Programmable Shape Recovery Process of Water-Responsive

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Programmable shape recovery process of water-responsive shape-memory polyvinyl alcohol by wettability contrast strategy Zhiqiang Fang, Yudi Kuang, Panpan Zhou, Siyi Ming, Penghui Zhu, Yu Liu, Honglong Ning, and Gang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14868 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Programmable shape recovery process of water-responsive shape-memory polyvinyl alcohol by wettability contrast strategy Zhiqiang Fang1,2‡, Yudi Kuang1‡, Panpan Zhou1, Siyi Ming1, Penghui Zhu1, Yu Liu1, Honglong Ning2*, Gang Chen1* 1. State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China 2. Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, China

*E-mail: [email protected], [email protected] ‡These authors contributed equally.

Abstract Water-responsive shape memory polymers (SMPs) are desirable for biomedical applications, but their limited shape recovery process is problematic. Herein, we demonstrate a shape-memory polyvinyl alcohol (SM-PVA) with programmable multi-step shape recovery processes in water via a wettability contrast strategy. A hexamethyldisilazane (HMDS)-treated SiO2 nanoparticle layer with varying loading weights was rationally deposited onto the surface of SM-PVA, aiming to create surface-wettability contrast. The varying wettability led to different water adsorption behaviors of SM-PVA that could be well described by the pseudo-first-order kinetic model. The results calculated from the kinetic model showed that both the pseudo-first order-adsorption rate constant and the saturated water absorption of 1

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SM-PVA demonstrated a declining trend as the loading weight of SiO2 increased, which laid the foundation for the local regulation of the water-responsive rate of SM-PVA. Finally, two proof-of-concept drug delivery devices with diverse three-dimensional

structures

and

actuations

are

presented

based

on

the

water-responsive SM-PVA with preprogrammed multi-step shape-recovery processes. We believe the programmable shape-memory behavior of water-responsive SM-PVA could highly extend its use in drug delivery, tissue engineering scaffolds, smart implantable devices, etc.

Keywords: shape-memory polymer; wettability contrast; shape recovery process; water-responsive; smart materials

Introduction Shape-memory polymers (SMPs) are one of most significant branches of smart materials that have garnered great attention from both academic and industrial communities owing to their ability of changing their shape upon exposure to external stimuli such

as heat,1

light,2-3

electrical fields,4-5

magnetic fields,6 and

water/moisture.7-10 A series of practical applications based on SMPs with diverse shape-memory effects (SMEs) have been demonstrated for biomedical devices,11-13 deployable structures,14 surface patterning,15-16 smart textiles,17-18 sensors and actuators.9, 19-30 Among these SMPs, thermal-responsive SMPs are the most widely investigated because of their simple triggering process, tunable transition temperature, 2


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and excellent SMEs.31-34 Active studies on thermal-responsive SMPs have been carried out all over the world in the past few decades, but there are several disadvantages for biomedical applications: (1) the switching temperature is above body temperature, which may damage the surrounding tissues and cells or the activity of the loaded drug; (2) thermal-responsive SMPs always possess a high Young’s modulus and may therefore limit the movement of surrounding tissues; (3) most heat-triggered SMPs exhibit quick shape recovery that is not suitable for some special clinical uses because they may cause insertion-induced tissue damage. In recent years, SMPs triggered by water, the most abundant solvent in nature, have been developed for biomedical applications to overcome the aforementioned challenges faced by thermal-sensitive SMPs. Water-responsive SMPs have much more moderate characteristics that will not cause unfavorable effects.31, 34 Huang and co-workers first found moisture/water-responsive SMEs in polyurethane SMP. The absorbed moisture molecules weakened the hydrogen bonds between the N–H and C=O groups and increased the mobility of the neighboring macromolecule chains, thus resulting in water-responsive SMEs.35-37 After that, similar moisture/water-responsive SMEs also have been found in other natural and synthetic polymers34, 38-44. Recently, one novel kind of water-responsive SMPs based on a percolation network of rigid cellulose nanocrystals and a thermal plastic elastomer was proposed. These water-responsive SMPs demonstrated unique 3



