A soft, ultra-tough and multifunctional artificial muscle for volumetric muscle loss treatment

Abstract

The escalating prevalence of skeletal muscle disorders highlights the critical need for innovative treatments for severe injuries such as volumetric muscle loss. Traditional treatments, such as autologous transplants, are constrained by limited availability and current scaffolds often fail to meet complex clinical needs. This study introduces a new approach to volumetric muscle loss treatment by using a shape-memory polymer (SMP) based on block copolymers of perfluoropolyether and polycaprolactone diol. This SMP mimics the biomechanical properties of natural muscle, exhibiting a low elastic modulus (2-6 MPa), high tensile strength (72.67 ± 3.19 MPa), exceptional toughness (742.02 ± 23.98 MJ m^-3) and superior biocompatibility, thereby enhancing skeletal muscle tissue integration and regeneration within 4 weeks. Moreover, the polymer's shape-memory behavior and ability to lift >5000 times its weight showcase significant potential in both severe muscle disorder treatment and prosthetic applications, surpassing existing scaffold technologies. This advancement marks a pivotal step in the development of artificial muscles for clinical use.

Keywords: scaffolds; shape-memory polymer; skeletal muscle; stimuli responsiveness; volumetric muscle loss.

Figure 1.

Multifunctional artificial muscles with tissue-like modulus achieved through biomimetic design. (A) Summary plot of the range of elastic moduli for PFPE₁–PCL₃ and representative biological tissues. (B) Schematic structure of the designed multifunctional artificial muscle. (C) Multifunctional artificial muscles can treat volumetric muscle loss, promote myogenic differentiation and angiogenesis (left) and mimic natural muscle function (right).

Figure 2.

General characterization and mechanical properties of PFPE_x–PCL_y. (A) XRD analysis of elastomers with varying molar ratios of PFPE–OH and PCL–OH. (B) Analysis conducted using a differential scanning calorimeter (DSC). (C) Transmission electron microscopy (TEM) images of (i) and (ii) PFPE₀–PCL₁ and (iii) and (iv) PFPE₁–PCL₃. (D) Stress–strain curves of PFPE_x–PCL_y and (E) corresponding stress, elastic modulus and toughness. (F) Cyclic stress–strain curves of PFPE₁–PCL₃ at various strain levels. (G) (i) and (ii) Photograph comparing the elastic modulus of PFPE₁–PCL₃ with that of artificial skin tissue and (iii) photograph showing 0.2 g of PFPE₁–PCL₃ elastomer lifting a 10-kg weight. (H) Comparison of the elastic modulus and tensile strength of PFPE₁–PCL₃ with those of shape-memory polymers reported in other literature. (I) Comparison of the elastic modulus and toughness of PFPE₁–PCL₃ with those of shape-memory polymers reported in other literature.

Figure 3.

Training reinforcement properties of PFPE₁–PCL₃ and mechanism study. (A–C) Cyclic stress–strain curves of PFPE₁–PCL₃ under repetitive mechanical-training process at different strains showing self-strengthening properties with the increased cycles. (D) Stress–strain curves of the PFPE₁–PCL₃ after mechanical training at different strains for 300 cycles. (E) Strengthened fiber (weight 5 mg, thickness 0.02 mm, width 0.6 mm) can lift a weight of 1 kg. (F) SEM pictures of PFPE₁–PCL₃ under different strains. (G) 2D SAXS scattering patters of PFPE₁–PCL₃ under stretching demonstrating strain-induced orientation with increasing strain.

Figure 4.

Shape-memory properties and application in artificial muscles. (A) Shape-memory program and recovery curves obtained by using DMA. (B) Stress–strain curves of the unnotched and notched samples. (C) Images displaying the puncture tests and the elastomer recovery to initial state upon heating. (D) Multiple program and actuation behaviors of PFPE₁–PCL₃ tested by using DMA. (E) Photographs of PFPE₁–PCL₃ at the initial state, stretched to 600% and contracted by heat stimuli. (F) Pre-stretched PFPE₁–PCL₃ artificial muscle (1.5 g) actuates a 350-g, 70-cm life-sized upper-limb model when heated. (G) Literature comparison of the actuation performance based on energy density and elastic modulus between different kinds of actuators. (H) Reversible actuation performance of the pre-stretched PFPE₁–PCL₃ at a fixed length under a repetitive heating–cooling process.

Figure 5.

Cell cytotoxicity, proliferation and differentiation of PFPE₁–PCL₃. (A) Live/dead staining of C2C12 cells cultured with PFPE₁–PCL₃ for 24 h (scale bar = 400 μm) and (B) statistical analysis of the ratio of live cells and dead cells. (C) Cell viability of C2C12 cells cultured with PFPE₁–PCL₃ for 1, 3 and 5 days detected by using CCK-8 assay. (D) MHC staining (red) and cytoskeleton staining by phalloidin (green) of C2C12 cells cultured on different samples for 7 days (scale bar = 150 μm). Nuclei were stained blue by using Hoechst. All data are presented as mean ± SD, n = 4. (E) Scheme of electromyography test was created using BioRender (biorender.com) and (F) statistical analysis of contractile force of the regenerative muscles at Week 1 and 4 of the normal, blank and PFPE₁–PCL₃ groups. *P < 0 .05; **P < 0.01, compared with the blank group.

Figure 6.

Promotion of angiogenesis and muscle repair 4 weeks after PFPE₁–PCL₃ implantation in vivo. (A) Representative images of HE staining of TA muscle treated in different groups at Week 4 post-injury (scale bar = 500 μm). (B) Representative images of immunofluorescence staining of TA remodeled muscle treated in different groups with MHC stained as red at Week 4 post-injury (scale bar = 250 μm). (C) Quantitative analysis of immunofluorescence staining of MHC. (D) Representative images of CD31 staining of TA muscle treated in different groups at Week 4 post-injury (scale bar = 250 μm). (E) Quantitative analysis of the expression of CD31. (F) Representative images of α-SMA staining of TA muscle treated in different groups at Week 4 post-injury (scale bar = 250 μm). (G) Quantitative analysis of the expression of α-SMA. All data are presented as mean ± SD. ns P > 0.05; *P < 0.05; **P < 0.01; ***P < 0.001. n = 4.