Recent advances in the processing and understanding of neural data has led to a pressure for improvements in materials that are able to chronically interface with soft neural tissue. Traditional electrodes including those patterned on silicon or polyimide, are stiff enough for insertion. However, these materials are grossly mismatched in terms of modulus when compared to the tissues they stimulate and record from. Shape memory polymers (SMPs) have proven a more compatible substrate, leading to neural prosthetics that can both insert reliably while softening considerably after some time in vivo. A challenge with these systems however is the chronic lifespan of these devices, due to the hydrolytic instability of previously used constituent monomers. Now using a rigid aliphatic dithiol, a chronically stable substrate that exhibits the shape memory effect is described for use as an ultrasoft long-term or permanent platform for neural electrodes. A trifunctional thiol, tris[2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TMICN), is substituted for an equivalently rigid dithiol, tricyclodecane dithiol (TCDDT), and added to a trifunctional alkene, 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO). 2,2-Dimethoxy-2-phenylacetophenone (DMPA) was used as a photoinitiator to catalyze the thiol-ene ‘click’ reaction between the comonomers.
Using this system, an order of magnitude drop in rubbery modulus is shown from the 10MPa to 1MPa regime, allowing for substrates which more closely resemble the stiffness of the penetrated tissue. In addition to the modulus drop, the aliphatic nature of the dithiol allows for a decreased swelling of the overall network when tested in physiologic conditions (1x PBS, 37°C). Most importantly, due to the removal of the ester linkages in the main chain of the polymer, no hydrolytic degradation was observed in the network under accelerated aging conditions, leading to chronically stable electrode substrates. These results show the ultra-softening nature of the polymer composition TCDDT TATATO and the possibility of using this network as a substrate which more closely matches the stiffness of the physiology with which it interacts. Overall, these polymers show promise as a reliable substrate for neural interfaces which can maximize insertion success while minimizing scar-formation in vivo long-term.