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mechanical adaptivity and excellent shape-recovery properties in water.8,

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45-50

Previous works have concentrated on the realization or enhancement of water-responsive SMEs through novel construction methods or materials. However, to date, the development of SMPs with programmable shape-recovery process in water has rarely been studied, which significantly hinders the extensive applications of water-responsive SMPs. Multi-segment composite system, which comprises of SMPs with different stimuli-responsive properties, is a promising way to program the shape-recovery process. Xie and coworkers reported an epoxy-based multicomposite SMP, consisting of three regions (Fe3O4-SMP region/neat SMP region/CNT-SMP region), exhibited well-controlled multiple shape recoveries due to the selective radiofrequency heating of Fe3O4 and CNT nanoparticles.51 Multi-segment composites with similar controllable shape-recovery process can also be prepared by introducing a reversible covalently cross-linking reaction, such as Diels-Alder reaction, disulfide exchange, transcarbamoylation and transesterification.52-57 Although programmable shape recovery processes have been realized in many stimuli-responsive polymer systems, the programmable shape-recovery process of water-responsive SMPs is still missing. In this paper, we propose a wettability contrast strategy to realizing programmable multi-step shape recovery of water-responsive shape-memory polyvinyl alcohol (SM-PVA) by strategically depositing hexamethyldisilazane (HMDS)-treated SiO2 on the SM-PVA surface. The fundamental concept behind this water-responsive 4


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shape-memory behavior was also investigated by using the pseudo-first-order kinetic model. Finally, as the proof of concept, two types of drug delivery devices with different three-dimensional (3D) structures and actuations are demonstrated on the basis of this SM-PVA.

Results and discussion SM-PVA is a kind of semicrystalline polymer with a nontoxic nature, biodegradability, biocompatibility, hydrophilicity, moderate mechanical properties, and thermal/moisture dual stimuli-responsive shape-memory effects. The cross-linked network and crystalline domains in the PVA act as “net points” that respond to the permanent shape, whereas the amorphous phase acts as the mainly “switching unit” that contributes to the stress fixation. Moisture absorption in PVA will weaken the intra/intermolecular hydrogen bonds and increase the mobility of the macromolecular chains, which decrease the glass transition temperature (Tg) and cause the shape recovery of the SM-PVA. In this study, the Tg of the SM-PVA employed here was approximately 67.8 °C, and its dual stimuli-responsive SMEs are shown in Figure S1, Video S1 and S2 (Supporting Information). To realize the regulation of the shape-recovery process in water, HMDS-treated SiO2 nanoparticles were used to modify the surface wettability of the SM-PVA. Figure 1a shows the regulation and programming process of water-responsive SM-PVA and its shape-recovery processes in water. In route I, the local area of the rod-shaped 5



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SM-PVA (permanent shape) was coated with HMDS-treated SiO2 nanoparticles at a loading weight of 0.75 g/m2 (red-colored region) by a spray-coating process. After air drying, the sample was programmed into an “N” shape (St1, temporary shape) at a temperature of 100 °C. The red region exhibited a superhydrophobic surface with a contact angle of 162°, whereas the pure SM-PVA corner showed a hydrophilic surface (approx. 32°). Upon immersion in water, the hydrophilic corner in the “N”-shaped SM-PVA initially recovered (Sr1), followed by the hydrophobic part (Sf1) (as shown in Figures 1a, b and Video S3, Supporting Information). In route II, the pure SM-PVA, acting as a control sample, was also programmed into an “N” shape, but it displayed a very different recovery process as compared to the HMDS-treated SiO2-coated SM-PVA after being immersed in water at room temperature. As seen from Figures 1a and c, the programmed “N”-shaped pure SM-PVA gradually stretched in the axial direction from shape St2 to Sf2. Finally, the two corners in the deformed control sample simultaneously regained their original straight shape (Sf2), indicating that they had the same shape-recovery rate. This phenomenon can be attributed to the same swelling rate in each direction of the pure and isotropic SM-PVA. The results shown in Figure 1 demonstrate that the shape-recovery behavior of water-responsive SM-PVA can be manipulated by regulating its surface-wettability contrast using superhydrophobic SiO2.

