 freestanding film. (c) Photograph of an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (d) Time-dependent release profile of ibuprofen from a (PAH-D/HA)*30 free-standing film in PBS solution. Error bars are for standard deviation (n ¼ 5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Free-standing PAH-D/HA films as adhesives A glass substrate deposited with a (PAH-D/HA)*30 film can adhere strongly with a bare glass substrate. More interestingly, ibuprofen-loaded (PAH-D/HA)*30 free-standing film can firmly glue two pieces of bare glass substrates in the presence of water and pressure. As shown in Fig. 5, the lap shear strengths for an ibuprofen-loaded (PAH-D/HA)*30 free-standing film and a (PAH-D/ HA)*30 film deposited on a glass substrate were measured to be 4.21 0.87 and 3.86 0.97 MPa, respectively. Taking into consideration of experiment errors, we regard that the lap shear strengths in these two cases are equal. Our previous study showed that PAH-D microgels in aqueous solution can deposit directly on both hydrophilic and hydrophobic surfaces without any surface
Fig. 5. (a) Lap shear strength of an ibuprofen-loaded (PAH-D/HA)*30 free-standing film for adhering of two glass substrates. (b) Lap shear strength of a (PAH-D/HA)*30 film deposited on glass substrate for adhering of the second glass substrate.
modification [30,32]. It is interesting to find that PAH-D microgels on the surface of PAH-D/HA free-standing film can adhere strongly on bare glass substrates in the presence of water and pressure. The adhesion of PAH-D microgels, and therefore PAH-D/HA films on glass was mainly achieved by electrostatic interaction and hydrogen bonding between amine groups of PAH-D microgels and silanol groups on glass surface [30]. PAH-D microgels are critically important to enable stable adhesion of PAH-D/HA free-standing films with bare glass substrates. The use of PAH-D/HA free-standing films makes the adhesion process convenient because deposition of PAH-D/HA films on objects to be glued is not required. Objects that are inconvenient for conducting LbL assembly can also be glued by PAH-D/HA free-standing films. The adhesion of periostea by ibuprofen-loaded PAH-D/HA freestanding films was then investigated to examine the potential of PAH-D/HA films as bioadhesives. The periosteum, which contains blood vessels and nerves, is the membrane that protects and nourishes the bone [38]. The outer layer of the periosteum is primarily made up of collagen while the inside layer contains osteoblasts which can produce new bone [39]. As schematically shown in Fig. 6a and b, periosteum was torn away from the fresh bovine rib and cut into pieces of 5 20 mm2. Two pieces of periostea were glued by a 5 5 mm2 ibuprofen-loaded PAH-D/HA free-standing film in the presence of water and pressure, with the inside layers of periostea contacting the PAH-D/HA film. These periostea were firmly adhered together by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film (Fig. 6c). Lap shear tests indicate that the lap shear strength of an ibuprofen-loaded (PAH-D/HA)*n free-standing film increases with increasing number of film deposition cycles (Fig. 6d). An ibuprofenloaded (PAH-D/HA)*30 free-standing films has an adhesion strength of 2.69 0.92 MPa for periostea, which is smaller than the adhesion strength of the same film adhering glass substrates. PAH-D layers have electrostatic interaction with the inside layer of periosteum because the osteoblast is negatively charged [39]. However, water
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Fig. 6. (aec) Processes to glue two pieces of periostea by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (a) Separating a piece of periosteum from bovine rib. (b) Periostea and ibuprofen-loaded (PAH-D/HA)*30 free-standing film are cut into required sizes. (c) Gluing two pieces of periostea by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (d) Lap shear strengths of ibuprofen-loaded (PAH-D/HA)*n films as a function of the number of film deposition cycles. (e) SEM image of a piece of glued periosteum after being separated.
contained in the periosteum might weaken the electrostatic interaction, which explains the smaller adhesive strength of the freestanding films for adhesion of periostea than for glass substrates. SEM image of glued periosteum after adhesive failure is shown in Fig. 6e. The partial tearing of PAH-D/HA film from periosteum occurs, as part of the PAH-D/HA film still adheres on periosteum. The strong adhesion of drug-loaded PAH-D/HA films toward periosteum promises their potential application as bioadhesives. The types of drugs capable of loaded in PAH-D/HA free-standing films are diverse and are not limited to ibuprofen. Moreover, the release kinetics of drugs from PAH-D/HA adhesive films can be regulated by controlling the parameters for the deposition of PAH-D/HA films, or by using prodrug strategy because amine groups of PAH-D are available for prodrug design. Bioadhesives with drug delivery ability are expected to be more effective in healing damaged tissues or organs than those without drug delivery ability.
loaded PAH-D/HA free-standing films enable direct adhesion of periostea, promising their applications as bioadhesives capable of drug delivery. Bioadhesives with drug delivery ability are expected to accelerate the healing of damaged tissues or organs. We anticipate that LbL assembled PAH-D/HA adhesive films with more desired functions can be fabricated because PAH-D microgels allow for the incorporation of various functional guest materials, e.g., conductive and antibacterial species. Moreover, the deposition of PAH-D/HA films is independent of morphologies and sizes of substrates. Therefore, PAH-D/HA adhesive films will be useful for adhesion of objects of tiny sizes with complicated morphologies. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC Grant 21225419, 21221063) and the National Basic Research Program (2013CB834503).
