The present disclosure discussed a handpiece for trans-canal delivery of a therapeutic substance to the inner ear. The handpiece can be inserted into the middle ear via a surgical tympanotomy approach. The handpiece can enable a controlled injection of a therapeutic substance directly through the round window membrane and into the inner ear. The direct delivery of the therapeutic substance to the inner ear can enable the delivery of a precise amount of therapeutic substance into the inner ear. Because the therapeutic substance is delivery directly to the inner ear the delivery of the therapeutic substance is not subject to limitations on molecule size and inconsistent diffusion rates that are present when therapeutic substances are diffused across the round window membrane.