6


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Figure 1. (a) Schematic showing the regulating and programming process of water-responsive SM-PVAs and their whole shape-recovery process in water. Digital photographs show the shape-recovery processes of (b) SM-PVA with an HMDS-treated SiO2 nanoparticle layer and (c) pure SM-PVA. Note that the images from left to right in Figures 1b and c represent the temporary shape, intermediate shape, and original shape. To better understand the wettability-contrast strategy for the SMEs of water-responsive SM-PVA, the surface wettability of SM-PVA films with and without HMDS-treated SiO2 nanoparticles was investigated in detail. A rectangular SM-PVA 7



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film was prepared by a casting method, and then half of its area was spray coated with HMDS-treated SiO2 nanoparticles at a coating weight of 0.75 g/m2. As shown in Figure 2a, it is quite difficult to visually distinguish the coated area from the uncoated region because the transparent appearance of the SiO2-coated sample was well maintained. However, an obvious difference in the surface wettability between the two dissimilar regions is observed when a droplet of water is separately dropped on their surfaces (shown in Figure 2b). Scanning electron microscopy (SEM) analysis was utilized to unveil the difference in water contact angle between the coated and uncoated surfaces of SM-PVA. The SEM images in Figures 2c and d show that the SiO2 nanoparticles formed a continuous yet porous layer over the surface of SM-PVA as a result of spray coating. Additionally, micro- and nanostructures were built on the surface of the SM-PVA film, resulting from the aggregation of HMDS-treated SiO2 nanoparticles during spray coating. In light of the basic theory of constructing a super-hydrophobic surface, the hydrophobicity of the HMDS-treated SiO2 nanoparticles combined with the micro- and nanostructures jointly contributed to the super-hydrophobicity of the SiO2-coated SM-PVA, with a water contact angle of approximately 162°. Interestingly, when the rectangular SM-PVA film was immersed into water, the SiO2-coated region seemed to be opaque, thus demonstrating a mirror effect due to the air gap at the interface between SM-PVA and water that resulted from the poor surface wettability. However, the uncoated region was transparent because of the good surface wettability 8


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(Figure 2e).

Figure 2. (a) Visual appearance and (b) surface wettability of SM-PVA with and without an HMDS-treated SiO2 nanoparticle layer. (c, d) SEM images showing the surface topography of the SiO2-coated SM-PVA under different magnifications. (e) Different phenomena were observed as SM-PVA with and without the HMDS-treated SiO2 nanoparticle layer was immersed in water. Although the SiO2-coated SM-PVA exhibited super-hydrophobic properties, with a water contact angle of 162°, the shape-recovery process will still occur when exposed to water. To explore the underlying principle for this phenomenon, the time-dependent swelling of the pure SM-PVA and the SiO2-coated SM-PVA (0.75 g/m2) was measured. As shown in Figure 3a, the SiO2-coated SM-PVA tended to swell with increasing immersion time, but its swelling rate was smaller than that of the pure SM-PVA owing to the super-hydrophobic performance of the SiO2 nanoparticle layer. To quantitatively evaluate the effect of HMDS-treated SiO2 on the swelling behavior of SM-PVA, the water adsorption kinetics during the swelling process were studied. We found that the water adsorption behavior of SM-PVA with and without the 9



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SiO2 nanoparticle coating can be well described by the pseudo-first-order kinetic model, as shown in Figure 3a. The differential equation can be expressed as

= ( − )

(1)

Integrating Eq. (1) and applying the boundary conditions = 0 at t = 0 and = at

t = t gives = −

(2)

where and (%) are the swelling ratio at equilibrium and at time t, respectively.

(min−1) is the rate constant of pseudo-ﬁrst-order adsorption. The results calculated from the pseudo-first-order kinetic model are listed in Table 1. We can conclude that the absorption rate and saturated water absorption of SM-PVA decreased by coating it with a thin layer of HMDS-treated SiO2 nanoparticles.

