In vivo optogenetic stimulation using Parylene photonic waveguides for light delivery

Flexible polymer neural probes enable minimally invasive interfacing with biological tissue. The smaller mechanical mismatch between soft polymer materials and the tissue reduces inflammation response and scarring in the tissue during chronic implantation of flexible neural probes compared to those made of rigid substrates, including Silicon, Silicon Dioxide, and Silicon Nitride. We have previously demonstrated a fully flexible Parylene photonic waveguide array platform for high-resolution targeted light delivery in tissue. Parylene photonics is a novel integrated photonic platform composed of flexible, biocompatible materials with a large refractive index contrast. The core of the photonic layer is Parylene C (n = 1.639) and the cladding is PDMS (n = 1.4), both two orders of magnitude more flexible than traditional Silicon substrates. Here, we perform optogenetic stimulation experiments using Parylene photonic waveguide arrays to deliver light to the brain in a transgenic mouse line expressing ReaChr; a red-shifted opsin. In this paper, we discuss, for the first time, the application of Parylene photonic waveguides for in vivo optogenetic stimulation of neurons in rodent models, evidenced by increased neural firing following light delivery. Spike sorting was performed to isolate neural units in the vicinity of the recording electrodes, demonstrating selective neural stimulation. Parylene photonic waveguide arrays were packaged with commercially-available single mode optical fibers and laser light sources operating at lambda=633 nm. Implantation of the flexible waveguide arrays was achieved via attachment to a rigid shuttle using bioresorbable polyethylene glycol (PEG) coating. Post implantation, Nissl staining was used to characterize neuronal damage following insertion. Neuroinflammation was also assessed using immunofluorescence.