Electrically Conductive Injectable Silk/<scp>PEDOT</scp>: <scp>PSS</scp> Hydrogel for Enhanced Neural Network Formation

With no effective treatments for functional recovery after injury, spinal cord injury (SCI) remains one of the unresolved healthcare challenges. Human induced pluripotent stem cell (hiPSC) transplantation is a versatile patient-specific regenerative approach for functional recovery after SCI. Injectable electroconductive hydrogel (ECH) can further enhance the cell transplantation efficacy through a minimally invasive manner as well as recapitulate the native bioelectrical microenvironment of neural tissue. Given these considerations, we report a novel ECH prepared through self-assembly facilitated in situ gelation of natural silk fibroin (SF) derived from mulberry Bombyx mori silk and electrically conductive PEDOT:PSS. PEDOT:PSS was pre-stabilized to prevent the potential delamination of its hydrophilic PSS chain under aqueous environment using 3% (v/v) (3-glycidyloxypropyl)trimethoxysilane (GoPS) and 3% (w/v) poly(ethylene glycol)diglycidyl ether (PeGDE). The resultant ECH formulations are easily injectable with standard hand force with flow point below 100 Pa and good shear-thinning properties. The ECH formulations with unmodified and GoPS-modified PEDOT:PSS, that is, SF/PEDOT and SF/PEDOTGoP maintain comparable elastic modulus to spinal cord (~10-60 kPa) under physiological condition, indicating their flexibility. The GoPS-modified ECHs also display improved structural recoverability (~70%-90%) as compared to the unmodified versions of the ECHs (~30%-80%), as indicated by the three interval time thixotropy (3ITT) test. Additionally, these ECHs possess electrical conductivity in the range of ~0.2-1.2 S/m comparable to spinal cord (1-10 S/m), indicating their ability to mimic native bioelectrical environment. Approximately 80% or more cell survival was observed when hiPSC-derived cortical neurons and astrocytes were encapsulated within these ECHs. These ECHs support the maturation of cortical neurons when embedded for 7 days, fostering the development of a complex, interconnected network of long axonal processes and promoting synaptogenesis. These results underline the potential of silk ECHs in cell transplantation therapy for spinal cord regeneration.