The

thin

HMDS-treated

SiO2

nanoparticle

layer

exhibited

super-hydrophobic properties, and it had the ability to protect the water-sensitive SM-PVA from liquid water upon immersion. In addition to the super-hydrophobicity, the thin SiO2 nanoparticle layer possessed a porous structure (Figures 2d and Figure S2, Supporting Information), permitting mass transfer at the interface between the SM-PVA and the liquid water (Figure 3b). The water vapor passed through the porous SiO2 nanoparticle layer and finally penetrated into the SM-PVA, causing it to swell. However, this whole process significantly increased the time and resistance to water absorption, leading to the decrease of the swelling rate and the equilibrium swelling ratio. 10


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Figure 3. (a) Time-dependent swelling of the pure SM-PVA and the SiO2-coated PVA (coating weight: 0.75 g/m2) and the corresponding fitting curves based on the pseudo-first-order kinetic model. (b) Schematic illustrating the mass transfer at the interface between the SM-PVA and the liquid water as the SiO2-coated SM-PVA was immersed in water. Table 1 The pseudo-first-order kinetic fitting results for the water absorption of pure SM-PVA and SiO2-coated SM-PVA Sample Neat SM-PVA 0.5 SiO2/SM-PVA 0.75 SiO2/SM-PVA 1.00 SiO2/SM-PVA 1.25 SiO2/SM-PVA

(min−1) 0.04577 0.03264 0.02425 0.01430 0.01190

(%) 1.4712 1.4246 1.2873 1.2864 1.2738

R2 0.9908 0.9938 0.9943 0.9940 0.9922

Dynamic mechanical analysis (DMA) testing was further carried out to better understand the shape-recovery mechanism of SiO2-coated SM-PVA in terms of its thermomechanical properties. The results show that the storage modulus of SM-PVA decreased with increasing water absorption (Figure S3a, Supporting Information). In comparison to SM-PVA without water, a decline in storage modulus of approximately two orders of magnitude (from nearly 6200 MPa to approximately 44 MPa) was 11



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observed when the water content in SM-PVA reached 0.2969 g H2O/g PVA at 20 °C. The released stress finally triggered the shape recovery of SM-PVA in water. Meanwhile, the decrease of storage modulus also indicates the enhanced polymeric segmental mobility of the SM-PVA, which is reflected in the decreasing Tg (Figure S3b, Supporting Information). Differential Scanning Calorimetry (DSC) testing was further utilized to investigate the melting transition of crystalline region of SM-PVA during water immersion (Figure S4, Supporting Information). The endothermic melting peak at 225 °C slightly decreases with increasing water content of SM-PVA, indicating a partial melting transition of the crystalline region of SM-PVA that increases the mobility of PVA molecule chains. Thus, the shape recovery of the SiO2-coated SM-PVA in water can be concluded as resulting from the plasticizing effect caused by water absorption, which is in good agreement with the results of previous studies regarding pure SM-PVA.34, 58-60 Furthermore, the water adsorption kinetics of SM-PVA were tuned by rationally regulating the coating weight of HMDS-treated SiO2, and the results are displayed in Figure 4a. Both the adsorption rate constant and the saturated water absorption decreased with increasing coating weight of SiO2 (Table 1), which could be ascribed to the increased diffusion path and mass transfer resistance in the SiO2 nanoparticle layer. The tunable water adsorption kinetics of SiO2-coated SM-PVA paves the way to realizing the multi-step shape-recovery process of SM-PVA in water. The complex shape-recovery process of SM-PVA was also realized by increasing 12


