top of page
Linktree
Mul-t hand.
MUL-T Hand is a prototype prosthetic arm developed as part of my mechanical engineering masters project at Imperial College London. The goal of this project was to develop an affordable, body-powered, trans-radial (below elbow) prosthesis able to perform simple daily tasks.
This project was intended as a resposnse to two of the most common complaints from people with a limb-difference about their prosthetics, namely the cost of the devices (which range from $4,000 - $8,000 for body-powered devices and $25,000 - $50,000 for externally powered ones) and the weight of them making them uncomfortable to use for extended periods of time. Two major goals were, therefore, to make the device as affordable and lightweight as possible.
Initial Prototype.
The origins of this project can actually be traced back the summer of my first year at Imperial. I created a basic articulating hand to practice my SOLIDWORKS and 3D printing skills.
This led into me doing a week of work shadowing at Rookwood hospital prosthetics department where I got to interact with prothetists, technicians, and patients doing rehabilitation and those getting fitted for new sockets. This gave me a great deal of insight into the most important aspects of a prosthetic device to the end user and the healthcare system and the considerations that must be made when designing them.
Development.
All aspects of the original prototype underwent major redesigns, one of the most important aspect of this was a total overhaul of the devices aesthetics. This is more important to prosthetic design than it may first appear as the device has to function as an extension of the user and often forms a part of their identity.
There are two common approaches in modern prosthetic design in regard to this aesthetic. The first is to simulate a real limb as closely as possible, often using silicone covers to simulate skin as closely as possible this is often favoured by people who are older or have had an injury recently that resulted in a limb difference. This runs the risk, however of falling into the 'uncanny valley' - a feeling of unease when looking at somthing that almost looks huma in its form, but is not quite right. The other approach is to embrace the difference in the prosthetic limb, often leaning into futuristic, sci-fi-esque, designs with solid colours and covers to suit the users taste.With my design I opted for the latter approach as I believed it would allow users a greater degree of self expression and customisation while circumventing the problem of the 'uncanny valley' by not attempting to look human in the first place.
Another important aspect of the device was, of course, the functionality. One of the factors that greatly affects the grip a hand has is the frictional force at the palm and fingertips.
To approximate this as closely as possible to a human hand silicone fingertips and palm pads were produced using a 3d printed mould. Making the whole fingertip and pad from silicone also allows it to deform slightly, creating a greater contact surface and further improving grip.
The MUL-T hand uses the same opening mechanism as the split hook prosthesis which is the most widely used and functional arm prosthesis.
This involves a harness on the opposite shoulder to the arm the device is on, the user then adducts their shoulder to create tension in a string or cable which opens or closes the device depending on if it is set up to be voluntary-open or voluntary-close
The finger mechanism uses muscles in the form of elastic bands and tendons in the form of fishing line. The tendons are controlled by the user and the muscles create opposing tension to either open or close the device (depending if it is set up to be voluntary-open or voluntary-close) when force is not applied.
The segments of the fingers snap fit together and are pinned using a small legth of 3D printer filament melted at each end.
The initial redesign was successful in achieving a greater degree of funtionality and improving the aesthetics of the device however including all 3 finger segments led to issues with the pinch grip collapsing and the construction of each digit was very labour intensive due to each joint needing several rubber bands. These problems were addressed in the final prototype version which is detailed below.
Final design.
The final prototype design incorporated a harness and Koalaa ALX sleeve, this allowed the device to be worn and used for testing.
The finger segments were simplified to reduce the occurance of hinging when trying to perform a pinch grip. The elastic band 'muscles were also exchanged for an elastic cord. This made it easier to assemble the fingers whille retaining a similar grip strength.
Bowden tubes were used to stop the 'tendon' fishing line from rubbing against clothes or skin as this was a common complaint amongst users.
The connection point of the 'tendons' to the harness also includes a tensioning mechanism so the individual digits can be independantly tightened to ensure they all move at once.
I presented my final prototype as a technical presentation at the end of the term and described the process in a detailed report for my masters thesis. I also had the opportunity to display my work as a scientific poster at an end-of-year show for which my hero image was chosen as the cover photo for the booklet.
Testing.
The device was tested on its suitability for use. the maximum tension needed to operate the device was measured using a force guage and then compared to a standard split hook prosthesis, both of which were approximately 110N. In other words the shoulder adduction force needed to actuate the device was equivalent to the force needed to lift an 11.2kg weight. This is within the standard, reasonable operationg range for a device of this type.
The ovarall weight of the device was also measured as this was a major consideration for user comfort. The assembled hand weighed 225g and the the entire device including sleeve and harness was 630g, making it lighter than most human shaped devices on the market.
A modified version of the Southhampton Hand Assessment Procedure SHAP wa used to evaluate the effectiveness of the final prototype. These tests simulate several daily activities that a person with a limb difference may use the device for such as picking up objects of various shapes, opening doors and buttoning a shirt.
The device performed well when manipulating larger objects but struggled with tasks requiring fine motor skills such as the afformentioned shirt buttoning due to lacking the required pinching force, this would need to be addressed in future models to make the device truly useful for everyday use. A few examples of the evaluation tasks can be seen here.
Even on the tasks the device performed well it was between 2-10 times slower than the control of using a human hand, this is expected of a prosthetic device but could likely be reduced with further practice.
The other evaluation criteria used was the cost of manufacture. The goal of keeping costs low achieved as the overall cost of the device was under £90 (under £65 per device if making multiple) and the total cost of the tools used to make it were just over £200. Therefore to aquire everything needed to make the device and then manufacture it the cost is under £300.
bottom of page