Soft, Flexible and Biocompatible Technologies for Optical Sensing and Stimulation in Biomedical Applications

Light-based therapies and optical sensing are rapidly gaining interest in the biomedical field. Currently, new therapeutic or diagnostic concepts are mainly relying on relatively large optical components made of standard rigid materials. For example in preclinical research, silica optical fibers are used for light delivery to and from brain tissue for both biosensing (fiber photometry, optoacoustic imaging,…) and therapy (optogenetics, optopharmacology,…). However, to bring these techniques to a clinical setting, the required technologies need to be made more biocompatible. Currently, chronic implantation of rigid large probes evokes gliotic scar formation, demyelination and neuronal loss near the probe. In addition, these biological responses have a negative influence on the efficacy and durability of triggered interventions. Therefore, small and soft probes are needed to limit gliotic scarring and cellular damage upon probe implantation. We address this need by developing miniaturized, soft wearable and implantable technology.<br/>We give an overview of our technologies for realizing soft waveguides allowing guiding light to stimulate deeper regions (e.g. in the brain), and technologies for integrating miniature sources and detectors on flexible foils allowing non-invasive stimulation or on-body sensing.<br/>To match mechanical properties of tissue, we investigate several (optically transparent) waveguide materials, ranging from ‘standard’ optical polymers (e.g. Ormocers®, epoxies), poly-dimethylsiloxane (PDMS), to ultra-soft hydrogels. Depending on the material of choice, and waveguide design (single mode or multimode), standard lithography, laser direct-write lithography, soft-lithography or moulding techniques are being used. We will report on planar multimode (stretchable) PDMS waveguides with low propagation loss for operation from near-UV (405nm), over visible, up to near-infrared wavelengths and planar single mode waveguides made of hybrid UV-curable polymers (OrmoClear®). Furthermore, the optically clear hydrogel materials that are under development (tuning mechanical, optical, biological properties) will allow even further improvements in softness of implantable waveguides.<br/>While softness and flexibility are desired for long term implantation, a higher rigidity is required to successfully implant the probes. We are tackling this challenge from different angles, i.e. from the material science side (e.g smart coatings) and the technological side (e.g implantation aids).<br/>For future wearable operation, implantable waveguides will need to be equipped with recording electrodes, driving electronics and light sources. Our focus is on combining such light sources in the least obtrusive way together with the flexible waveguides relying mainly on thin bare die (e.g. LEDs and VCSELs) integration on flexible foils. Similar in-house technologies can be applied to integrate the electrodes and driving circuitry. Alternatively, without waveguides, such patches can serve as standalone phototherapeutic minimally invasive flexible devices (e.g. for cortical stimulation or for skin monitoring).<br/>Although the presented soft technologies have a wide applicability, our current focus is on optogenetic or optopharmacologic therapies of the brain e.g. in the context of epilepsy.