Skeletal Muscle Fibers Inspired Polymeric Actuator by Assembly of Triblock Polymers

Abstract Inspired by the striated structure of skeletal muscle fibers, a polymeric actuator by assembling two symmetric triblock copolymers, namely, polystyrene‐b‐poly(acrylic acid)‐b‐polystyrene (SAS) and polystyrene‐b‐poly(ethylene oxide)‐b‐polystyrene (SES) is developed. Owing to the microphase separation of the triblock copolymers and hydrogen‐bonding complexation of their middle segments, the SAS/SES assembly forms a lamellar structure with alternating vitrified S and hydrogen‐bonded A/E association layers. The SAS/SES strip can be actuated and operate in response to environmental pH. The contraction ratio and working density of the SAS/SES actuator are approximately 50% and 90 kJ m−3, respectively; these values are higher than those of skeletal muscle fibers. In addition, the SAS/SES actuator shows a “catch‐state”, that is, it can maintain force without energy consumption, which is a feature of mollusc muscle but not skeletal muscle. This study provides a biomimetic approach for the development of artificial polymeric actuators with outstanding performance.

Scheme S1 Synthesis route for the SES triblock copolymer.
Scheme S2 Synthesis route for the SAS triblock copolymer.

Supporting Videos
Video S1 The dilation of the SAS/SES strip as pH increasing.

Video S2
The contraction of the SAS/SES strip as pH decreasing.

Video S3
The SAS/SES strip alternately dipping into pH = 1 solution and pH = 13 solution.

Video S4
The contraction and dilation of SAS/SES by adding acid and base into the solution.

Video S5
The breakage of A/SES strip as pH increasing to pH = 13.

Video S6
The elongation of SAS/E strip as pH increasing to pH = 13 but no contraction as pH decreasing to pH = 1. were purified by passing a basic alumina column to remove the inhibitor before use.

SES
Scheme S1 Synthesis route for the SES triblock copolymer.
When the reaction was finished, the crude production was purified by column chromatography on a silica gel column with a mixture of PE and EA as the eluent (10:1, v/v).
The product RAFT-yne was collected as a yellow powder with a yield of 91%. 1  Discussions on the synthesis of SES. The synthetic procedure of SES triblock copolymer is shown in the Scheme S1. The chemical structure of RAFT-yne was proved by 1 H NMR ( Figure S1a). The "click" reaction of RAFT-yne with N 3 -PEO 909 -N 3 afforded the macromolecular RAFT agent RAFT-PEO 909 -RAFT, the structure of which was confirmed by FT-IR and 1 H NMR spectra. As shown in Figure S2, the vibrational peak of the azide group at 2111 cm -1 totally disappeared after the "click" reaction, confirming the successful reaction with RAFT-yne. Moreover, in the 1 H NMR spectra in Figure S1b, besides the strong resonance at 3.63 ppm from the methylene protons in the PEO main chain, the chemical shifts of methylene protons adjacent to the alkyne group moved from 2.51 ppm to 3.06 ppm, and the methylene protons close to the ester group shifted from 4.20 ppm to 4.35 ppm. In addition, the proton in the alkynyl group at 1.95 ppm fully disappeared, while a new weak resonance peak appeared at 7.54 ppm, which was attributed to the proton in the newly generated heterocyclic rings. Finally, the macromolecular RAFT agent was applied to polymerize styrene to prepare the target triblock copolymer SES. As shown in Figure S1c, a S-7 Supporting Information set of resonances in the range of 6.44 and 7.08 ppm appeared, which can be attributed to the protons of benzene ring in the PS block. Combined with GPC curves (Figure S3), the SES triblock copolymer showed a monodisperse peak, and its retention time was smaller than that of RAFT-PEO 909 -RAFT, due to the increase in molecular weight. Based on these data, the successful preparation of the SES triblock copolymer was validated. The molecular weight of a PS segment determined by 1 H NMR was 15k, and average polymerization degree was 144.
PDI of SES determined by GPC with PS standard sample was 1.09. Scheme S2 Synthesis route for the SAS triblock copolymer.

Synthesis of PS-RAFT-PS.
The SAS triblock copolymer was synthesized according to the previously reported procedure with modifications. [1,2] First, the RAFT agent HOOC-RAFT-COOH (0.010 g, 0.035 mmol), initiator AIBN (0.020 g, 0.010 mmol) and 50 mL of styrene monomer were added into a 100 mL of Schlenk flask with a magneton. After degassed three times by freeze-vacuum-thaw cycles, the flask was placed in an oil bath at 75 ℃ for polymerization. The degree of polymerization was monitored by GPC. When reaching the desired PS molecular weight, the polymerization was stopped by quenching the mixture into liquid nitrogen. Then the mixture was precipitated into methyl alcohol three times to thoroughly remove the unreacted styrene monomer. After filtration, the target product of PS-RAFT-PS was collected. The molecular weight of a PS segment determined by GPC was 10k, average polymerization degree was 96, and PDI was 1.07.  Figure S4 confirmed a monodispersed peak for PS-PtBA-PS, which shifted to a lower retention time relative to that of PS 96 -RAFT-PS 96 due to an increase in molecular weight.

Synthesis of SAS triblock copolymer.
The target triblock copolymer SAS was obtained by hydrolysis of the PtBA block to remove the tert-butyl ester groups. [2] 3 g PS-PtBA-PS was dissolved in a mixture of 30 mL DCM and 30 mL TFA. The mixture was stirred at room temperature for 48 h and the hydrolysis reaction was monitored by FT-IR and 1 H NMR spectra. In the FT-IR data (Figure S5), the C=O absorbance peak moved from 1721 cm -1 to 1697 cm -1 , and the peak of tert-butyl groups at 1365 cm -1 thoroughly disappeared after the reaction. After the deprotection reaction, the peak of the tert-butyl groups at around 1.40 ppm fully disappeared ( Figure S6). After the reaction finished. the solvent was removed by rotary evaporator, and the crude product was dissolved in THF and precipitated into cooled diethyl ether to obtain white powder of SAS. Figure S1 1 H NMR spectra of (a) RAFT-yne, (b) RAFT-PEO-RAFT and (c) SES.

Supporting Figures
Assignments of major peaks were marked in the spectra.