Methods, systems, and compositions are provided for implanting an implantable device into a biological tissue (e.g., muscle, brain). A subject implantable device includes: (i) a biocompatible substrate, (ii) a conduit (e.g., an electrode, a waveguide) that is disposed on the biocompatible substrate, and (iii) an engagement feature (e.g., a loop) for reversible engagement with an insertion needle. The biocompatible substrate can be flexible (e.g., can include polyimide). The implantable device is implanted using an insertion needle that includes an engagement feature corresponding to the engagement feature of the implantable device. To implant, an implantable device is reversibly engaged with an insertion needle, the device-loaded insertion needle is inserted into a biological tissue (e.g., to a desired depth), and the insertion needle is retracted, thereby disengaging the implantable device from the insertion needle and allowing the implantable device to remain implanted in the biological tissue.