All experiments were performed on adult male mice (8 to 14 weeks of age) and were randomly assigned to control or experimental groups and device implantation. Sample sizes were determined from preliminary experiments using the software G*Power for the appropriate type of statistical comparison. For place preference and thermal sensitivity assays, data were excluded from animals where the device was obviously damaged. All data were collected in a nonbiased manner, and the experimenter was blinded to treatment for histological analysis and to animal genotype for the place preference assay. One-way and two-way analyses of variance (ANOVAs) were performed as appropriate. P values <0.05 were considered statistically significant. All graphs are represented as means ± SEM. Further statistical details can be found in tables S3 and S4.