Unusual and Superfast Temperature‐Triggered Actuators

A superfast actuator based on a bilayer fibrous mat shows folding/unfolding and the formation of 3D structures in a fraction of a second. The actuation is reversible for many cycles without losing its form and size, with unfolding at room temperature and folding above 35 °C. The system is promising for making 3D bioscaffolds, electrodes, and micro-/macroactuators. Stimuli-responsive actuation behavior of materials is highly interesting for a variety of applications including controlled drug release, tissue engineering, encapsulation of cells, biomedicine, microfabrication, and micromanipulation.1-17 Particularly interesting kind of actuating materials are the polymers.18 Among all examples of stimuli-responsive polymeric actuators, thermoresponsive ones deserve a particular attention19 because temperature is the most suitable stimulus for working with living object-cells withstand changes of temperature between 4 and 37 °C. Thermally triggered actuating polymeric thin films generally have a layered or patterned structure where one of the layers is thermoresponsive polymers showing temperature dependent reversible swelling and deswelling in water. Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermoresponsive polymer showing temperature dependent phase separation in water (lower critical solution temperature; LCST = 32 °C), which is widely used for design of thermoresponsive actuators.15, 20 The crosslinked PNIPAM swells and deswells in water below and above LCST, respectively. Deformation of thermoresponsive films crosslinked in dry/non-swollen state takes place at lower temperatures (below LCST) and derolling happens above LCST when the polymer is dehydrated in a bilayer structure. The actuation time of such actuators depend upon dimensions and increases from several seconds for very thin objects to many minutes for centimeter thick ones. Actuators based on synthetic materials with a combination of fast response and large displacements are rare. In general, fast actuation requires either small size due to enhanced fluid transport or elastic instabilities such as snap buckling and explosive fracture as shown by analysis on naturally occurring movements in plants and fungi.21 The synthetic actuators with fast response make use of porous materials which increase the rate of diffusion of solvent and facilitates fast interaction with the material. Asymmetric polyaniline films made by flash welding of corresponding nanofiber mats show reversible actuation in ≈20 s in presence of camphorsulfonic acid and sodium hydroxide for a 2.5 cm long sample. The casting of aqueous dispersion of polyaniline fibers made during polymerization by fast mixing of aniline and oxidants provided porous fibrous film.22 Recently bending of a 20 mm long porous membrane based on cationic poly(ionic liquid), poly(3-cyanomethyl-1-vinylimidazolium bis(trifuoromethanesulfonyl)imide) and a carboxylic acid-substituted pillar arene into multiply wound coils in presence of vapors of organic solvents such as acetone was shown in 0.4 s, and the original shape was recovered in ≈3 s in air.23 Electrospinning is an effective method to fabricate nanofiber nonwoven mats with large surface area and porosity.24 Several studies have been carried out in the recent time regarding use of temperature responsive polymers like PNIPAM in making nanofiber nonwovens for applications such as drug release, cell culture, etc.25-29 Electrospun nanofibers with very fine diameter, high surface to volume ratio and porosity are highly promising for transferring the stimulus and mass transport in comparison to the corresponding bulk films.30, 31 In this work, we report simple but versatile method of fabrication of superfast temperature-triggered actuators using electrospun porous fibrous double layer membranes based on crosslinked thermoresponsive PNIPAM. The state-of-the-art method of making actuators when applied to fibrous bilayer membranes provided new and unexpected results demonstrating a number of advantages: i) the actuation of mats is very fast (<1 s) even for thick and large sized samples; ii) the formation of 3D structures could be OPEN-CLOSED for