4. Conclusions and outlook In the present study, we have successfully fabricated waterbased highly adhesive films by LbL assembly of PAH-D microgels and HA in aqueous solutions. Substrates deposited with micrometer-thick PAH-D/HA films can strongly adhere together in the presence of water and slight pressure. Because of the strong interaction of PAH-D microgels with various surfaces, PAH-D/HA free-standing films which are flexible and mechanically robust can be used directly as adhesives. Free-standing PAH-D/HA films as adhesives avoid the processes of depositing PAH-D/HA films on objects to be glued, and make the adhesion convenient. Ibuprofen-
References [1] Adams RD, editor. Adhesive bonding. England: Woodhead Publishing Limited; 2005. [2] Luo XF, Lauber FE, Mather PT. Polymer 2010;51:1169e75. [3] Alam MO, Bailey C, editors. Advanced adhesives in electronics. England: Woodhead Publishing Limited; 2011. [4] Boesel LF, Greiner C, Arzt E, del Campo A. Adv Mater 2010;22:2125e37. [5] Huber G, Mantz H, Spolenak R, Mecke K, Jacobs K, Gorb SN, et al. Proc Natl Acad Sci U S A 2005;102:16293e6. [6] Greiner C, del Campo A, Arzt E. Langmuir 2007;23:3495e502. [7] Mahdavi A, Ferreira L, Sundback C, Nichol JW, Chan EP, Carter DJD, et al. Proc Natl Acad Sci U S A 2008;105:2307e12.
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[8] Duarte AP, Coelho JF, Bordado JC, Cidade MT, Gil MH. Prog Polym Sci 2012;37: 1031e50. [9] Lee H, Scherer NF, Messersmith PB. Proc Natl Acad Sci U S A 2006;103:12999e 3003. [10] Lee H, Dellatore SM, Miller WM, Messersmith PB. Science 2007;318: 426e30. [11] Lee BP, Messersmith PB, Israelachvili JN, Waite JH. Annu Rev Mater Res 2011;41:99e132. [12] Lee H, Lee BP, Messersmith PB. Nature 2007;448:338e42. [13] Asoh TA, Kawai W, Kikuchi A. Soft Matter 2012;8:1923e7. [14] Kobayashi M, Terada M, Takahara A. Soft Matter 2011;7:5717e22. [15] Foster AB, Lovell PA, Rabjohns MA. Polymer 2009;50:1654e70. [16] Decher G. Science 1997;277:1232e7. [17] Zhang X, Chen H, Zhang HY. Chem Commun 2007;14:1395e405. [18] Li Y, Wang X, Sun JQ. Chem Soc Rev 2012;41:5998e6009. [19] Ariga K, Ji QM, Hill JP, Bando Y, Aono M. NPG Asia Mater 2012;4:e17. [20] Schlenoff JB. Langmuir 2009;25:14007e10. [21] Jiao Q, Yi Z, Chen YM, Xi F. Polymer 2008;49:1520e6. [22] Chen QX, Ma N, Qian HJ, Wang LY, Lu ZY. Polymer 2007;48:2659e64. [23] Sheng KX, Bai H, Sun YQ, Li C, Shi GQ. Polymer 2011;52:5567e72. [24] Kim YS, Davis R, Cain AA, Grunlan JC. Polymer 2011;52:2847e55.
[25] Picart C, Mutterer J, Richert L, Luo Y, Prestwich GD, Schaaf P, et al. Proc Natl Acad Sci U S A 2002;99:12531e5. [26] Porcel C, Lavalle P, Ball V, Decher G, Senger B, Voegel JC, et al. Langmuir 2006;22:4376e83. [27] Wang X, Liu F, Zheng XW, Sun JQ. Angew Chem Int Ed 2011;50:11378e81. [28] Matsukuma D, Aoyagi T, Serizawa T. Langmuir 2009;25:9824e30. [29] Zou Y, Renneckar S, Pillai KV, Li Q, Lin Z, Church WT. BioResources 2010;5: 1530e41. [30] Wang L, Wang X, Xu MF, Chen DD, Sun JQ. Langmuir 2008;24:1902e9. [31] Mamedov AA, Kotov NA. Langmuir 2000;16:5530e3. [32] Wang L, Chen DD, Sun JQ. Langmuir 2009;25:7990e4. [33] Awaja F, Gilbert M, Kelly G, Fox B, Pigram PJ. Prog Polym Sci 2009;34:948e68. [34] Ninan L, Monahan J, Stroshinea RL, Wilker JJ, Shi RY. Biomaterials 2003;24: 4091e9. [35] Wang JH, Niu H, Dong JY, Du J, Han CC. Polymer 2012;53:1507e16. [36] Shackelford JF, Alexander W, editors. Materials science and engineering handbook. Boca Raton: CRC Press LLC; 2001. [37] Qiu X, Leporatti S, Donath E, Möhwald H. Langmuir 2001;17:5375e80. [38] Colnot C, Zhang XP, Tate MLK. J Orthop Res 2012;30:1869e78. [39] Gongadze E, Kabaso D, Bauer S, Slivnik T, Schmuki P, Van Rienen U, et al. Int J Nanomed 2011;6:1801e16.
