Development of a peripheral nerve cuff for optogenetic stimulation and detection of host encapsulation in rats, chronically

Abstract

Optogenetics is a recent technology which allows neural modulation using light. Optogenetics requires both expression of a light sensitive protein called an opsin and sufficient light to activate it. Tissue encapsulation is a known response of the body to foreign objects, including implantable medical devices. Tissue encapsulation has been widely observed in electrical stimulation systems where it degrades stimulation and recording performance. Optogenetic stimulators are also encapsulated by the body and a reduction in light power is expected due to the strong attenuation characteristics of the encapsulation tissue. This light power reduction could be sufficient to reduce opsin activation and might explain the reduction or loss of optogenetic response reported in some studies. However, as there are currently no studies quantifying the attenuation from encapsulation and the relative contribution to reduction of optogenetic effect are uncertain. Consequently, a peripheral nerve cuff for optogenetic stimulation incorporating a light power detector was developed to evaluate the impact of tissue encapsulation on optogenetic stimulation. This cuff was designed to be placed on the superior cervical ganglion, a promising target for understanding and treating high blood pressure. A concept was developed of a cuff for a rat superior cervical ganglion (SCG) which could deliver sufficient light to activate opsins, and which could also quantify tissue encapsulation based on light transmission. First, the optical properties of fibrous scar tissue and the SCG were determined from tissue samples, as these were not available. A design process was created to predict the measurement sensitivity in terms of tissue thickness resolvable by the system. A Monte-Carlo ray-tracing model was implemented in TracePro to enable simulation of the light transmission though the SCG and scar tissue. Transmission through the SCG was evaluated in the model to determine the sensitivity required to detect tissue encapsulation under varying SCG geometry. This allowed system specifications to be determined such that a cuff and recording electronics could be designed based around a photodiode. It also allowed a suitable LED source for optogenetic stimulation to be chosen which could illuminate the SCG. The photodiode and LED were encapsulated in glass tubes held in place by a titanium nerve saddle and enclosed by a silicone cuff. Leads were attached and routed to a transcutaneous port to allow in vivo measurements in later studies. A signal conditioning system was designed to convert the photo current into a voltage suitable for digitisation. The large range of geometry and tissue properties possible in vivo required the measurement system to handle a range of input photocurrent and demonstrated at least 45 dB signal to noise ratio across the input photocurrent range of 0.752 μA – 17.1 μA. Detection of thin host encapsulation was confirmed through in vitro measurement of a 50 μm change in tissue thickness; the resulting 60.9% change in the full-scale output was easily detectable. Comparison of benchtop measurements to the Monte Carlo model indicate ~10% agreement when considering the initial transmittance. The nerve cuff’s measurement system is evaluated in a saline bath and demonstrated 50 stable measurements over 25 hrs. The lifetime of the nerve cuff was demonstrated to be over 2 weeks. Water ingress was identified as a potential problem and a measurement approach, based on output power variation is suggested to increase robustness to water ingress. A pilot study of the peripheral nerve cuff was performed in a small number of animals (n=2) allowing the change due to the attenuation of host encapsulation to be estimated as a ~13% (0.05 mm³) reduction in illuminated volume. While a study in a larger number of animals is required to confirm the results, the preliminary study confirms the feasibility of performing tissue encapsulation measurement in vivo. The cuff is also capable of stimulation with the model estimating 78% ii (0.39 mm³) of the SCG volume was exposed to greater than 1mW/mm².This work presents progress in quantification of the host encapsulation attenuation which is an important step toward precisely controlled light delivery for optogenetic investigation of peripheral neural circuits in chronic animal studies.