many cycles without losing its form and size; iii) contrary to already reported PNIPAM-based bilayer films, fibrous mats are unfolded at room temper­ature and folded at elevated temperature; iv) they demonstrate anisotropic actuation behavior. Change in fiber diameters, swelling/shrinking, and morphology at different temperatures correlates to the unusual actuation behavior. The developed approach would be highly useful in design of porous 3D bioscaffolds and electrodes, catalysis, drug release, energy storage, etc. in the future. For our approach we have used two photo-crosslinkable polymers. First polymer is hydrophobic, non-stimuli-responsive thermoplastic polyurethane (TPU) with small addition of photo-crosslinker (4-acryloylbenzophenone, ABP). Second polymer is thermoresponsive photo-crosslinkable copolymer of N-isopropylacrylamide with 2 mol.% of photo-crosslinker acryl­oylbenzophenone (P(NIPAM-ABP)). P(NIPAM-ABP) was prepared by free radical polymerization according to the literature procedure.32 P(NIPAM-ABP) is stimuli-responsive polymer and demonstrates pronounced thermoresponsive properties and has LCST around T = 29 °C in pure water (Figure S1, Supporting Information). The LCST of P(NIPAM-ABP) is slightly lower than LCST of pure PNIPAM (T = 32 °C) that is due to presence of hydrophobic ABP moieties. Polymer mats made from individual polymers (TPU+ABP and P(NIPAM-ABP)) were prepared by electrospinning from concentrated polymer solutions. The TPU fibers had an average diameter 238 ± 69 nm whereas crosslinked poly(NIPAM-ABP) fibers were 477 ± 69 nm thick (Figure S2, Supporting Information). Electrospun mats had randomly oriented fibers with smooth surface without beads. We could not observe any predominant mole­cular orientation of polymer chains along the fibers using X-ray diffraction (XRD) and polarized Fourier transform infrared (FTIR) spectroscopy (not shown here), which is consistent with previous studies regarding electrospun PNIPAM fibers.27 The TPU fiber mat did not show any volume shrinkage at temperatures till 40 °C both in air and in water (Figures S3 and S4, Supporting Information), while P(NIPAM-ABP) fiber mat demonstrate reversible change of the size below and above LCST in water. The exposure of P(NIPAM-ABP) mat to cold water (4 °C) results in slight swelling and the length and width of the sample increase by 7%–8% (Figure 1a,b and Figure S5, Supporting Information). The total area increase is ≈15% (Figure 1b, Table 1 and Table S1, Supporting Information). The thickness of mat considerably increased in cold water (590%). Increase of the temperature above LCST results in considerable contraction of mat and area at 40 °C is only ≈42% of the area in dry state (Figure 1c, Table 1 and Table S1, Supporting Information). The switching of size in cold and warm water is reproducible and can be repeated many times (Figure S5, Supporting Information). The thickness of mat also decreased in warm water from 590% to 280% but still significantly more than that of dry mat (Figure 1d–f and Table 1). We calculated volumes of the mats, which is area × thickness, in different conditions. Volume of the mat increases 6.8 times due to swelling in cold water (4 °C) and remains almost unchanged in warm water (40 °C). It is observed that the diameters of fibers increase to 1338 nm in cold water and reduces to 752 nm after exposure to warm water (Figure 1g–i, Table 1 and Table S1, Supporting Information). In both cases the diameter was more than the dry fibers. Thus, swelling of P(NIPAM-ABP) mats is highly anisotropic. It is very well known that the randomly oriented electrospun fibers are deposited in non-equilibrium stretched state due to very fast deposition process. The stretched PNIPAM chains cannot relax to more entropic favorable conformation of random coils because of very high Tg of polymer (Tg (PNIPAM) = 132 °C).33 Exposure to water makes polymer chains mobile and can lead to contraction in planar direction due to relaxation to random coils. This effect is similar to the mechanism of activation of solvent-driven shape memory poly­mers34 and temperature driven contraction of electrospun fibers.35 The small dimensional change in planar directions