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Layer-by-layer assembled highly adhesive microgel films Jianfu Zhang, Dongdong Chen, Yang Li, Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2013 Received in revised form 19 May 2013 Accepted 2 June 2013 Available online 10 June 2013
Water-based adhesives which have strong adhesion and can simplify the adhesion process, endow the adhesives with desired functions are important for various applications. In this work, water-based highly adhesive films with drug delivery ability are fabricated by layer-by-layer (LbL) assembly of chemically cross-linked poly(allylamine hydrochloride)-dextran (PAH-D) microgels and hyaluronic acid (HA). Strong adhesion as high as 6.95 0.92 MPa can be achieved when glass substrates deposited with LbL assembled PAH-D/HA films are slightly pressed together. Confocal laser scanning microscope (CLSM) measurements disclose that the strong adhesion originates from the intermixing of HA with PAH-D microgels at the interface of two contacted PAH-D/HA films. Free-standing PAH-D/HA films are released from substrate under assistance of a sacrificial layer for direct use as adhesives because PAH-D microgels have strong interactions with various surfaces. PAH-D/HA adhesive films can load negatively charged drugs such as ibuprofen based on electrostatic interaction between PAH-D microgels and ibuprofen molecules and release them in physiological conditions. Ibuprofen-loaded PAH-D/HA freestanding films can strongly glue periostea, promising their potential application as bioadhesives capable of accelerating the healing of damaged tissues or organs. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Layer-by-layer assembly Free-standing films Adhesive films
1. Introduction Adhesives which can bond materials of different shapes and properties to achieve connection and package are widely used in daily life [1]. Adhesives can be either pastes or films based on their physical forms. Compared with pastes, adhesive films have the advantages of convenience in storage and handling, and conformal adhesion due to their homogenous thickness. Traditional adhesive films composed of thermoplastic polymers usually require thermal treatments or organic solvents in the adhesive process, which make the adhesion process complicated and environmentally unfriendly [2,3]. Recently, various novel adhesive films have been produced, with their functions being much better than those of traditional adhesive films. For example, gecko-inspired synthetic adhesives composed of microsized non-sticky plastic pillars with high aspect ratios of heights to radii were fabricated by mimicking the nanotopography of gecko feet [4e7]. These gecko-inspired adhesives can achieve reversible adhesion on flat surface based on van der Waals and capillary forces. More importantly, they can act as tissue adhesives when biocompatible and biodegradable elastomer is used for producing micropillar arrays [7]. Tissue adhesives are important
* Corresponding author. Tel.: þ86 431 85168723; fax: þ86 431 85193421. E-mail address: [email protected] (J. Sun). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.06.002
in healing of tissues and organs where mechanical fastening using sutures might lead to unexpected damage [8]. The adhesive proteins secreted by marine mussels adhere strongly and rapidly to a variety of surfaces even in sea water. Inspired by a key composition in adhesive proteins, amino acid 3,4-dihydroxy-L-phenylalanine (DOPA) has been widely employed for the fabrication of adhesives suitable for use in aqueous environments [9e11]. As reported by Messersmith and co-workers, gecko-inspired adhesives lose most of their adhesion when immersed in water. However, when microsized pillars of gecko-inspired adhesives are coated with a DOPA-containing polymer layer, wet adhesion increases as much as 15-fold [12]. Meanwhile, substrates bearing oppositely charged polyelectrolytes, such as hydrogels [13] and polyelectrolyte brushes [14] can adhere strongly to each other based on electrostatic interaction between them. Adhesive films based on electrostatic interaction can be designed to bond/debond multiple times by an externally applied electric field [13]. With the demand to bond complex objects in a precise and controlled way, adhesive films with novel or unexplored functions are highly required. In particular, water-based adhesive films which can simplify the adhesive process, enable strong adhesion [15], and impart the adhesives with new functions are highly desirable but rarely reported. The layer-by-layer (LbL) assembly, which involves the alternate deposition of species with complementary interactions, is currently one of the most efficient methods to fabricate composite films with
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precisely controlled film structure and compositions [16e24]. By deliberately tailoring deposition parameters, the LbL assembly enables controlling the interdiffusion of polyelectrolytes within polyelectrolyte multilayer films [25,26]. Taking advantage of the interdiffusion of polyelectrolytes, we successfully fabricated polyelectrolyte films capable of healing damage such as deep cuts and scratches [27]. Adhesion of two physically contacted solid substrates covered with LbL assembled polyelectrolyte films such as poly(diallyldimethylammonium chloride) (PDDA)/poly(sodium styrenesulfonate) (PSS) or poly(allylamine hydrochloride) (PAH)/ poly(acrylic acid) (PAA) was conveniently realized in the presence of water and pressure, thanks to the interdiffusion of polyelectrolytes [28,29]. These previous studies show that LbL assembled polyelectrolyte films are promising as adhesives. However, considering the wealth of functions of LbL assembled polyelectrolyte films, their functions as adhesive films have not been fully explored because no other function has been integrated into the LbL assembled adhesive films. Moreover, the objects to be bonded must be pre-covered with polyelectrolyte adhesive films, making the adhesion process largely inconvenient. It will be convenient if free-standing polyelectrolyte films can be directly used for adhesion purpose. Here, a new type of highly adhesive microgel films are fabricated by LbL assembly of chemically crosslinked poly(allylamine hydrochloride)-dextran microgels (named PAH-D) with hyaluronic acid (HA). The PAH-D/HA adhesive films deposited on solid substrates can adhere strongly when they are slightly pressed with their surfaces getting wet with water. More importantly, free-standing PAH-D/HA films can be directly used as adhesives. The abundance of amine groups in PAH-D microgels allows the incorporation of drugs in PAH-D/HA adhesive films, imparting the adhesive films with drug delivery ability. Ibuprofenloaded PAH-D/HA free-standing films can firmly bond periostea, promising their application as bioadhesives with drug delivery ability.
