The term “bionic” evokes visions of humans enhanced to superhuman levels through science fiction. Advances in engineering, such as improved motors and batteries, along with modern computing, have removed the mechanical and electronic barriers to advanced prostheses. However, integrating these powerful machines with the human body has been a challenge. This is now changing. A recent trial tested a new integration technique involving the surgical reconstruction of muscle pairs to give recipients a sense of the position and movement of a bionic limb. This system allows the prosthesis to be fully controlled by the user’s brain, enabling individuals with below-knee amputations to walk more naturally and navigate slopes, stairs, and obstacles better, as reported in the July issue of Nature Medicine.
Traditionally, engineers have viewed biology as a fixed limitation to be engineered around. However, if we consider the body as part of the system to be engineered alongside the machine, the interaction between the two can improve significantly. This perspective is driving a wave of techniques that reengineer the body to better integrate with the machine. These techniques, termed “anatomics,” aim to exploit bones for stable anchors, reroute nerves to create control signals for robotic limbs, and co-opt muscles as biological amplifiers. These methods enhance the connection and communication between a robotic limb and the human nervous system, pushing the capabilities of bionic prostheses.
While anatomics-based devices have been slow to transition from labs to commercial and clinical use, the field is moving closer to the sci-fi vision of seamlessly integrated, brain-controlled bionic limbs. Proprioception, the body’s awareness of itself in space, is crucial for movement, especially walking. Traditional amputations discard this important feedback, but the agonist-antagonist myoneural interface (AMI) technique reconstructs these muscle pairs to control prosthetic joints, allowing recipients to “feel” their prosthetic limb.
The recent study, part of a clinical trial, found that recipients of the AMI-based system increased their walking speed by about 40 percent, comparable to that of people without amputation. Common complaints from prosthetic users include pain and discomfort, often due to the attachment point. Techniques like osseointegration, which uses titanium bolts to anchor the prosthesis, offer greater strength, stability, and comfort, though they carry infection risks. Bionicists have also sought to tap into the body’s nerves for prostheses that communicate with the brain, but early efforts were hampered by weak signals.
Modern bionic prostheses primarily communicate with muscles, which emit larger electrical signals. Targeted muscle reinnervation (TMR) reroutes severed nerves to freshly cleared muscles, creating sources of control signals. However, TMR limits the number of signals that can be created. A new technique, the regenerative peripheral nerve interface (RPNI), surgically inserts small muscle grafts and reroutes nerves to these, allowing researchers to create as many signals as needed. Combining RPNIs with osseointegration can provide higher quality signals through implanted electrodes.
Researchers are also exploring wireless systems and new attachment methods that avoid permanent holes. At MIT, Hugh Herr is developing a technique called magnetomicrometry, which uses magnetic spheres inside muscles to control bionic prostheses. Herr, who lost both his legs in a mountain climbing accident, envisions a future where these techniques will lead to the Hollywood version of brain-controlled robotic limbs, restoring proprioception and making recipients feel as though the prosthetic is part of their own body.
