Photopatternable Biodegradable Aliphatic Polyester with Pendent

Sep 23, 2015 - Programming temporal shapeshifting. Xiaobo Hu , Jing Zhou , Mohammad Vatankhah-Varnosfaderani , William F. M. Daniel , Qiaoxi Li ...
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Photopatternable Biodegradable Aliphatic Polyester with Pendent Benzophenone Groups Dayong Chen, Chia-Chih Chang, Beth Cooper, Angela Silvers, Todd Emrick,* and Ryan C. Hayward* Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States ABSTRACT: Highly efficient photo-cross-linking reactions enable numerous applications in biomaterials. Here, a photopatternable biodegradable aliphatic polyester with benzophenone pendent groups was synthesized by coppercatalyzed alkyne−azide cycloaddition, affording polyesters that undergo UV-induced cross-linking to yield photopatterned films. Using this material, a self-folding multilayer structure containing polyester/hydrogel bilayer hinges was fabricated. Upon swelling of the hydrogel layer, the construct folds into a triangular tube, which subsequently unfolds due to lipase-catalyzed degradation of the polyester layer. The ability to precisely design such degradation-induced structural changes offers potential for biomaterials and medical applications, such as evolving and responsive 2D and 3D tissue engineering scaffolds.

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INTRODUCTION Photo-cross-linking reactions are important for the design of biomaterials for drug delivery,1 cell encapsulation,2 and tissue engineering,3,4 because they can enable spatial and temporal control of material properties under mild reaction conditions, that is, with low energy input, at room or physiological temperature, and at neutral pH.5 Approaches suitable for application to biodegradable materials, including natural polymers such as chitosan, and synthetic polyesters prepared from lactides or lactones, are of special interest due to the ability to control release of encapsulated materials and ensure eventual degradation of the scaffolds. Indeed, photo-crosslinking of biodegradable materials has opened opportunities for controlled drug delivery,6 tissue adhesives,7−9 functional cell culture substrates,10 and 3D tissue engineering scaffolds.11 A promising avenue for photopatternable polymers in the design of biomaterials involves the patterning of films that can self-fold into complex 3D structures,12−18 with potential advantages for the fabrication of advanced tissue scaffolds and deployable biomedical devices. However, such work has focused largely on nondegradable materials, with only a few examples where biodegradable polymers have been patterned into self-foldable bilayers or hinged structures.19−22 In these cases, degradation of one or more components led to complete unfolding or breakup of the folded structures. Controlling the placement and relative decomposition rates of the components to allow self-folded structures that controllably evolve into different 3D configurations in response to biodegradation is an important goal that would be facilitated by simple approaches to photopattern degradable materials. Two general approaches have typically been employed for photo-cross-linking: (1) photopolymerization of a reaction mixture containing cross-linkers, prepolymers, and photoinitiating species, and (2) irradiation of polymers bearing © XXXX American Chemical Society

photoreactive groups that undergo intermolecular cross-linking. The majority of research efforts on photo-cross-linkable biodegradable polymers have taken the first approach.4,7,8,16,19,21−32 For example, Li et al. synthesized biodegradable and photo-cross-linkable polyphosphoesters grafted with acrylate-terminated polyethylene glycol chains.25 However, this approach also suffers from several disadvantages. Apart from systems where polymerization is conducted in solution at low polymer volume fraction,33 it is difficult to reach full monomer conversion due to limited chain mobility as the polymer gels. Incomplete conversion could ultimately lead to nonreproducible mechanical properties, poor biocompatibility34 and even uncontrolled biodegradation. Residual unreacted monomers and photoinitiators could be toxic to cells and thus detrimental to biomedical applications if leached from the gel.34,35 The sensitivity of free radical polymerization to oxygen can affect the pattern resolution and introduce structural defects,36 while the rapid diffusion of radicals in low viscosity systems can also degrade resolution.36 To overcome these limitations, micro mold and inert gas protection are often required to generate 2D and 3D patterns with good fidelity.37−41 The second approach, irradiation of functional polymers containing photo-cross-linkers such as phenyl azide,42 benzophenone,14,43−47 or cinnamoyl48 groups, offers advantages of no small molecule residuals, tolerance to oxygen and water, and improved spatial resolution. Among these chromophores, benzophenone (BP) is of particular interest due to its good chemical stability, compatibility with long-wavelength UV light (∼360 nm) and preferential reactivity toward otherwise Received: July 22, 2015 Revised: September 12, 2015