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of water and finally dried by N2 flow. Caution: Piranha solution reacts violently with organic material and should be handled carefully. The newly cleaned glass or silicon substrate was first immersed in PAH-D aqueous solution (w1 mg mL1, pH 7.4) for 15 min to obtain a layer of PAH-D, followed by a rinsing and drying step using water and N2 flow. The substrate was then transferred to an aqueous HA solution (1.0 mg mL1, pH 7.4) for 15 min, followed by another water rinsing and N2 drying step. (PAH-D/HA)*n multilayer films can be deposited on the substrate by repetition of the above deposition processes in a cyclic fashion until a desirable thickness is achieved (n refers to the number of film deposition cycles). Freestanding PAH-D/HA adhesive films were produced by employing a sacrificial cellulose acetate (CA) layer [31]. Acetone solution of CA (30 mg mL1) was spin-coated on a newly cleaned glass substrate at 1000 rpm to obtain a homogeneous CA layer (w1 mm). The CA layer was further treated with oxygen plasma for 3 min to obtain a negatively charged hydrophilic surface, which can facilitate the deposition of PAH-D layer. The deposition process of the PAH-D/HA film on the oxygen plasma-treated CA layer was the same as that on the bare glass substrate. Large-area free-standing PAH-D/HA film was exfoliated from the substrate by immersing the resultant film in acetone solution to dissolve the sacrificial CA layer. 2.3. Loading and release of ibuprofen
2. Experimental section
Ibuprofen was loaded into PAH-D/HA films by immersing the films into an aqueous solution of ibuprofen sodium (15 mM, pH 7.4) for 3 h [32]. The release of ibuprofen from PAH-D/HA film was conducted by immersing the film into a vial containing 3 mL of phosphate buffered saline (PBS) at 37 C, which was replaced by a fresh one after an appropriate time to ensure constant release conditions. The releasing profile of ibuprofen was obtained by monitoring the absorbance of ibuprofen at 220 nm in PBS. As the absorbance of ibuprofen at 220 nm in PBS obeys LamberteBeer law, the amount of released ibuprofen can be determined using the calibration curve for ibuprofen in PBS.
2.1. Materials
2.4. Characterization
Poly(allylamine hydrochloride) (PAH, Mw ca. 56 000), hyaluronic acid sodium (HA, Mw ca. 1 630 000) were purchased from SigmaeAldrich. Lucifer yellow cadaverine (LYC) was purchased from Biotium. Dextran (Mw ca. 40 000) was purchased from Tokyo Chemical Industry Co., Ltd. Ibuprofen sodium was purchased from Fluka. All chemicals were used without further purification. PAH-D microgels were synthesized by chemical cross-linking of PAH and dextran according to our previously reported method [30]. The cross-linking was conducted by first partially oxidizing hydroxyl groups of dextran to produce aldehyde groups, which then reacted with amine groups of PAH via the formation of imine (eNeCe) bonds. Finally, the eNeCe bond in the cross-linked PAH and dextran microgel was reduced into eNeCe. The synthesized PAH-D microgels contain PAH and dextran with a monomer molar ratio of 1.5:1. LYC-labeled HA (HA-LYC) was synthesized according to a literature method and was briefly described in our previous work [27]. The molar ratio of LYC to carboxylate groups of HA was chosen at 1:50. The unreacted LYC was removed from the reaction solution by dialyzing against deionized water for one week. Periostea were torn away from fresh bovine ribs.
Scanning electron microscopy (SEM) images were recorded on a XL30 ESEM FEG scanning electron microscope. Ultraviolet-visible (UVevis) absorption spectra were collected on a Shimadzu UV2550 spectrophotometer. The cross-section of glued (PAH/HA)*30 films deposited on two silicon wafers was viewed by an Olympus FV1000 confocal laser scanning microscope (CLSM). The thickness of the films deposited on glass substrates was determined by a Veeco Dektak 150 surface profiler. Adhesion strength measurements were taken with an autograph universal testing machine (Shimadzu AG-I 1 kN) at ambient environment. The stretching velocity was set as 1 mm min1. A PDC-002 plasma cleaner (Harrick Plasma company, US) was used for oxygen plasma treatment of CA films. The stressestrain curves of PAH-D/HA free-standing films were measured with a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN).
