Soft Three-dimensional self-opening intraneural peripheral interface for optic nerve stimulation

Retinitis Pigmentosa and age-related macular degeneration are two of the principal causes of blindness in industrialized countries, for which there is still no established prevention, treatment or cure. In the past decade, retinal prostheses emerged as promising technology to restore vision1. On the contrary, optic nerve stimulation has been proposed as an attractive alternative to retinal prostheses, by acting directly on the axons of the ganglion cells2. Our research group is currently testing a self-opening intraneural electrode array for optic nerve stimulation (OpticSELINE)2. The implant consists of a flexible polyimide-based structure embedding gold interconnects and active sites. Although polyimide provides flexibility to the device, the use of a softer and stretchy material would greatly help in the integration of the array inside the nerve tissue. A commonly used material to fabricate soft neural interfaces is polydimethylsiloxane (PDMS); however, it presents challenging steps in the microfabrication process, especially for the encapsulation of the active sites. On the other hand, off-Stochiometric-Thiol-Ene Epoxy polymers (OSTE+)3 represent a promising replacement for the polyimide structure thanks to their tunable mechanical properties. In fact, by adjusting the stoichiometry of the constituting monomers, the Young’s modulus of OSTE+ can be decreased down to a few MPa at physiological temperature4. The preparation of OSTE+ consists of two curing steps3 in which (1) thiol-ene photopolymerization allows to pattern the polymer surface and remove the non- polymerized areas before (2) thermal curing. Thus, it is possible to encapsulate the electrode array simply using photolithographic techniques. Furthermore, the off-stoichiometric formulation introduces an excess of reactive thiol groups in the bulk and on the surface3. In this way, OSTE+ can covalently bind to other surfaces without the use of bonding strategies such as plasma activation. Therefore, the versatility of OSTE+ makes it a promising alternative to PDMS. Taking advantage of these properties, we used direct writing photolithography to fabricate implantable multielectrode arrays in OSTE+ (Figure 1). The functional characteristics of the array have been tested by electrochemical characterization. In conclusion, the introduction of OSTE+ in the OpticSELINE aims at minimizing the mechanical mismatch between the electrode array and the optic nerve. This technology has the potential to allow stable and selective stimulation (or recording) of the optic nerve during chronic implantations. The advancement will improve the use of the OpticSELINE as visual prosthesis for blind patients and as tool to further investigate the effect of the electrical stimulation in the visual system.