By Erin Mackey (Pitzer College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only].
Our sense of touch is one of the most important senses. Without it, we wouldn’t be able to grasp objects with the correct amount of force, or discern subtle differences in the properties of objects, such as texture or hardness. It might be hard to imagine how you could live your life without a sense of touch in your hands or legs, but for many people, it is an unfortunate reality. Today, there are about 2 million amputees in the United States. Currently, one out of 190 Americans lives with the loss of a limb. By 2050, the projected number will almost double to 3.6 million people (Ziegler-Graham et al., 2008). Limb loss is mostly caused by diabetes mellitus, dysvascular disease, trauma, and the malignancy of the bone and joint (Ziegler-Graham et al., 2008). It has also been shown that without prosthetic care, individuals begin to live more sedentary lifestyles, which are known to lead to secondary complications such as diabetes and otherwise preventable hospital visits (Dobson et al., 2013).
What if their sense of touch could be restored in the limbs of amputees? What if prosthetic limbs were not hard and plastic, but flexible and could feel things just as a real hand could? This large prevalence of limb loss has had many research groups working to create sophisticated and technologically advanced prosthetic limbs to give amputees back the sense of touch that they have lost.
The most interesting research currently going on in the field of prosthetic limbs is in designing a prosthetic that contains a sensory feedback system to deliver real-time sensory information to the patient’s brain. An ideal prosthetic would be one that could receive signals from the muscles to order to move and send signals through the patient’s nerves to relay sensory information to the brain about the object that it is interacting with. A sensory-equipped prosthesis could give back the normalcy of having a sense of touch to many amputees.
Previous studies have attempted to make sensory prostheses. A study done at the University of Utah in 2005 was one of the first to demonstrate that a prosthetic is able to be controlled by a human mind. Dr. Ken Horch and his research group implanted LIFE electrodes (longitudinal intrafascicular electrodes) into the nerves located in an amputee’s arm. The electrodes were connected to a computer, which then signaled a prosthetic hand, which was located on a table in sight of the patient. The patient was able to open and close the prosthetic hand by only thinking about opening and closing his hand. The patient was also able to feel different sense perceptions, such as touch, pressure, and movement in the finger regions of his “phantom limb”. However, the sensory prosthetic was not attached directly to his arm (Horch et al., 2005).
A research group at the BioRobotics Institute in Rome, Italy recently published a paper regarding a case study for a new bidirectional and sensory-equipped prosthesis (Raspopovic et al., 2014). The patient had suffered a left transradial (=forearm) amputation 10 years ago in a traumatic event. He was chosen for the study because his arm still had significant musculature in his arm and he was also experienced in using hand prostheses. Raspopovic et al. developed a prosthetic hand with sensors embedded in the index and pinky finger. These artificial hand sensors were connected to transversal intrafascicular multichannel electrodes (TIMEs), which relay the signal to the nerves within the arm of the patient. The TIMEs were surgically implanted into the median and ulnar nerves of the remaining forearm of the patient (Figure 2). These electrodes were also connected to a computer system, which would “translate” the sensory feedback from the artificial hand sensors into a signal that the arm nerve could relay back to the brain (Figures 3 and 4). In the future, this machinery would be miniaturized and simplified so it can fit inside the patient’s arm. If this technology is developed, the sensors inside of the amputee’s hand could provide real-time sensory information to the brain, giving the amputee their sense of touch back again in that limb.
The patient was asked to perform over 700 (!) tasks with the hand, all while he was acoustically and visually blocked. One task that the patient performed was discerning between three different objects of differing stiffness. He was given a piece of wood (hard), a stack of plastic glasses (medium), and a cotton pack (soft). By grasping the object and applying force, the patient was able to distinguish the stiffness of the object within 3 seconds (Raspopovic et al., 2014). Another test was done to see if the patient could differentiate between three objects of varying shape and size. The researchers presented him with three objects throughout the trials: a cylindrical object (a bottle), a large spherical object (a baseball), and a smaller spherical object (a mandarin orange) while he was still blindfolded and acoustically inhibited (Figure 5). He was able to correctly classify all three shapes with an average accuracy of 88%. The patient’s performance also increased rapidly over the weeklong testing period, showing that he intuitively and correctly integrated the information provided from the feedback (Raspopovic et al., 2014). What an amazing result!
Put in dry medical terminology, Raspopovic et al. concluded that they accomplished creating a real-time closed-loop sensory feedback communication between the prosthetic hand and the patient’s nerves and brain. The hand is a complicated body part because it has to intricately and precisely combine its muscle movements with the sensory input it receives. This study shows that it is possible to obtain sensory information from a prosthetic arm. Also, it is possible that if the prosthesis could stimulate more nerves in the arm, a greater variety of sensations could be relayed to the user. In order for this to be possible, the machinery needs to be miniaturized and fully implantable. The control unit for the decoding of muscle signals and sensation by stimulation needs to be programmed into a chip and placed inside of the prosthetic hand. At this point, this study has opened the door for the development of more natural, dexterous, and competent bidirectional control of prostheses.
For further information on the rapidly developing industry of prostheses, check out the Open Hand Project. The Open Hand Project (OHP) is an open-source project, where programs and software are available for you to improve on designs for 3-D printable hands. The hands are quite dexterous and agile for being made out of plastics, but do not contain the same type of sensory system as mentioned above. The prostheses only contain sensors that sense the movement of the muscles within the arm, which are specific enough to discern and execute complex finger movements. The OHP also accepts donations so that they can make prostheses more affordable.
These cutting-edge prostheses have a promising future. I think that these new technologies, such as the sensory feedback loop innervation prosthesis and the Open Hand project prostheses, are going to be absolutely crucial in changing the lives of many amputees in the near future. The ability to give a sense back to a person that has lost it is a great accomplishment, and I hope to see much more research and support in this field in the future (Figure 6).
Dobson, A., El-Gamil, A., Shimer, M., and DaVanzo, J. E. 2013 Retrospective cohort study of the economic value of orthotic and prosthetic services among medicare beneficiaries. American Orthotic and Prosthetic Association: The Amputee Coalition 11(114), 1-7. (http://www.amputee-coalition.org/content/documents/dobson-davanzo-report.pdf)
Horch, K.W., Dhillon, G. S. 2005 Direct neural sensory feedback and control of a prosthetic arm. IEEE Trans Neural Syst Rehabil Eng 13(4): 468-472. (PMID:16425828)
Raspopovic, S., Capogrosso, M., Petrini, F. M., Bonizzato, M., Rigosa, J., Pino, G., Carpaneto, J., Controzzi, M., Boretius, T., Fernandez, E., Granata, G., Oddo, C., Citi, L., Ciancio, A., Cipriani, C., Carrozza, M., Jensen, W., Guglielmelli, E., Stieglitz, T., Rossini, P., Micera, S. 2014 Restoring natural sensory feedback in real-time bidirectional hand prostheses. Science Translational Medicine 6(222), 1-10. (DOI 10.1126/scitranslmed.3006820)
Ziegler-Graham, K., MacKenzie, E., Ephraim, P., Travison, T., Brookmeyer, R. 2008 Estimating the prevalence of limb loss in the United States: 2005 to 2050. Archives of Physical Medicine and Rehabilitation 89(3), 422-429. (DOI 10.1016/j.apmr.2007.11.005)