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DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules unreactive C−H bonds even in the presence of water.43 Another merit of BP is that cross-linking ceases after removal of UV radiation, minimizing post curing effects.49 The typical mechanism of photo-cross-linking via pendent BP groups is shown in Scheme 1,44,47 in which UV absorption leads to triplet

drugs, and oligopeptides using copper(I)-catalyzed alkyne− azide click cycloaddition.50−53 Herein, we exploit this versatile platform to prepare polyesters functionalized with BP photocross-linkers. We demonstrate the synthesis of a photopatternable biodegradable aliphatic polyester, characterize its degradation profile, and use it to fabricate self-folding structures that unfold upon lipase-catalyzed degradation. This simple approach to photopatternable biodegradable polymers enables self-folding 3D structures that controllably morph, offering promise for biomedical applications such as tissue engineering scaffolds and implantable devices.

Scheme 1. Primary Mechanism of Cross-Linking by Pendent Benzophenonea

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EXPERIMENTAL METHODS

Materials. Benzyl alcohol, copper(I) bromide(Cu(I)Br, 98%), propargyl bromide (80% solution in toluene), N,N,N′,N″,N″pentamethyldiethylenetriamine (PMDETA, 99%), sodium azide (98%), 4-(bromomethyl)benzophenone(96%), and lipase from Pseudomonas cepacia (powder, light beige, ≥30 U/mg) were purchased from Sigma-Aldrich, while δ-valerolactone, ε-caprolactone (CL), tin(II) 2-ethylhexanoate(Sn(Oct)2, 95%), diisopropylamine, and nbutyllithium in hexanes (2.9 M) were purchased from Alfa Aesar. Pooled normal human serum (Innovative Research) was used as received. Diisopropylamine, benzyl alcohol, methylene chloride, δvalerolactone, and ε-caprolactone were dried and distilled over calcium hydride. THF was distilled over sodium/benzophenoneketyl (caution: this should be done carefully following proper safety practices). All other materials were used without further purification. Literature procedures were used to prepare 4-(azidomethyl)benzophenone,54 αpropargyl-δ-valerolactone (AVL), and poly(α-propargyl-δ-valerolactone-co-ε-caprolactone) (poly(AVL-co-CL)).50 Instrumentation. NMR spectra were recorded on a Bruker DPX300 or BrukerAvance400 spectrometer. Chemical shifts were calibrated to residual solvent signals. Molecular weights and dispersities were measured by gel permeation chromatography in THF relative to polystyrene standards on an Agilent 1260 infinity system equipped with a three-column set (PLgel 5 μm mixed-c (7.5 × 300 mm), PLgel mixed-d (7.5 × 300 mm), and a guard column (7.5 × 50 mm)), a G1310B isocratic pump, and a refractive-index detector (G1362A) at a flow rate of 1.0 mL/min. Synthesis of Poly(α-propargyl-δ-valerolactone-co-ε-caprolactone) (Poly(AVL-co-CL)). Benzyl alcohol and tin(II) 2-ethylhexanonate (Sn(Oct)2) were added in anhydrous THF to prepare 1.7 and 0.5 M stock solutions, respectively. Benzyl alcohol (106 μL, 0.18 mmol), Sn(Oct)2(36 μL, 0.018 mmol), α-propargyl-δ-valerolactone (0.50 g, 3.63 mmol), and ε-caprolactone (0.84 g, 7.37 mmol) were added under a flow of nitrogen to a dry Schlenk tube. The tube was evacuated for 20 min at room temperature, then sealed and heated to 100 °C for 16 h. After cooling to room temperature, the polymer was precipitated twice into cold methanol and dried under vacuum at room temperature overnight to afford poly(AVL-co-CL) as a viscous oil, 1.2 g (90%). GPC (THF): Mn = 14.6 kDa, Đ = 1.18. 1H NMR (CDCl3, 300 MHz): δ (CHCl3 = 7.26 ppm) 4.07 (m, 4H), 2.58 (m, 0.33H), 2.45 (m, 0.66H), 2.29 (m, 1.34H), 2.03 (s, 0.33H), 1.64 (br m, 8H), 1.39 (br m, 1.34H). The obtained poly(AVL-co-CL) has a composition of 33 mol % AVL and 67 mol % CL, determined by integration of the terminal alkyne proton at 2.03 ppm and the oxymethylene protons from the polymer backbone at 4.07 ppm. Click Cycloaddition of 4-(Azidomethyl)benzophenone to Prepare Poly(α-propargyl-δ-valerolactone-g-benzophenoneco-ε-caprolactone) (Poly(AVL-g-BP-co-CL)). To a solution of poly(AVL-co-CL) (0.281 g, 33 mol % AVL, 0.76 mmol alkyne) and 4-azidomethylbenzophenone (54.6 mg, 0.23 mmol), in degassed DCM, copper(I) bromide (33 mg, 0.23 mmol) and PMDETA (39.8 mg, 0.23 mmol) were added, and the resulting mixture was stirred at room temperature for 3 h, while protected from light using aluminum foil. The benzophenone-grafted polymer was isolated by column chromatography using DCM as eluent to remove copper catalyst to afford a pale yellow solid poly(AVL-g-BP-co-CL) in 70% yield. GPC (THF): Mn = 15.0 kDa, Đ = 1.17. 1H NMR (CDCl3, 300 MHz): δ