for PNIPAM at temperature < LCST could be due to the balance between the contraction of polymer chains due to relaxation and swelling of PNIPAM. The higher expansion in thickness than in the planar (length and width) direction was also previously observed for electrospun fibrous mat of a mixture of poly(vinyl alcohol) and poly(acrylic acid) in buffer solutions.30 The layer-by-layer structure of randomly oriented fibers with voluminous inter-fiber spaces in planar directions with lots of contact points between fibers in different layers along thickness also provide anisotropic swelling. The randomly distributed fibers in planar directions swell in inter-fiber spaces reducing overall expansion in length and width whereas swelling of individual fibers is added on in the thickness direction due to contact points.30 Increase of temperature above LCST results in the dehydration of the polymer and expulsion of polymer from inter-fiber spaces leading to shrinkage in planar direction. The bilayered fibrous nanomats were prepared by sequential electrospinning of TPU mixed with ABP and P(NIPAM-ABP) (Figure S6, Supporting Information). The thickness of the TPU layer was fixed to 40 ± 10 μm whereas the thickness of P(NIPAM-ABP) layer was varied (15 ± 5, 30 ± 7, 70 ± 12, and 100 ± 13 μm) by spinning different amounts of P(NIPAM-ABP) solution of a fixed concentration. The corresponding samples were denoted as TPU40-NIPAM15, TPU40-NIPAM30, TPU40-NIPAM70, and TPU40-NIPAM100, respectively. The polymers were then crosslinked by irradiation with UV light. We found that the sample preparation is highly critical. The bilayer prepared by sequential spinning of TPU (without ABP) and P(NIPAM-ABP) easily delaminates upon immersion in water (Figure S7 and Movie S1, Supporting Information). Additionally, pressing of two layers at 200 bars for 5 min at room temperature helped in getting stable bilayers without delamination in water for many cycles. We observed that crosslinked bilayer mat actuate upon immersion in cold (T = 4 °C) and warm (T = 40 °C) water (Figure 2). The bilayer mat fixed to a needle is slightly bent in cold water below LCST in the direction of TPU layer (Figure 2c,f). Heating above LCST leads to bending of the film in the direction of thermoresponsive polymer (Figure 2b,e,g). The bending and restoration of the shape of a bilayer by changing temperature of water is instant, complete and could be repeated at least 20 times. The actuation properties of the mat bilayer arise from responsive properties of P(NIPAM-ABP) which is contracted in warm water (Figure 1c). TPU, on the other side, restricts expansion/shrinking of the P(NIPAM-ABP) mat that results in the bending of the film. As curvature of bending scales inversely with membrane thickness, we estimated actuation velocity as ratio kh/t ≈ 0.3 s−1 (where k is the curvature, h is the bilayer thickness and t is the time), which is much faster that other actuators (10−5–10−2 s−1) and is comparable to the most fast ones (0.1 s−1).23, 31 The free standing bilayer mat (TPU40-NIPAM70 with planar size of 25 mm × 5 mm) rolled in less than one second (Figure 3c and Movie S2, Supporting Information). The rolling direction can also be precisely controlled. For example, rolling of bilayer mat around the solid wire results in the formation of porous tubes (Figure S9, Supporting Information). Such tubes can be used for encapsulation of cells and design of scaffolds. This actuation rate is much higher than of nonfibrous bilayer made from similar materials. For example, thinner bilayer consisting of 4 μm PNIPAM and polycaprolactone layers fold within 20–30 s.32 The higher actuation rate of fibrous mats can be explained by higher intrinsic surface area and porosity which facilitates water diffusion in and out of thermoresponsive layer. Finally, we demonstrated actuation of more shapes than rectangle such as star (Movie S3, Supporting Information). The actuation was instant even in presence of water vapors. The star-like bilayer (the maximal diameter is ≈1 cm) was fixed on a substrate by its central part in the way that TPU is bottom layer and P(NIPAM-ABP) is upper one. Larger structures also showed immediate change of the