Highly flexible and resilient elastin hybrid cryogels with shape memory, injectability, conductivity and magnetic responsive properties

Yuqing Liu (University of Manitoba)
Malcolm Xing (University of Manitoba)

Introduction

Hydrogels have wide applications in tissue engineering and bioelectronics. In spite of widespread applications of functional hydrogels with superior conductive or magnetic responsive performances, there are still some limitations caused by insufficient flexibility, resilience and responsive sensitivity in some fields, such as highly flexible electronics, organic pressure sensitive sensors, and responsive actuators. In this paper, an ultra-flexible elastin-peptide based hybrid (elastin-gelatin-CNT, EGC) cryogel was fabricated as scaffold to load large amount of rigid functional components, including carbon nanotube (CNT), polypyrrole (PPY) and/or iron oxide magnetic nanoparticles (IONP), for combining excellent conductivity or magnetic responsive property with high elasticity, flexibility, shape memory property and injectable property together. EGC loaded with dispersed PPY aggregates showed highly flexible and injectable property with a moderate conductivity. However, when PPY formed a second rigid network on this soft scaffold, the bicontinuous network exhibited an extraordinarily high resilience enduring an extraordinary compressive strain and an outstanding conductivity as well as excellent bulk pressure sensitive conductivity.

Materials and Methods

High elastic scaffold was prepared by cryogelation of methacrylated elastin peptide (elastin-MA, hydrolyzed from bovine neck ligament and then methacrylated), methacrylate gelatin (gelatin-MA) and multi-wall CNT. Elastin-MA and gelatin-MA were prepared by conjugation of the primary amine groups from elastin or gelatin backbones with methacrylic anhydride (MA) in ice bath or 40°C respectively. CNT was dispersed by F-127-DA (PEO-PPO-PEO) triblock amphiphilic copolymer in water, which could covalently bond with gelatin-MA and elastin-MA to form hydrogel. PPY coating on scaffold were through (1) fast crosslinking deposition (EGC-PPY-FD) or (2) fast crosslinking and then slow aging (EGC-PPY-SA). In EGC-PPY-FD process, pyrrole monomers absorbed in the EGC hydrogels was initiated, aggregated, precipitated out and rapidly absorbed onto EGC scaffold in-situ to form dispersed PPY aggregates. In EGC-PPY-SA process, pyrrole was initiated by deficient amount of Fe(NO3)3, which left large amount of unpolymerized monomers for slow oxidation polymerization by (NO3)3 to form continuous network structures on the scaffold and the slow process will allow more intermolecular hydrogen bond interactions between PPY and gelatin/elastin substrate.

Results

The SEM morphology of EGC hydrogel looks similar to honeycomb-like macroporous structure with an interconnectivity of higher than 99%, which could allow deformation and water drainage rapidly, and CNT were embedded in the substrate layers like a mesh network, as shown in Figure 1 (a) and (d). Figure 1(b)/(e) or Figure 1(c)/(f) showed respectively dispersed distribution of PPY nanoparticle aggregates or well-developed PPY network deposited on EGC scaffold. With coating of dispersed PPY aggregates or continuous PPY network, EGC(30)-PPY-FD and EGC(30)-PPY-SA showed compression strength of 26.7 and 58.4kPa at 80% strain respectively without any fracture, as shown in Figure 2(a). From Figure 2(b), the addition and fraction of elastin component played a critical role on the flexibility of EGC scaffold. EGC(30)-PPY-SA cryogel endured a 80% deformation at the rate of 20mm/min and 200mm/min for 100 cycles respectively. After 100 cycles, the recovery loss is 8.8% for 20mm/min rate and 13.8% for 200mm/min rate, and the hydrogel still kept good shape and elasticity, provided in Figure 2(c) and 2(d). When EGC(30)-PPY-SA cryogel endured a ultra-high compressive strain of 97.5% at the rate of 2mm/min for 5 cycles, it could afford 97.5% compressive strain with a 6.35MPa stress and cyrogel scaffold kept well with only limited partial fracture of rigid PPY layer, according to Figure 2(e) and 2(f). Figure 3(a) -3(d) shows the LED changed its light intensity with hydrogel deformation, suggesting pressure sensitive conductivity of EGC-PPY-SA, which could be used as potential biosensors. Due to the well-formed network of PPY on EGC scaffold, the EGC-PPY-SA showed an excellent conductivity, which was dramatically higher than EGC-PPY-FD hydrogel loaded with dispersed PPY nanoparticles (Figure 3(e)). In pressure dependent cycling test, the hydrogel relative resistance was proportional to hydrogel strain and close to 25% of initial value of initial value at 50% strain, as shown in Figure 3(f). According to compressive stress- strain curve of EGC(30)-PPY-SA, the pressure sensitivity (DR/R0 per kPa) is around 0.086 per kPa. The conductivity vs. strain profile of EGC(30)-PPY-SA did not increased linearly, and the highest conductivity of 50.1±2.9 S/cm at 90% strain was obtained for the best sample (Figure 3g).

Discussion and Conclusion

So it can be concluded, water soluble elastin peptide based cryogel macroporous scaffold loading rigid conductive PPY or IONP could combine high elasticity, injectable capability and shape memory property with conductivity or magnetic property together. All PPY coated EGC hydrogels exhibited excellent elasticity, flexibility, shape memory behavior and stress dependent conductivity. In particular, when EGC scaffold loaded with dispersed PPY, the resulted conductive hydrogel with a fixed shape showed excellent flexibility and injectable property, suggesting its potential application as a syringe-injectable biosensor or bioelectronics; with formation of EGC-PPY soft-hard bicontinuous network, the product exhibited high elastic modulus and outstanding resilience with fast recovery with an excellent conductivity of 50.1 ± 2.9 S/cm at 90% strain, and even can afford 97.5% compressive deformation, indicating a good candidate for the cardinal tissue engineering materials or pressure sensitive biosensors.


Figure 1. SEM images of EGC-PPY cryogels.


Figure 2. Mechanical properties of EGC cryogels.


Figure 4. Pressure sensitive conductivity of EGC cryogels.

Acknowledgements

M.X. would like to thank NSERC Discovery Grant for the research support.

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