a (1) Absorption of photon at ∼365 nm excites the carbonyl group to a triplet state. (2) The electron deficient oxygen radical participates in hydrogen atom abstraction from a proximal alkyl chain. (3) Recombination results in cross-linking.

formation, and oxygen abstracts a hydrogen atom from a proximal alkyl chain. The two resultant radicals recombine, leading to covalent cross-linking. Notably, Ionov and coworkers showed that simple blending of small-molecule BP derivatives with polycaprolactone yielded photopatternable, degradable, and biocompatible polymer films.19,21,22 However, as cross-linking relied exclusively on dimerization of aliphaticcentered radicals generated on the polymer chains upon hydrogen abstraction by BP (rather than coupling of aliphatic and ketyl radicals as in Scheme 1, or dimerization of ketyl radicals), the cross-linking yield is expected to be low relative to polymers having pendent BP.47 Furthermore, cross-linking by this method inevitably results in small molecule byproducts that may leach from the material. Despite its potential advantages, this second approach to cross-linkable polymers via covalently attached photoactive groups has found limited applicability to biodegradable materials, likely due to the challenge of functionalizing biodegradable polymers with suitable chromophores.42,48 While conventional aliphatic polyesters only offer chain end substituents, incorporation of suitable moieties along the polymer backbone increases the number of chromophores per chain, and thus improves cross-linking efficiency. Notably, alkyne functionalized polyesters have been successfully modified with poly(ethylene glycol), phosphorylcholine, B

DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules (CHCl3 = 7.26 ppm): δ (CHCl3 = 7.26 ppm) 7.76−7.28 (m, 1.2H), 5.59 (s, 0.24H), 4.07 (m, 4H), 2.03 (s, 0.21H), 1.61(br m, 8H), 1.38 (br m, 1.34H). Poly(benzophenone-grafted-α-propargyl-δ-valerolactone-co-ε-caprolactone) (poly(BP-co-CL)) was synthesized from poly(AVL-co-CL) containing 12 mol % AVL using the same approach. GPC (THF): Mn = 22.0 kDa, Đ = 1.22. The obtained poly(BP-co-CL) has 12 mol % BP as determined by 1H NMR spectroscopy. Thermal Characterization of Poly(AVL-g-BP-co-CL). The thermal properties of poly(AVL-g-BP-co-CL) were evaluated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). A TGA 2950 (TA Instruments) with a 5 mg sample in aluminum pans was employed with a dynamic nitrogen atmosphere between 20 and 450 °C, with a heating rate of 10 °C/min. DSC was performed using DSC Q200 (TA Instruments). Accurately weighted (5 mg) samples were placed into aluminum cups and sealed. An empty cup was used as reference. The experiment consisted of two runs: the first from −100 to 200 °C to remove thermal history and the second from −100 to 250 °C, both at a heating rate of 10 °C/min. Film Casting and Photopatterning. Silicon wafers were cleaned with acetone, ethanol, and water and dried under a stream of nitrogen, followed by oxygen plasma for 30 min and silanized with [3(methacryloxy)-propyl]trimethoxysilane for at least 5 h to promote adhesion between the polyester and silicon substrates.55 Poly(AVL-gBP-co-CL) in chloroform (20 mg/mL) was drop cast onto clean silicon wafers. To minimize film thickness gradients, the samples were prepared in a closed glass Petri dish to slow solvent evaporation. The thickness of the polyester films was determined using an optical reflectometer (StellarNet TF-VIS system). Photopatterning was carried out on an inverted optical microscope (Zeiss Axiovert200) equipped with a UV source (X-Cite 120Q, Lumen Dynamics). The photomasks (custom-designed and ordered from Front Range Photomask) were placed directly in contact with the polyester film surface, and the UV dose (365 nm) controlled by the shutter time. The samples were developed in a toluene/dichloromethane (2:3 vol) mixture for 1 h to remove any non-cross-linked material, dried with a nitrogen gas stream, and baked on a hot plate at 40 °C for 30 min. The gel fraction as a function of applied UV dose was determined by measuring the film thickness with an optical reflectometer (StellarNet TF-VIS system). Specifically, the polyester films were exposed to different UV doses by controlling the exposure time. The gel fraction was determined as the ratio of the film thickness following exposure, development and baking, to the initial film thickness. Biodegradation. Circular samples of poly(AVL-g-BP-co-CL) with a diameter of 1 mm and a thickness of ∼10 μm were prepared by casting and cross-linking with a UV dose of 3 J/cm2. The thickness of the films was the same before and after development, indicating that the polymer was fully gelled by UV-exposure. To measure degradation, each sample was incubated in 3 mL of pooled human serum (not heat inactivated) at 37 °C. Every 7 d over a period of 2 months, the samples were taken out, gently rinsed with deionized water, dried, and then characterized using an upright optical microscope (Zeiss AxiotechVario) in reflected mode. The human serum was replaced with a fresh solution at each measurement. Degradation-Induced Unfolding of Self-Folded Polymer Structures. Poly(p-methylstyrene-co-benzophenone acrylamide) (PpMS) and poly(N-isopropylacrylamide-co-sodium acrylate-co-benzophenone acrylamide) (PNIPAM) were synthesized according to previous reports.17,56 As shown in Figure 5, a sandwiched trilayer structure was constructed,17 with 1 μm swellable PNIPAM film as the middle layer and two stiff PpMS films of 70 nm with symmetrical cuts as the top and bottom layers. A fourth layer of poly(AVL-g-BP-co-CL) with thickness of 100 nm was photopatterned on top of the symmetric trilayer. To pattern and align the shapes layer by layer, the sample was illuminated with a pattern of UV light (365 nm, pE-100, CoolLED) generated using a digital micromirror array (DLP Discovery 4100, 0.7 XGA, Texas Instruments) attached to an inverted optical microscope (Nikon ECLIPSE Ti) with a 4× or 10× objective lens (S Fluor, Nikon). After patterning, PpMS layers were developed in toluene/ dichloromethane (2:3 vol.), PNIPAM in ethanol/water (1:1 vol.), and