shape in water vapor (Figure S10 and Movies S4 and S5, Supporting Information). In summary, we report superfast actuators with large scale movements in less than 1 s using highly porous electrospun fibrous mats. The actuator is a bilayer nanofiber mat made by sequential electrospinning and subsequent photo-crosslinking of a thermoresponsive polymer (P(NIPAM-ABP)) and TPU. TPU can also be replaced by other non-responsive polymers such as nylon-6, polysulfonamide, etc. with similar actuation behavior and time (data not shown here). The state-of-the-art method of making actuators combined with fibrous bilayer membranes provides altogether new and unexpected results. We demonstrate for the first time the unusual responsive properties of thermoresponsive P(NIPAM-ABP) that results in actuation of bilayer mat in warm water and restoration of its shape in cold water. The actuation of the mats with the thickness more than 100 μm and planar size of 25 mm × 5 mm is very fast and occurs in less than one second that puts electrospun actuators among the fastest ones. We also demonstrate formation of 3D structures with different shape by self-folding of electrospun bilayers. We expect that developed approach can be used for design of porous 3D bioscaffolds and electrodes as well as superfast actuators. Materials: N-Isopropylacrylamide (NIPAM) (Aldrich), 4-hydroxybenzophenone (Fluka), TPU (Desmopan DP 2590, Bayer Materials Science), N,N′-dimethylformamide (DMF) (99.8%, Aldrich), benzophenone (Aldrich), and N,N′-diisopropylethylamine (Aldrich) and acryloyl chloride (Fluka) were used as received. Azobisisobytyronitrile (AIBN) (Fluka) was recrystallized from ethanol before use. The preparation of polymerizable photo-crosslinker (ABP) and photo-crosslinkable P(NIPAM-ABP) (NIPAM:ABP = 98:2 molar ratio; Mn = 1.61 × 105 and Mw = 3.14 × 105) was done according to a previous report.32 Electrospinning: TPU (18 wt%) and P(NIPAM-ABP) (35 wt%) solutions were prepared in DMF. Rhodamine B (0.4 wt% with respect to the weight of TPU) and ABP (4 wt% with respect to the weight of TPU) was added to TPU solution to get color contrast and crosslinking with P(NIPAM-ABP) layer, respectively. The applied voltage on the needle (outer diameter: 0.9 mm) for the electrospinning was 22 and 10.9 kV for TPU and P(NIPAM-ABP), respectively. All the fiber samples were collected by a horizontally rotated grounded disc (diameter of 15 cm) with a rotating speed of 30 rpm. A bilayer nanofiber mat was made in which one layer was from P(NIPAM-ABP) (active polymer) and other layer was from TPU (passive polymer). The bilayered samples were prepared by spinning different amounts (0.1, 0.5, 0.9, and 1.3 mL) of P(NIPAM-ABP) on electrospun TPU (0.5 mL). For comparison purposes pure TPU, P(NIPAM-ABP), and bilayered TPU/P(NIPAM-ABP) nanofiber mats without ABP crosslinker in TPU were also prepared by electrospinning 0.5 mL of the corresponding solutions. The flow rate was set to 0.6 mL min−1 for each experiment. The bilayer nanofiber mats were pressed at 200 bars for 5 min at room temperature. The nanofiber mats were dried in vacuum oven at 50 °C for 12 h to remove the residual solvent. The photo-crosslinking of nanofiber mats was performed with an UV lamp (Honle UVAHAND 250 GS) for 4 h. Characterization: A digital microscope (Keyence VHX-100K) was used to observe the bending behavior. The morphology of nanofibers and the thickness of fiber mats were studied by scanning electron microscopy (SEM) (Zeiss Leo 1530). Before scanning, the samples were coated with 3.0 nm of platinum (Pt) to increase the conductivity of the samples. Image J software was used to measure the diameter of fibers and the thickness of the fiber mat. Micro-DSC was performed on a Setaram Micro-DSC III at a heating/cooling rate of 0.25 °C min−1. The molecular weight of P(NIPAM-ABP) was measured by gel permeation chromatography (GPC) using DMF as the eluent at a flow rate of 0.5 mL min−1 at 25 °C and poly(methyl methacrylate) standards were used for calibration. S.A. and L.I. are thankful to DFG for the financial support. The work was financially supported under the frame of SFB 840 and IO 68/1-3. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.