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the size of the SiO2-coated areas or by simply altering the sequence of SiO2-coated regions with different coating weights along the axial direction. As shown in Figure 4b and c, the rod-like SM-PVA was divided into three parts (R1, R2, and R3), where we adjusted their surface wettability values by coating the samples with HMDS-treated SiO2 of different coating weights. After drying, the SiO2-coated SM-PVA samples were programmed into “W”-like shapes using the same procedures as mentioned previously. When the “W”-like SM-PVA samples were immersed in water, the region without the SiO2 nanoparticle layer recovered first, followed by the 0.5 g/m2 SiO2-coated region, and finally the 1.0 g/m2 SiO2-coated region. By changing the order of the aforementioned three regions, SM-PVA with the desired shape recovery process was achieved (Figures 4d and e). The only difference between strategy I and strategy II was the sort orders of the 0.5 g/m2 and 1.0 g/m2 SiO2-coated regions, but the recovery sequences of the corresponding SM-PVA were totally different. That is to say, when exposed to water, although the whole recovery process for both strategies was the same bent-to-straight change, the temporary shapes before regaining the permanent shapes were quite different owing to the different arrangement of SiO2-coated regions with dissimilar surface wettability values.

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Figure 4. (a) Time-dependent swelling of SM-PVA with different coating weights of SiO2 nanoparticle layer and their corresponding pseudo-first-order kinetic fitting curves. (b, c) Schematics illustrating the two simple strategies for achieving SM-PVA with different shape recovery processes (insets represent the corresponding temporary shape of SM-PVA after programming), and (d, e) show their corresponding shape-recovery processes upon exposure to water. Recently, growing attention has been paid to the application of water-responsive SMPs in medical industries.31,

34, 38, 42, 61-62

The water-responsive SM-PVA with a

programmable shape-recovery process, prepared by a wettability contrast, has the ability to expand its use in the field of biomedical applications, such as drug delivery and release. Figure 5a demonstrates a strategically assembled SM-PVA cube for a 14


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sustained-release capsule. To control the releasing behavior of the medicine in the capsule upon exposure to body fluid, each edge of the predesigned SM-PVA cube was spray coated with different weights of SiO2 to realize the surface wettability contrast before programming it into a cube. When the cube was immersed in water, the edges without HMDS-treated SiO2 first deformed to release the medicine. Over time, the other SiO2-coated edges opened successively according to the coating weight of the SiO2, further increasing the release of medicine as a result of the expanded contact area between the medicine and the body liquid (Video S4, Supporting Information). Moreover, a spiral drug-delivery stent was also fabricated based on a similar regulation strategy, releasing the drug by gradually unwinding when it was subjected to water (Figure 5b and Video S5, Supporting Information).

15



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Figure 5. Two proof-of-concept drug delivery devices based on multi-step water-responsive SM-PVA. (a) Schematic illustration of building a sustained-release capsule and its corresponding deforming process in water. (b) A spiral drug-delivery stent that releases the drug by gradually unwinding in water, note that the coating weight of SiO2 layer reduces as the color changes from red to cyan.

Conclusion In summary, water-responsive SM-PVA with a programmable shape-recovery process is proposed for the first time using a wettability-contrast strategy, which allows defining regional water-responsive property of SM-PVA after it was fabricated. HMDS-treated SiO2 nanoparticles were coated on the surface of SM-PVA to create a surface wettability contrast. As a result, the water absorption behavior of SM-PVA changed, but it could be well described by the pseudo-first-order kinetic model. By strategically regulating the sequence of the various coated regions with different coating weight of SiO2 or increasing the SiO2-coated area, the water-responsive SM-PVA exhibited a programmable multi-step shape-recovery process. Finally, two types of proof-of-concept drug delivery devices with different 3D structures and actuations were proposed based on the water-responsive SM-PVA with programmable multi-step shape-recovery behavior. It is believed that the strategy proposed here will significantly extend the potential applications of water-responsive SMPs, especially in the field of biomedical applications such as drug delivery, tissue engineering scaffold, 16


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and smart implantable devices.