2.2. Preparation of PAH-D/HA adhesive films Glasses and silicon wafers were first immersed in piranha solution (1:3 mixtures of 30% H2O2 and 98% H2SO4) and heated until no bubbles released. Then they were rinsed with copious amounts
3. Results and discussion 3.1. Preparation of PAH-D/HA adhesive films on solid substrates Glass and silicon wafers are selected as substrates to examine the adhesive ability of LbL assembled PAH-D/HA films. Aminecontaining PAH-D microgels and glucuronic acid-containing HA are positively and negatively charged in aqueous solution with pH of 7.4, respectively. Therefore, PAH-D and HA can be LbL assembled on glass or silicon substrates based on electrostatic interaction
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between them [32]. The thicknesses of the PAH-D/HA films with different deposition cycles deposited on glass substrates, determined from their corresponding cross-sectional images measured by a surface profiler, are shown in Fig. 1. The as-prepared PAH-D/HA films exhibit a typical exponential deposition behavior in the initial 10 deposition cycles and thereafter a rapid linear growth with an increment of approximately w190 nm per deposition cycle. A (PAHD/HA)*30 film has a thickness of w4.2 mm. The “in-and-out” diffusion of at least one kind of polyelectrolytes during the LbL deposition process is usually required for achieving exponential growth of LbL assembled polyelectrolyte films [25,26]. CLSM image of a (PAH-D/HA)*30 film with the top layer of HA being conjugated with LYC shows that HA-LYC can diffuse deeply into the (PAH-D/HA) *30 film (Fig. 3c). Therefore, the exponential growth of PAH-D/HA films is ascribed to the “in-and-out” diffusion of HA during the film deposition process. The adhesion strength of PAH-D/HA films with different deposition cycles deposited on glass substrates was investigated by lap shear tests [13,14,33]. As schematically shown in Fig. 2a and b, glass substrates with a size of 5 10 mm2 are deposited with PAH-D/HA films and used for adhesion tests. Two glass substrates are glued together with a contact area of 5 5 mm2 in the presence of 5 mL deionized water, which provides a water rich condition for polyelectrolyte diffusion. To enhance the adhesion, a weight of 500 g was placed on top of the glued substrates for 2 h at room temperature, which produces a pressure of w200 kPa. Two pieces of iron substrates with a hole in the end were glued on both ends of the sample by commercially available cyanoacrylate glue to enable fixation of the sample with clamps of the tension apparatus. A uniformly increasing force was loaded until debonding between the film and the glass substrates occurred. Fig. 2c shows the lap shear strengths as a function of film thickness, which is controlled by the number of film deposition cycles. Adhesion between (PAHD/HA)*10 films is observable, but the lap shear strength is too weak to be measured. The lap shear strength increases with increasing number of film deposition cycles and reaches 6.95 0.92 MPa for two glued (PAH-D/HA)*30 films. In other words, the glued (PAH-D/ HA)*30 films with an area of 5 5 mm2 can hold a weight of approximate 17 kg. Such a high adhesive strength of PAH-D/HA films is important for their practical applications. Although the adhesive strength of (PAH-D/HA)*30 films is smaller than commercially available cyanoacrylate glue (10e30 MPa), but it is stronger than adhesives of marine mussel extracts (2.97 MPa) [34],
electrophoretically adhered hydrogels (60.1 7.3 kPa) [13] and polyelectrolyte brushers (1.52 0.43 MPa) [14]. SEM image of the debonded substrate in Fig. 2d indicates that the adhesive failure is not due to the separation of two glued films, but originates from the detachment of the glued PAH-D/HA films from the surface of the substrates. Amine and hydroxyl groups in PAH-D layer ensure strong electrostatic and hydrogen bonding interactions of the PAHD/HA film with silanol groups on a glass surface [30]. However, these interactions can be broken under an externally applied force. This result confirms that the adhesion between the glued films is stronger than that between the film and the substrate surface. Fig. 2d also shows clearly that a thicker PAH-D/HA film has a higher adhesive strength because thick films are difficult to rip and then tear away from the substrate surface. The adhesion of two PAH-D/HA films deposited on glass was further confirmed by SEM measurements. Fig. 3a is the crosssectional SEM image of a (PAH-D/HA)*30 film on a glass substrate, which has a constant thickness of w4.5 mm. This value is consistent with the thickness measured by a surface profiler in Fig. 1. When two (PAH-D/HA)*30 films are adhered together, they get a thickness of w9 mm (Fig. 3b). The adhered films shows no visible interface or separation at the points where they adhere, confirming that these two (PAH-D/HA)*30 films are well connected together. To understand the adhesion mechanism of two PAH-D/HA films in the presence of water and pressure, the outmost layer of one (PAH-D/HA)*30 film was deposited with LYC-labeled HA. Crosssectional CLSM image in Fig. 3c shows that HA-LYC can diffuse deeply into the (PAH-D/HA)*30 film. The film thickness determined from the cross-sectional CLSM image is w5.2 mm, which is larger than w4.5 mm determined from cross-sectional SEM image. The overestimated thickness by CLSM is due to “bleeding effect” which occurs for thin films [28]. A (PAH-D/HA)*30 film with the outmost layer being HA-LYC was glued with the same (PAH-D/HA)*30 film without fluorescent labels and their cross-section was examined by CLSM. As shown in Fig. 3d, CLSM image shows that the luminescent region in the glued film increased to be w6.9 mm. This result discloses that diffusion of HA-LYC into the glued (PAH-D/HA)*30 film occurs, where the presence of water and pressure can facilitate the diffusion process. The diffusion of polyelectrolytes enables the intermixing of HA with PAH-D at the interface of two contacted PAH-D/HA films, which reforms electrostatic interaction between HA and PAH-D and glues these two PAH-D/HA films together. Water is critically important to enable the tight adhesion of two pieces of PAH-D/HA films. 3.2. Free-standing PAH-D/HA films for loading and release of ibuprofen
Fig. 1. Thicknesses of (PAH-D/HA)*n films increase as a function of the number of deposition cycles.
