Several invasive approaches to creating sensory feedback from upper limb prostheses have been tested during the past 50 years, involving various types of neural interface. The quality of studies has varied — studies with more participants and longer durations have provided the most important contributions.

One approach is the use of longitudinal intrafascicular electrodes, which are flexible wires that are used to stimulate individual fascicles within a nerve. One of the most notable studies of this technology, published about 20 years ago, involved six people with upper limb amputations at the elbow or below4. In this study, the interface enabled force and position feedback to be provided by modulating the frequency and amplitude of the stimulation current applied to the nerves.

An alternative approach is the use of flat interface electrodes that wrap around the nerve and flatten it to increase the proximity to the fascicles. In a study of this approach published around 10 years ago, an interface consisting of flat interface electrodes was implanted in the median and ulnar nerves of two people with upper limb amputations for a period of more than 1 year5. Electrical stimulation via this interface improved the participants’ ability to control the grasping strength of the prosthesis and enabled them to better manipulate delicate objects, and this benefit remained stable during the course of the study. Other studies have demonstrated the use of similar technology, but the duration of this study makes it particularly notable.

Multielectrode arrays can also be used to stimulate nerves. In a study published in 2016, a 96-channel electrode array was implanted for 30 days in the median or ulnar nerve of two people with upper limb amputation, and stimulation evoked more than 80 sensory percepts at different locations6. Modulation of stimulation parameters varied the quality of these percepts (for example, tingling, pressure or vibration).

An alternative approach is a technique called targeted reinnervation, in which the remaining arm nerves are transferred to residual chest areas after amputation. This procedure was originally intended to aid motor control but leads to touch on the chest area being perceived as touch on the missing limb7, meaning this area can be used as the target of sensory stimulation.