Hydrogel-Elastomer System as pH-Responsible Soft Actuator

Soft actuators are stimulus-responsive materials that change shape in response to external stimuli such as pH, temperature, and light.¹ Hydrogels based on chitosan methacrylate (ChiMa) are pH-responsive biomaterials that present biocompatibility, high water content, and permeability to chemical and biological molecules.² However, these hydrogels often exhibit poor mechanical properties, and fast biodegradability, which limit their use as soft actuators.² On the other hand, elastomers based on methacrylated poly(glycerol sebacate) (MPGSe) and Pluronic F127 dimethacrylate (F127DMa) were mechanically robust materials and stable in various environments, although they are inert to pH changes.^3,4 Therefore, hydrogels and elastomers present complementary properties that, when combined in a hydrogel-elastomer system (HES), become suitable for the development of pH-responsive soft actuators.^1,5 Herein, we present the preparation of a hydrogel-elastomer system via photopolymerization of an oil-in-water emulsion (30:70volume ratio) composed of a chitosan methacrylate aqueous phase (2 w/v%, total), a methacrylated poly(glycerol sebacate) solution in ethyl acetate (6 w/v%, total) as the organic phase, and Pluronic F127 dimethacrylate (1 w/v%, total) as a crosslinkable surfactant. ChiMa (degree of methacrylation = 20%) hydrogel exhibited a swelling degree in water up to 7000%, a compressive modulus around 30 kPa, and strain at break of 15%. MPGSe elastomer (degree of methacrylation = 6, 13, and 23%) showed a swelling degree in water up to 100%, a compressive modulus around 100 kPa, and a strain at break above 99%. The HES successfully integrates the desirable features of both materials, and its properties can be tuned by varying the concentrations of ChiMa and MPGSe, as well as the degree of methacrylation of MPGSe. The HES exhibits pH-responsive and reversible swelling behavior and mechanical properties. For instance, chitosan is deprotonated at pH = 7.0 and the HES exhibited swelling degree, compressive modulus, and strain at break in the range from 700 - 1500%, 10 - 80 kPa, and 60 - 81% respectively; at pH = 4.5, chitosan is protonated, and the swelling degree, compressive modulus, and strain at break range from 1000 - 2700%, 25 - 140 kPa, 40 - 60%, respectively. Moreover, increasing the pH from 4.5 to 7.0 resulted in a reversible decrease of the diameters of the HES cylindrical specimen up to 33%. These findings demonstrate the potential of the HES to combine the advantages of hydrogels and elastomers in a single material for the development of pH-responsive soft actuators.

References: [1] https://doi.org/10.1021/acs.chemrev.2c00418; [2] https://doi.org/10.1039/D1MA00765C; [3] https://doi.org/10.1021/bm070423u; [4] https://doi.org/10.1557/s43579-024-00618-4; [5] https://doi.org/10.3390/polym14091822