The abundance of protonated amine groups in PAH-D microgels allow the incorporation of negatively charged drugs into PAHD/HA films [32]. Herein, ibuprofen is selected as a model drug because it is widely used as non-steroidal anti-inflammatory drug and is easily detected by spectroscopic means. Ibuprofen is a weak acid and is hardly soluble in water. Herein ibuprofen sodium is used for incorporation into PAH-D/HA films. Ibuprofen sodium in aqueous solution contains carboxylate and carboxylic acid groups and can have electrostatic and hydrogen bonding interactions with amine groups of PAH-D microgels. Fig. 4a shows that after immersing an as-prepared (PAH-D/HA)*30 film in aqueous ibuprofen solution for 3 h, a characteristic absorption peak of ibuprofen at 220 nm appears, confirming the successful loading of ibuprofen into the film. The pH of aqueous ibuprofen sodium solution for drug loading is 7.4, which is the same with the pH of PAH-D and HA dipping solutions. Therefore, dissociation of PAH-D/ HA films is largely avoided during the loading of ibuprofen.
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Fig. 2. (a, b) Schematic illustration of the adhesion process (a) and the following lap shear tests (b) of glass substrates coated with PAH-D/HA films. (c) Lap shear strengths for glass substrates coated with PAH-D/HA films of different deposition cycles. (d) SEM image of a glued glass after being separated.
Moreover, the incorporation of ibuprofen does not produce an influence on the stability of PAH-D/HA films, meaning that the dissociation can be neglected. Large-area free-standing PAH-D/HA films can be released from substrate by dissolving the sacrificial CA layer in acetone as PAH-D/HA films are stable in acetone. The stressestrain curve in Fig. 4b shows that the (PAH-D/HA)*30 freestanding films are mechanically robust, which satisfies their applications as adhesive films. The calculated ultimate tensile strength and Young’s modulus of (PAH-D/HA)*30 free-standing films are 144.1 19.5 MPa and 3.4 0.4 GPa, respectively. The ultimate tensile strength of (PAH-D/HA)*30 free-standing films
Fig. 3. (a, b) Cross-sectional SEM images of a (PAH-D/HA)*30 film deposited on glass substrate (a) and two glued (PAH-D/HA)*30 films (b). (c, d) Confocal laser scanning microscope characterization of LYC-labeled HA diffusion in PAH-D/HA films. (c) A (PAH-D/HA)*30 film with the outmost layer of HA conjugated with LYC. (d) The (PAHD/HA)*30 film in (c) glued with a (PAH-D/HA)*30 film without luminescent labels.
exceeds the value for some daily used industrial plastics, which usually have a value of 20e66 MPa [35,36]. Fig. 4c shows the digital image of a 3 3 cm2 free-standing (PAH-D/HA)*30 film loaded with ibuprofen, which was obtained by first loading of ibuprofen and then released it from substrate. The free-standing (PAH-D/HA)*30 film with ibuprofen is transparent and defectfree. A w20% thickness increase was found after ibuprofen loading in (PAH-D/HA)*30 free-standing film, implying that a large amount of ibuprofen molecules were loaded into the film. When an ibuprofen-loaded (PAH-D/HA)*30 free-standing film was immersed in PBS solution, ibuprofen molecules were released from the film due to the dissociation of electrostatic and hydrogen bonding interactions of ibuprofen with PAH-D component. Fig. 4d indicates the time-dependent release profile of ibuprofen molecules from a (PAH-D/HA)*30 free-standing film in PBS. The PBS solution was replaced by a fresh one after each measurement was performed. Approximately 67% and 84% of the loaded ibuprofen was released from the film within 1 h and 24 h, suggesting a rapid release of ibuprofen in the beginning. The rest of ibuprofen was slowly released in a sustainable way. The rapid release of ibuprofen in the beginning is related to its high concentration in the film. The rapid release of drugs is practically useful in case that healing of the wounds or killing of bacteria requires high dosage of drugs. By summing up the amount of released ibuprofen in PBS solutions, the total amount of ibuprofen loaded in the (PAH-D/HA) *30 film was calculated to be w106 mg cm2. Previous study showed that ibuprofen loaded in layer-by-layer assembled polyelectrolyte multilayer capsules released more rapidly in an aqueous solution of higher pH value [37]. Therefore, the release of ibuprofen is pH dependent and can be slowed down in an acidic medium.