polyester in toluene/dichloromethane (2:3 vol.). All film thicknesses were determined by optical reflectometry. The sample was then soaked in phosphate buffered saline (137 mM sodium chloride) (PBS) to drive self-folding. To keep the self-folded polymer object anchored to the substrate, a poly(acrylic acid) sacrificial layer was prepared57 and scratched with a razor blade to induce adhesion at the desired position. The degradation of the polyester layer, and therefore the unfolding of the shape was induced by culturing the sample with 10 mL of solution containing 1 mg/mL lipase from Pseudomonas cepacia in PBS (pH = 7.4) at 37 °C. Optical images of the sample were recorded every 3 days using a Zeiss Axiotech Vario upright microscope at room temperature (22 °C), with fresh lipase/PBS solution exchanged each time.

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RESULTS AND DISCUSSION

Synthesis and Characterization of Poly(AVL-g-BP-coCL). Our approach to photo-cross-linkable polyesters relies on attachment of benzophenone moieties using copper catalyzed alkyne−azide cycloaddition, which allows for nearly quantitative functionalization of polymers featuring pendent alkyne groups.50 A poly(δ-valerolactone) copolymer, poly(AVL-coCL), containing 33 mol % of alkyne groups, was synthesized by Sn(Oct)2-mediated ring-opening polymerization, and then functionalized with 0.3 equiv of 4-(azidomethyl)benzophenone relative to alkyne groups in cycloaddition catalyzed by Cu(I)Br and PMDETA. Fourier-transform infrared spectroscopy revealed complete consumption of azide after 3 h, as confirmed by the disappearance of the azide stretch at 2105 cm−1. Gel permeation chromatography showed a slight increase in number-average molecular weight from 14.6 to 15.0 kDa, while the dispersity remained nearly unchanged (Đ = 1.17). As shown in Figure 1, successful attachment of BP to the polyester backbone was confirmed by the appearance of the triazole proton at 7.76 ppm in the 1H NMR spectrum, while the benzylic protons of 4-(azidomethyl)benzophenone at 4.46 ppm shifted to 5.59 ppm in the polyester product. The measured

Figure 1. 1H (top) and 13C (bottom) NMR spectra of poly(AVL-gBP-co-CL). Peak integration provides a molar composition of m = 12%, n = 21%, and p = 69%. C

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Biomacromolecules incorporation of BP was 12 mol %, representing an average of 7 BP groups per polymer chain, close to the intended loading of 10 mol %. Thermal gravimetric analysis (TGA) revealed thermal stability to about 295 °C, while differential scanning calorimetry (DSC) showed a glass transition at −2 °C and no melting transition. While aliphatic polyesters are typically semicrystalline, the random copolymerization and pendent substitution disrupt crystallinity, resulting in rubbery materials at room temperature. This is ideal for photopatterning, as it leads to films with minimal residual stress. Moreover, the presence of residual alkynes is attractive in that it would allow for incorporation of additional functionality to the polyester. Photopatterning of Poly(AVL-g-BP-co-CL). We next considered the UV-induced cross-linking of BP-functionalized polyesters by studying the gel fraction as a function of applied dose. Polyester films of ∼10 μm thickness were drop cast onto [3-(methacryloxy)-propyl]trimethoxysilane functionalized silicon wafers. The polyester films were irradiated with a series of different UV doses, followed by developing in a toluene/ dichloromethane (2:3 vol.) mixture. To ensure complete development and prevent delamination due to excessive swelling, this mixture of poor solvent (toluene) and good solvent (dichloromethane) functioned as a marginal solvent.58 The thickness of polyester films after development was monitored as a function of UV dosage by optical reflectometry. The gel fraction was quantified by the ratio of the developed film thickness to the initial film thickness. As shown in Figure 2,

Figure 3. Optical micrographs show photopatterned stripes and circular pads of poly(AVL-g-BP-co-CL). Scale bar represents 100 μm.

