Optical neuromodulation techniques, like optogenetics and photopharmacology, as a treatment for neurological disorders have two major advantages in comparison to other neuromodulation techniques: a very high spatiotemporal resolution and cell-type specificity. A major hurdle for the practical use of optical neuromodulation techniques is the mechanical mismatch between soft neural tissue and stiff optical probes, leading to a severe Foreign Body Reaction, which decreases the probe efficacy over time. Hydrogel materials are ideal candidates for soft optical probe materials, as they display stiff characteristics in dehydrated state, which is desirable for the implantation process, and soft characteristics in hydrated state, to limit the Foreign Body Reaction. In literature, only existing hydrogel materials with suboptimal properties for this application have been used. Furthermore, in literature only one fabrication method for hydrogel optical probes has been described, with a severe drawback: it limits the minimal obtainable probe diameter to 300 µm. In this proposed project, new hydrogel materials are developed with tailored mechanical, optical and biological properties for optical neuromodulation uses. Afterwards, two novel fabrication methods, Two-Photon Polymerization and Self-Written Waveguides, will be explored to design soft optical probes. The effect of the low stiffness of the probes on the Foreign Body Reaction will be investigated during in vivo experiments.