Posted on May 15, 2020, 1 p.m.
Those with arm amputations can now for the first time experience the sensations of touch in a mind controlled arm prosthesis that can be used for everyday life. A recent study published in the New England Journal of Medicine reports on 3 patients who have been living with this new technology for several years
This new technology may be one of the world’s most integrated interfaces between human and machine, and this advance is unique, these patients have been using this mind controlled prosthesis in their everyday professional and personal lives for as long as 7 years, of which for the last few years they have also experienced the new function of the sensations of touch in the prosthetic hand. The new concept is called neuromusculoskeletal prostheses, and they are connected to the user’s nerves, muscles, and skeleton.
"Our study shows that a prosthetic hand, attached to the bone and controlled by electrodes implanted in nerves and muscles, can operate much more precisely than conventional prosthetic hands. We further improved the use of the prosthesis by integrating tactile sensory feedback that the patients use to mediate how hard to grab or squeeze an object. Over time, the ability of the patients to discern smaller changes in the intensity of sensations has improved," says Max Ortiz Catalan who is an Associate Professor at Chalmers University of Technology.
"The most important contribution of this study was to demonstrate that this new type of prosthesis is a clinically viable replacement for a lost arm. No matter how sophisticated a neural interface becomes, it can only deliver real benefit to patients if the connection between the patient and the prosthesis is safe and reliable in the long term. Our results are the product of many years of work, and now we can finally present the first bionic arm prosthesis that can be reliably controlled using implanted electrodes, while also conveying sensations to the user in everyday life," continues Max Ortiz Catalan.
The unique neuromusculoskeletal prosthesis delivers several different features that have not been presented together in any other known prosthetic technology. It has direct connection to the user’s nerves, muscles and skeleton. It is mind controlled. It delivers sensations that are perceived by the users as if coming from a missing hand. It is self contained, meaning all the electronics needed are within the prosthesis so there is no need to carry additional equipment or batteries. Additionally, the technology is safe and stable for long term use as demonstrated in this study being used with no interruptions, without supervision, while not being restricted to confined or controlled environments.
Force sensors located in the thumb of the prosthesis measure contact and pressure that is being applied to an object while grasping, this information is transmitted to the user’s nerves leading to their brain; this process of nerve stimulation allows the user to feel when they are touching an object, its characteristics, and how hard they are pressing on it which is important for imitating a biological hand.
"Currently, the sensors are not the obstacle for restoring sensation," says Max Ortiz Catalan. "The challenge is creating neural interfaces that can seamlessly transmit large amounts of artificially collected information to the nervous system, in a way that the user can experience sensations naturally and effortlessly."
"Right now, patients in Sweden are participating in the clinical validation of this new prosthetic technology for arm amputation," says Max Ortiz Catalan. "We expect this system to become available outside Sweden within a couple of years, and we are also making considerable progress with a similar technology for leg prostheses, which we plan to implant in a first patient later this year."
The implant system for this arm prosthesis is called the e-OPRA and it is based on the OPRA implant system created by Integrum AB. This implant anchors the prosthesis to the skeleton in the stump of an amputated limb using an osseointegration process where in electrodes are implanted in muscles and nerves within the stump and the system send signals in both direction between the prosthesis and the brain in a similar manner as a biological arm.
The electric muscle and nerve signals sent through the arm stump are captured by the electrodes to control the prosthesis with the mind. Each signal passes into the implant which goes through the skin and connects to the prosthesis, these signals are interpreted by an embedded control system that was developed by the research team. The small control system is inside of the prosthesis and it processes signals using artificial intelligence algorithms which results in control signals for the movement of the prosthetic hand.
Sensations of touch arise from force sensors in the thumb of the prosthetic: Signals are sent from the sensors to be converted by the control system into electrical signals which are sent to stimulate a nerve in the arm stump, this nerve leads to the brain which will perceive the pressure levels against the hand.
This neuromusculoskeletal implant is suggested to be able to connect to any commercially available arm prosthesis to allow them to operate more effectively.
When a person loses an arm or leg they often experience phantom sensations from the missing body part. As the force sensors in the prosthetic react, the patients in this study feel that same sensations that comes from their phantom hand. However, where on the phantom hand varies between patients; the lowest level of pressure can be compared to touching the skin with the tip of a pencil and as the pressure increases the feeling becomes stronger and increasingly electric.
This study focused on patients with above elbow amputations, and the technology is approaching becoming a finished product. A parallel new system is being developed for amputation below the elbow, and the team is working on adapting the system for leg prostheses. The permanent interface between human and machine also provides new opportunities for research into how the human muscular and nervous system work.
This research was in collaboration with Sahlgrenska University Hospital, University of Gothenburg, and Integrum AB, all in Gothenburg, Sweden; and researchers at Medical University of Vienna in Austria and the Massachusetts Institute of Technology in the USA were also involved.
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