emzymolysis60 over desirable time scales is crucial for their use as degradable drug delivery systems and tissue engineering scaffolds. Previously, accelerated degradation of poly(AVL) in human serum, relative to PBS, was observed,61 suggesting that enzyme catalyzed depolymerization is the dominant degradation mechanism. Based on these findings, pooled human serum (not heat inactivated) was used as an incubation medium to test the degradation of fully cross-linked poly(AVL-g-BP-coCL) samples. As shown in Figure 4, the original flat polyester surface developed holes after 20 d, which subsequently grew in size. This observation agrees with autoacceleration of degradative fragmentation commonly observed for polyesters such as polycaprolactone.62 After 35−40 d, significant fragmentation of samples was apparent along with the formation of large holes. After 50 d, only a trace of material remained on the substrate. Self-Folding of 2D Sheets into 3D Objects. The use of photo-cross-linkable polymers to prepare patterned 2D sheets that undergo controlled folding into 3D objects represents an attractive platform for tissue engineering scaffolds, deployable biomedical devices, microrobotics and tunable optics. It is of great interest that the folded 3D objects can transform into different configurations upon the presence of different stimuli or environmental cues.63,64 The use of photopatternable biodegradable polymer affords the possibility to construct “smart” 3D structures that evolve into new configurations as the local mechanical properties are altered by degradation. As schematically shown in Figure 5a, using degradable polymer, a four-layered self-folding structure can be constructed, which unfolds as the top polymer layer degrades. This self-folding structure can be fabricated by patterning a symmetric trilayer with stiff thin films as the top and bottom layers, sandwiching a hydrogel film. The top and bottom layers are patterned with matching and aligned open stripes, leaving only a gel layer in this region. Finally, a fourth layer of degradable polymer is photopatterned across the entire top surface of the trilayer, breaking the top-to-bottom symmetry and therefore leading to bending of the structure in the central region due to swelling contrast between the gel polymer and the top degradable polymer film. Although poly(AVL-g-BP-co-CL) is amenable to photo-crosslinking and photopatterning, we noted a limited shelf life of several weeks despite minimal exposure to ambient light during storage. The copolymer became insoluble presumably due to the reactivity of the alkynes to cross-linking by radicals generated by benzophenone. BP grafted polyester, poly(BPco-CL) (Mn = 22.0 kDa, Đ = 1.22), containing the same amount of photosensitive BP groups but without any residual alkyne (as shown in Scheme 2), proved stable over at least 3

Figure 2. Gel fraction of poly(AVL-g-BP-co-CL) film as a function of irradiated UV dose.

poly(AVL-g-BP-co-CL) exhibits a gel point of 80 mJ/cm2, beyond which the gel fraction increased with the UV dose until the polyester completely gelled at 2 J/cm2. Moreover, the film thickness of cross-linked poly(AVL-g-BP-co-CL) remained unchanged even after irradiation with a very high UV dose of 8 J/cm2. No degradation due to UV irradiation at 365 nm was observed, an advantage of using benzophenone as a crosslinker, compared to approaches relying on short wavelength UV irradiation (e.g., 254 nm), which can cause significant degradation of cured polymer networks.59 Photopatterning of poly(AVL-g-BP-co-CL) was next demonstrated by UV irradiation through photomasks placed directly on top of the polyester film. As shown in Figure 3, features with characteristic dimensions of ∼100 μm were fabricated successfully, with the patterned polyester films showing good registry with the photomask. For example, when a photomask with parallel open stripes with widths of 60 μm and a pitch of 140 μm was used, the resulting polyester stripes had a width of 60 ± 2 μm. Degradation of Photo-Cross-Linked Poly(AVL-g-BPco-CL). The ability of polyesters to degrade by hydrolysis or D

DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules

Figure 4. Optical micrographs showing the degradation of a fully cross-linked poly(AVL-g-BP-co-CL) film cultured in pooled human serum at 37 °C over 50 d. Initially, the film surface was flat and smooth. Holes were observed to nucleate (20 d) and grow (30 d), leading to fracture and delamination (35−50 d), After 50 d, almost no material remains on the substrate.