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Fig. 4. (a) UVevis absorption spectra of a (PAH-D/HA)*30 film before (black line) and after (red line) loading of ibuprofen. (b) Typical stressestrain curve for a (PAH-D/HA)*30 freestanding film. (c) Photograph of an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (d) Time-dependent release profile of ibuprofen from a (PAH-D/HA)*30 free-standing film in PBS solution. Error bars are for standard deviation (n ¼ 5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.3. Free-standing PAH-D/HA films as adhesives A glass substrate deposited with a (PAH-D/HA)*30 film can adhere strongly with a bare glass substrate. More interestingly, ibuprofen-loaded (PAH-D/HA)*30 free-standing film can firmly glue two pieces of bare glass substrates in the presence of water and pressure. As shown in Fig. 5, the lap shear strengths for an ibuprofen-loaded (PAH-D/HA)*30 free-standing film and a (PAH-D/ HA)*30 film deposited on a glass substrate were measured to be 4.21 0.87 and 3.86 0.97 MPa, respectively. Taking into consideration of experiment errors, we regard that the lap shear strengths in these two cases are equal. Our previous study showed that PAH-D microgels in aqueous solution can deposit directly on both hydrophilic and hydrophobic surfaces without any surface
Fig. 5. (a) Lap shear strength of an ibuprofen-loaded (PAH-D/HA)*30 free-standing film for adhering of two glass substrates. (b) Lap shear strength of a (PAH-D/HA)*30 film deposited on glass substrate for adhering of the second glass substrate.
modification [30,32]. It is interesting to find that PAH-D microgels on the surface of PAH-D/HA free-standing film can adhere strongly on bare glass substrates in the presence of water and pressure. The adhesion of PAH-D microgels, and therefore PAH-D/HA films on glass was mainly achieved by electrostatic interaction and hydrogen bonding between amine groups of PAH-D microgels and silanol groups on glass surface [30]. PAH-D microgels are critically important to enable stable adhesion of PAH-D/HA free-standing films with bare glass substrates. The use of PAH-D/HA free-standing films makes the adhesion process convenient because deposition of PAH-D/HA films on objects to be glued is not required. Objects that are inconvenient for conducting LbL assembly can also be glued by PAH-D/HA free-standing films. The adhesion of periostea by ibuprofen-loaded PAH-D/HA freestanding films was then investigated to examine the potential of PAH-D/HA films as bioadhesives. The periosteum, which contains blood vessels and nerves, is the membrane that protects and nourishes the bone [38]. The outer layer of the periosteum is primarily made up of collagen while the inside layer contains osteoblasts which can produce new bone [39]. As schematically shown in Fig. 6a and b, periosteum was torn away from the fresh bovine rib and cut into pieces of 5 20 mm2. Two pieces of periostea were glued by a 5 5 mm2 ibuprofen-loaded PAH-D/HA free-standing film in the presence of water and pressure, with the inside layers of periostea contacting the PAH-D/HA film. These periostea were firmly adhered together by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film (Fig. 6c). Lap shear tests indicate that the lap shear strength of an ibuprofen-loaded (PAH-D/HA)*n free-standing film increases with increasing number of film deposition cycles (Fig. 6d). An ibuprofenloaded (PAH-D/HA)*30 free-standing films has an adhesion strength of 2.69 0.92 MPa for periostea, which is smaller than the adhesion strength of the same film adhering glass substrates. PAH-D layers have electrostatic interaction with the inside layer of periosteum because the osteoblast is negatively charged [39]. However, water
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Fig. 6. (aec) Processes to glue two pieces of periostea by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (a) Separating a piece of periosteum from bovine rib. (b) Periostea and ibuprofen-loaded (PAH-D/HA)*30 free-standing film are cut into required sizes. (c) Gluing two pieces of periostea by an ibuprofen-loaded (PAH-D/HA)*30 free-standing film. (d) Lap shear strengths of ibuprofen-loaded (PAH-D/HA)*n films as a function of the number of film deposition cycles. (e) SEM image of a piece of glued periosteum after being separated.
contained in the periosteum might weaken the electrostatic interaction, which explains the smaller adhesive strength of the freestanding films for adhesion of periostea than for glass substrates. SEM image of glued periosteum after adhesive failure is shown in Fig. 6e. The partial tearing of PAH-D/HA film from periosteum occurs, as part of the PAH-D/HA film still adheres on periosteum. The strong adhesion of drug-loaded PAH-D/HA films toward periosteum promises their potential application as bioadhesives. The types of drugs capable of loaded in PAH-D/HA free-standing films are diverse and are not limited to ibuprofen. Moreover, the release kinetics of drugs from PAH-D/HA adhesive films can be regulated by controlling the parameters for the deposition of PAH-D/HA films, or by using prodrug strategy because amine groups of PAH-D are available for prodrug design. Bioadhesives with drug delivery ability are expected to be more effective in healing damaged tissues or organs than those without drug delivery ability.
loaded PAH-D/HA free-standing films enable direct adhesion of periostea, promising their applications as bioadhesives capable of drug delivery. Bioadhesives with drug delivery ability are expected to accelerate the healing of damaged tissues or organs. We anticipate that LbL assembled PAH-D/HA adhesive films with more desired functions can be fabricated because PAH-D microgels allow for the incorporation of various functional guest materials, e.g., conductive and antibacterial species. Moreover, the deposition of PAH-D/HA films is independent of morphologies and sizes of substrates. Therefore, PAH-D/HA adhesive films will be useful for adhesion of objects of tiny sizes with complicated morphologies. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC Grant 21225419, 21221063) and the National Basic Research Program (2013CB834503).
