Prosthetic Arm

In January 2013, a 5-year-old boy who was born without fingers on one hand was given a customizable 3-D printed prosthetic hand, built for only $150 in parts. 3-D printing, by which a 3-dimensional object is created by successive layering of horizontal cross-sections of the object as dictated by a computer file, is an inexpensive method to produce such customized items. The designers, Richard Van As and Ivan Owens, work together on Robohand, a prosthetics project whose aim is to provide low-cost accessible prosthesis designs for finger amputees world-wide [1]. Currently, they have the most developed prosthetic platform in the world that is open source, meaning the program information is freely available to the public. Advancements like this are needed to provide a cost-effective means to meet the rising demand for artificial limbs – there were nearly 1.6 million people in the US who had lost their limbs in 2005, and an estimated 185,000 new amputations occur annually in the US [2]. Innovators like Van As and Owens are working to meet the demand for better and cheaper prosthetics, but how exactly do these advanced artificial limbs work?

History of Prosthetics

The use of prosthetics dates back to the early Egyptians between 950 and 710 B.C., according to the discovery in 2000 of the world’s first prosthetic toe, composed of wood and leather, from the mummified remains of a noblewoman [3]. In 1871, James Edward Hanger, the first amputee of the Civil War, patented his prosthetic leg, constructed from barrel staves and metal (for figure from patent, refer to: His prosthetic company continues to thrive to this day [4]. The original aim of prosthetics was simply to replace the missing limbs in order to allow amputees to lead active lives. However, as both surgical techniques and technological innovations improved over time, prosthetics advanced to allow patients to control their artificial limbs and move them like they would a regular arm or leg.

Targeted Muscle Reinnervation (TMR)

The goal of developing prosthetics advanced enough to mimic the limbs they replace is a realistic one for two key reasons. First, after amputation, the brain continues to send signals to the amputated muscle even though the muscle is no longer present to receive them. Second, amputation does not remove all the nerves from the limb; as such, there are working nerves present at the stub of the amputated limb. The idea behind a bionic limb is that the working nerves at the stub of the amputated limb can be redirected to a working muscle. Signals sent from the brain regarding movement of the amputated muscle will then trigger movement of the new muscle group [5].

Currently, the most innovative technique for rerouting the nerves is accomplished with a procedure known as “targeted muscle reinnervation,” developed by Dr. Todd Kuiken of the Rehabilitation Institute of Chicago. The surgical procedure for a prosthetic arm consists of dissecting the shoulder to access the nerve endings, and then redirecting the four major brachial nerves to distinct regions of the chest muscles. After several months, the nerves from the shoulder will become fully innervated into the chest muscles. Afterwards, electrodes, which can conduct electric current, are placed on the surface of the chest muscles – each controlling a motor that moves joints in the prosthetic arm. When the brain sends a signal to the nerve, the chest muscle that it is connected to contracts and the electrode on that muscle detects the contraction, resulting in movement of the prosthetic arm. The range of movement observed with use of a prosthetic is due to the integration of each nerve ending from the shoulder to different regions of the chest muscle. [5,6]

Prosthetic Arm
Figure 1: Targeted muscle reinnervation in a patient with an amputated shoulder. Schematic diagram that illustrates the reinnervation of the major brachial nerves – musculocutaneous, median, radial (colored green) – into distinct regions of the pectoralis major muscles of the chest (colored red). Upon reinnervation, electrodes (colored blue) are used to relay information detected in the chest muscles to the microprocessor in the prosthetic arm, which then translates the electrical impulses into movement. Image modified from:

Future of Prosthetics – Bionic Arm Technology

Current research aims to allow the prosthetic limbs to read commands directly from the brain. The ultimate goal would be a bionic limb that responds accurately and precisely to transmitted sensory information from the patient. The barriers to achieving this goal are our limited understanding of how a thought becomes translated into a movement at the neural network level, and the technological complexity required to engineer a prosthetic that is able to appropriately interpret such brain signals [2].

Despite these obstacles, researchers have made great strides toward development of bionic limbs. To provide patients greater dexterity of their prostheses, researchers are currently assessing whether severely paralyzed patients can control motor function of prosthetic limbs by neural implantation. In 2004 at Brown University, a quadriplegic human patient was able control a computer cursor to watch television, check e-mail, and play video games [7]. Taking this concept further, in December 2012 at the University of Pittsburgh Medical Center, Jan Scheuermann, a 52-year old quadriplegic for 9 years, was able to control movement of a robotic arm through brain-computer interface technology. To prepare for this feat, in February of that year two computer chips had been implanted, using image-guided technology, in her motor cortex – the region of the brain that controls movement. Each chip comprises 96 microelectrodes that are stationed near a group of neurons known to control motion of the right arm and hand. Based on the firing patterns associated with particular tasks, computer algorithms are used to translate the neuronal signals detected by the microelectrodes into movement of the robotic prosthesis. Consequently, the desire to move is translated into actual movement – the patient is able now able to shake hands with others and feed herself a chocolate bar. One of the major breakthroughs that made this achievement possible is improvement in the translation of brain signals into computer signals that can accurately control a robotic prosthesis [8, 9]. For quadriplegics and amputees, such advancements demonstrate the promise of being able to perform daily functions independently.

The current state of research in limb prosthetics has come a significant way since the days of James Hanger. However, before such technologically-advanced prosthetics become commonplace for quadriplegics and amputees, the bionic technology used in Jan Scheuermann’s case must be tested in more patients. These additional cases will help to tease out nuances of the brain-computer interface technology, such as whether the electrical signals detected from the individual microelectrodes of the implanted computer chip originates from one or several neurons, and will thereby help to ensure the success of these advanced prosthetics on a larger scale.

Jessica W. Chen is a PhD student in the Biological and Biomedical Sciences Program at Harvard University.

Supplemental Videos

”Breakthrough: Robotic Limbs Moved by the Mind.” CBS News’ 60 Minutes.

“TedMed Talk: Climber Hugh Herr’s Robotic Legs Help Him Scale New Heights.”

“Ted Talk: Dean Kamen Previews a New Prosthetic Arm.”


[1] Richard Van As and Ivan Owen. “Coming Up Short Handed (the Robohand Blog).”

[2] Michael Chorost, 2012. “A True Bionic Limb Remains Far out of Reach.” WIRED.

[3] Isaac Perry Clements, 2012. “How Prosthetic Limbs Work.”

[4] “Hanger Prosthetics and Orthotics, Inc.”

[5] Julia Layton, 2011. “How Can Someone Control a Machine with Her Thoughts?”

[6] Erik Scheme and Kevin Englehart, 2011. Electromyogram Pattern Recognition for Control of Powered Upper-Limb Prostheses: State of the Art and Challenges for Clinical Use. Journal of Rehabilitation Research and Development 48(6): 643-660.

[7] 2004. “Pilot Study of Mind-to-Movement Device Shows Early Promise.”

[8] David Templeton, 2012. “Pitt Team Inserts Computer Chip in Brain so a Person’s Thoughts can Instigate Motion.”

[9] Jennifer L Collinger, Brian Wodlinger, John E Downey, Wei Wang, Elizabeth C Tyler-Kabara, Douglas J Weber, Angus JC McMorland, Meel Velliste, Michael L Boninger, Andrew B Schwartz, 2012. High-Performance Neuroprosthetic Control by an Individual with Tetraplegia. The Lancet 48)6: 643-660.

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