Scheme 2. Synthesis of Poly(AVL-g-BP-co-VL) and Poly(BP-co-CL)

potential complications associated with nonspecific protein adsorption on the self-folded structure. Such controllably selffolding and unfolding 3D structures have the potential to serve, for example, as scaffolds for tissue culture that encapsulate cells and molecules of interest, with degradation-triggered unfolding providing a mechanism for release. For example, one could envision in vitro tissue engineering of blood vessels using a polymer film that first self-folds into a tubular scaffold to guide growth of seeded cells, followed by degradation-induced unfolding to release the blood vessel.

months without notable cross-linking, and therefore was used in the fabrication of self-folded structures. Specifically, a symmetric trilayer is photopatterned with stiff thin PpMS films as the top and bottom layers, sandwiching a PNIPAM-based hydrogel film. Finally, a fourth layer of poly(BP-co-CL) is photopatterned across the entire top surface of the trilayer. Good adhesion between each of the layers is ensured by BP cross-linking, which allows for covalent grafting to the underlying layer. Upon immersion into an aqueous medium, the structure partially detaches from the substrate as the patterned poly(acrylic acid) release layer dissolves, and the structure self-folds into a nearly closed triangular tube due to swelling of the PNIPAM layer. Degradation of poly(BP-co-CL) was achieved by incubating the sample in a 1 mg/mL solution of lipase at 37 °C for 1 month,65,66 leading to restoration of top-bottom symmetry and opening of the structure to a nearly flat, but still otherwise intact, state. In this case, we use purified lipase to facilitate the degradation of the polyester layer, since it is the key enzyme responsible for ester hydrolysis; this increases the rate of degradation compared to serum and also avoids

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CONCLUSION In conclusion, a photo-cross-linkable aliphatic polyester containing pendent benzophenone chromophores was synthesized by click cycloaddition, and shown to be suitable for fabrication of cross-linked films and photopatterned features with sizes of ∼100 μm. Degradation of the cross-linked films was modulated by enzyme-catalyzed depolymerization. Our selection of benzophenone allows facile high-resolution photoE

DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX

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Biomacromolecules

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Figure 5. (a) Schematic showing the steps of microfabrication via photolithography (1−4): photopatterning of a PpMS layer (dark blue) with open stripes on a silicon wafer coated with a scratched poly(acrylic acid) sacrificial layer (purple) (1), photopatterning of the swellable PNIPAM layer (light blue) (2), photopatterning of a second PpMS layer (dark blue) with matching and aligned open stripes (3), and photopatterning of the poly(BP-co-CL) top layer (green) (4), self-folding upon PNIPAM layer swelling (5), and unfolding upon degradation of the poly(BP-co-CL) top layer (6). (b) Top view optical micrographs showing the four layer structure selffolding into a nearly fully closed triangular tube and unfolding to almost flat configuration upon the lipase catalyzed degradation of the poly(BP-co-CL) top layer.

patterning at ambient conditions with 365 nm UV light, with good adhesion between adjacent photopatterned layers due to covalent grafting. Based on these merits, a self-foldable structure was designed and its unfolding upon lipase catalyzed polyester degradation was demonstrated, showing the potential of using this photopatternable polyester for biomedical applications such as 3D tissue engineering scaffolds that undergo degradation-induced structural morphing. While our experiments have demonstrated only rudimentary changes in shape, the incorporation of multiple photopatternable degradable materials with different degradation profiles would potentially enable complex programmed 3D morphing over time.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the U.S. Army Research Office through Grant W911NF-11-1-0080 (crosslinking and photopatterning studies), and the Materials Research Science and Engineering Center (MRSEC) on Polymers at UMass-Amherst (NSF-DMR-0820506; polymer synthesis). The authors thank Adam Hauser for providing poly(p-methylstyrene-co-benzophenone acrylamide).

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DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.biomac.5b00991 Biomacromolecules XXXX, XXX, XXX−XXX