4. Conclusions and outlook In the present study, we have successfully fabricated waterbased highly adhesive films by LbL assembly of PAH-D microgels and HA in aqueous solutions. Substrates deposited with micrometer-thick PAH-D/HA films can strongly adhere together in the presence of water and slight pressure. Because of the strong interaction of PAH-D microgels with various surfaces, PAH-D/HA free-standing films which are flexible and mechanically robust can be used directly as adhesives. Free-standing PAH-D/HA films as adhesives avoid the processes of depositing PAH-D/HA films on objects to be glued, and make the adhesion convenient. Ibuprofen-
References [1] Adams RD, editor. Adhesive bonding. England: Woodhead Publishing Limited; 2005. [2] Luo XF, Lauber FE, Mather PT. Polymer 2010;51:1169e75. [3] Alam MO, Bailey C, editors. Advanced adhesives in electronics. England: Woodhead Publishing Limited; 2011. [4] Boesel LF, Greiner C, Arzt E, del Campo A. Adv Mater 2010;22:2125e37. [5] Huber G, Mantz H, Spolenak R, Mecke K, Jacobs K, Gorb SN, et al. Proc Natl Acad Sci U S A 2005;102:16293e6. [6] Greiner C, del Campo A, Arzt E. Langmuir 2007;23:3495e502. [7] Mahdavi A, Ferreira L, Sundback C, Nichol JW, Chan EP, Carter DJD, et al. Proc Natl Acad Sci U S A 2008;105:2307e12.
4226
J. Zhang et al. / Polymer 54 (2013) 4220e4226
[8] Duarte AP, Coelho JF, Bordado JC, Cidade MT, Gil MH. Prog Polym Sci 2012;37: 1031e50. [9] Lee H, Scherer NF, Messersmith PB. Proc Natl Acad Sci U S A 2006;103:12999e 3003. [10] Lee H, Dellatore SM, Miller WM, Messersmith PB. Science 2007;318: 426e30. [11] Lee BP, Messersmith PB, Israelachvili JN, Waite JH. Annu Rev Mater Res 2011;41:99e132. [12] Lee H, Lee BP, Messersmith PB. Nature 2007;448:338e42. [13] Asoh TA, Kawai W, Kikuchi A. Soft Matter 2012;8:1923e7. [14] Kobayashi M, Terada M, Takahara A. Soft Matter 2011;7:5717e22. [15] Foster AB, Lovell PA, Rabjohns MA. Polymer 2009;50:1654e70. [16] Decher G. Science 1997;277:1232e7. [17] Zhang X, Chen H, Zhang HY. Chem Commun 2007;14:1395e405. [18] Li Y, Wang X, Sun JQ. Chem Soc Rev 2012;41:5998e6009. [19] Ariga K, Ji QM, Hill JP, Bando Y, Aono M. NPG Asia Mater 2012;4:e17. [20] Schlenoff JB. Langmuir 2009;25:14007e10. [21] Jiao Q, Yi Z, Chen YM, Xi F. Polymer 2008;49:1520e6. [22] Chen QX, Ma N, Qian HJ, Wang LY, Lu ZY. Polymer 2007;48:2659e64. [23] Sheng KX, Bai H, Sun YQ, Li C, Shi GQ. Polymer 2011;52:5567e72. [24] Kim YS, Davis R, Cain AA, Grunlan JC. Polymer 2011;52:2847e55.
[25] Picart C, Mutterer J, Richert L, Luo Y, Prestwich GD, Schaaf P, et al. Proc Natl Acad Sci U S A 2002;99:12531e5. [26] Porcel C, Lavalle P, Ball V, Decher G, Senger B, Voegel JC, et al. Langmuir 2006;22:4376e83. [27] Wang X, Liu F, Zheng XW, Sun JQ. Angew Chem Int Ed 2011;50:11378e81. [28] Matsukuma D, Aoyagi T, Serizawa T. Langmuir 2009;25:9824e30. [29] Zou Y, Renneckar S, Pillai KV, Li Q, Lin Z, Church WT. BioResources 2010;5: 1530e41. [30] Wang L, Wang X, Xu MF, Chen DD, Sun JQ. Langmuir 2008;24:1902e9. [31] Mamedov AA, Kotov NA. Langmuir 2000;16:5530e3. [32] Wang L, Chen DD, Sun JQ. Langmuir 2009;25:7990e4. [33] Awaja F, Gilbert M, Kelly G, Fox B, Pigram PJ. Prog Polym Sci 2009;34:948e68. [34] Ninan L, Monahan J, Stroshinea RL, Wilker JJ, Shi RY. Biomaterials 2003;24: 4091e9. [35] Wang JH, Niu H, Dong JY, Du J, Han CC. Polymer 2012;53:1507e16. [36] Shackelford JF, Alexander W, editors. Materials science and engineering handbook. Boca Raton: CRC Press LLC; 2001. [37] Qiu X, Leporatti S, Donath E, Möhwald H. Langmuir 2001;17:5375e80. [38] Colnot C, Zhang XP, Tate MLK. J Orthop Res 2012;30:1869e78. [39] Gongadze E, Kabaso D, Bauer S, Slivnik T, Schmuki P, Van Rienen U, et al. Int J Nanomed 2011;6:1801e16.