4D-printed multifunctional hydrogels as flexible strain sensors and nerve conduits

Conductive hydrogels are critical for advanced bioelectronics and the repair of electroactive tissues. However, developing conductive hydrogels into complex biomimetic shapes with good flexibility and bioactivity poses a major biofabrication challenge. This study utilizes dual-component hydrogel inks based on alginate incorporating conductive fCNT (acid-functionalized carbon nanotube) nanofillers, with the composite gel exhibiting an electrical conductivity of 6.6 ± 0.5 mS cm^-1 at 2 mg ml^-1 fCNT loading. Owing to their good combination of electrical conductivity and mechanical properties, the (three-dimensional) 3D-printed gels were successfully applied as strain sensors to sense subtle human motions, such as finger and elbow bending. Bilayered hydrogels prepared through four-dimensional (4D) printing exhibited programmable shape changes owing to differential swelling post-printing to yield nerve guidance conduits (NGCs) of intricate and tissue-adaptable designs, such as single and multichannel and bifurcated designs, based on accurate prediction by finite element analysis. The proliferation of neural cells was enhanced on the fCNT-gel compared to the neat gel. Sutureless deployment and enhanced peripheral nerve regeneration were established for the fCNT-gel in a rat sciatic nerve injury model. Overall, this work presents the fabrication of 4D-printed multifunctional conductive hydrogels, which can find diverse applications ranging from implantable nerve conduits to strain sensing.