Experimental Materials PVA-171 with water and thermal dual stimuli-responsive shape-memory properties was purchased from Sinopec Sichuan Vinylon Works (China). The polymerization and the saponification degrees of PVA-171 were 1700 and 99%, respectively. Hexamethyldisilazane (HMDS)-treated SiO2 with a Brunauer–Emmett–Teller (BET) specific area of 220 m2/g was purchased from Evonik Degussa AG (Germany). Preparation of SM-PVA PVA solution (8 wt%) was prepared by adding 32 g of PVA-171 to 400 mL of deionized water. The solution was maintained at 95 °C with a stirring speed of 300 rpm for 1 h, followed by moderate stirring at room temperature at a speed of 100 rpm for 24 h. After that, the homogenous PVA solution was poured into a 15 × 15 cm square mold and placed into a vacuum oven at −0.1 MPa for 4 h. Then, a solid SM-PVA film with a grammage of 200 g/m2 was prepared by drying the vacuum-treated PVA solution for 48 h in a constant-temperature humidity chamber at 50% relative humidity (RH%) and 45 °C. Finally, the SM-PVA film was cut into the specific shapes and sizes, and placed in a vacuum oven to dry for 8 h at 45 °C before use. Preparation of HMDS-treated SiO2 and spray coating An HMDS-treated SiO2 suspension was prepared by adding HMDS-treated SiO2 to 17



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ethanol liquid, followed by sonication for 20 mins. After that, the SiO2 suspension was manually sprayed onto the surfaces of the SM-PVA samples. The resulting samples were placed in a vacuum oven at 45 °C for 1 h. The coating weight was controlled by regulating the spray coating times. The HMDS-treated SiO2 suspension was used immediately after it was ready. Water absorption behavior of SM-PVA SM-PVA samples with different SiO2 coating weights were immersed into 30 °C deionized water and weighed every 10 min with an analytical balance (precision 0.0001 g) until the variation of swelling ratio was less than 1.5%. The swelling ratio of the SM-PVA was calculated as follows: Swelling ratio (%) = ( − ) × 100/ Where and are the initial mass of the SM-PVA sample and that at time t, respectively. Other characterizations The contact angles of 3 µL of water on the SM-PVA samples were measured using a contact angle tester (OCA40 Micro, Germany) under a dosing rate of 2 µL/s. The surface images of the SiO2-coated SM-PVA samples were obtained by utilizing a field emission scanning electron microscope (ZEISS Merlin, Germany). The storage modulus and tan δ of the SM-PVA samples (specimen size was 25 × 5 × 0.05 mm) with different moisture contents were determined by using a DMA 242C (NETZSCH, Germany) instrument in tensile mode at a frequency of 1 Hz in N2 atmosphere with a 18


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heating rate of 2 °C/min. The amorphous transition and melting transition of PVA before and after water immersion were measured by a Differential Scanning Calorimeter (TA Q200, America) at a heating rate of 10 °C/min from -40 °C to 250 °C.

Acknowledgments Zhiqiang Fang would like to acknowledge financial support from the Training Program of State Key Laboratory of Pulp and Papermaking Engineering (2016PY01), China Postdoctoral Science Foundation (2015M570716), and the Fundamental Research Funds for the Central Universities (2015ZM156). Yudi Kuang would like to acknowledge the financial support from China Scholarship Council (201606150041).

Supporting Information Detail information of the water/heat dual stimuli-responsive shape-memory effect of SM-PVA; surface morphology of the super-hydrophobic SiO2 nanoparticle layer before and after water immersion; DMA analysis of SiO2-coated SM-PVA with different water contents; DSC analysis of SM-PVA with different water contents; Video S1: Heat-induced shape recovery process of SM-PVA. Video S2: Water-induced shape recovery process of SM-PVA. Video S3: Water-induced shape recovery processes (100 times fast) of the SM-PVA with (Route I) and without (Route II) surface wettability modified. Video S4: Water-induced shape recovery processes (100 times fast) of the strategically assembled cubic SM-PVA sustained-release capsule. 19



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Video S5: Water-induced drug releasing process (100 times fast) of the strategically assembled spiral SM-PVA drug-delivery stent. This material is available free of charge via the Internet at http://pubs.acs.org.

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22 (31), 3344-3347. 62. Liu, Y.; Li, Y.; Yang, G.; Zheng, X.; Zhou, S., Multi-Stimulus-Responsive Shape-Memory Polymer Nanocomposite Network Cross-Linked by Cellulose Nanocrystals. ACS Appl. Mater. Inter. 2015, 7 (7), 4118-4126.

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Graphic for manuscript

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Programmable Shape Recovery Process of Water-Responsive

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