Design Project on Prosthetic Hand for

Freestyle swimmingDingwen Cheng, Chengjun Liu, Rajarshi Roy, Samuel Turner

August 11th 2016

Prof. Catherine Twyman

Executive Summary

Limb loss takes people’s ability to work and their abilities to enjoy many types of

entertainment away from their lives. Amputation brings not only the physical disadvantage, but

also psychological trauma that is derived from the inability of doing certain things that are easier

for non-disabled people. Around the world between 0.5% ~0.8% of the global population are

amputees. [1]

Giving amputees a chance to swim through a swimming prosthetic, they can retake their

joy of swimming and hope for life. Currently designs for forearm and hand prosthetics specific

for swimming are largely made of simply a paddle. Other designs contain complex technology

that also means they are expensive. Many people may have difficulties in familiarizing using

paddles, and also may not be able to afford other expensive professional swimming prosthetics.

This design team is formed to design an affordable, easy to use prosthetic hand for

freestyle swimming.

Following the research on the existing design, the team developed three design options

which were evaluated in a decision matrix with five criteria: installation maneuverability,

efficiency or speed, manufacturing ability, degree of comfort, and balance between the hand and

the prosthetic. The weight for each criteria is determined based on the people’s requirements for

swimming prosthetics. The final decision is that the design with a curved face, which is similar

in shape to human hand, is the optimal design.

The leftover work is to make the prototype full sized and test some of its properties.

Further experiments are needed to find out the best structure, such as the fingers’ length and the

radius of the curve for the face. The prosthetic hand should be able to fulfill the major

requirements for unilateral hand amputees for swimming.

1

Table of Contents Page number

● Background Information--------------------------------------------------------3

● Use Case Scenario---------------------------------------------------------------3

● Constrains-------------------------------------------------------------------------4

● Criteria----------------------------------------------------------------------------4

● Potential Design Solutions-----------------------------------------------------5

● Design Evaluation---------------------------------------------------------------6

● Explanation of the Final Design-----------------------------------------------7

● Testing-----------------------------------------------------------------------------7

● Future Plan & Limitation-------------------------------------------------------8

● Conclusion------------------------------------------------------------------------9

● References------------------------------------------------------------------------10

● Appendix A: Chosen Prototype-----------------------------------------------11

● Appendix B: Design Alternative & Other Graph---------------------------13

● Appendix C: Working Hour Count-------------------------------------------15

2

Background Information

Limb loss is a serious problem in US. In 1996 there were 1.3 million people in US that

had to live with at least one lost limb and this number is increasing at 50,000 per year. [2] About

30% of the people with amputations lost limbs on their upper body. This enormous number of

people lost their hand and forearm or even the whole arm, and also lost their chance to work and

to play. Especially the loss of a dominant hand will give more difficulties in their lives.

Use Case Scenario

Unilateral hand amputees who are

passionate about swimming, but struggling

with their disabled arms are currently seeking

for a functional prosthetic hand that can help

them swim in the pool water with a boost on

both their swimming speed and swimming

convenience.

According to the department of Exercise and

Sport Science of Manchester Metropolitan

University, motion of arms and motion of

legs during swimming depend on two totally

different systems, and as a result from their

research, it is concluded that “unilateral arm

amputee swimmers functionally adapt their

motor organisation to swim front crawl [3].”

In other words, hand amputee swimmers are

able to swim freestyle strokes better with

functioning motor organic systems, whereas

swimmers with disabled legs https://www.pinterest.com/pin/561401909779453380/

will comparatively have a harder time in

swimming freestyle strokes. Nonetheless, prosthetic hands will still come in handy, as it gives

more force when pushing the water to move forward. Based on Richard Stark's Neptune concept,

3

“swimming with one leg is like rowing with one oar [4].” Thus, any amputee swimmers need to

have a stronger mind to overcome some difficulties that most people do not have. Richard claims

that even though help from prosthetics is prohibited in the competition, they are still useful in

practice, as prosthetics “develop muscular strength and mobility to use in competition [4].”

Hence, in this project, a prosthetic hand is intended to be designed that can help hand amputee

swimmers to enjoy swimming, and especially to help them find joy in swimming freestyle.

Constraints

We found five constraints that limit our design options. First, it should weigh 2.3% of the

person’s weight [5]. A human hand comprises 2.3% of a person’s body weight, so we want the

prosthetic to be as close to this for our average user as possible. Next, its density should be near

to that of water, because this would allow the user to use the prosthetic more comfortably

without feeling any excess force due to the buoyancy of the prosthetic. Also, it should not rust or

react chemically with the pool water. We want the prosthetic to last for a long time and not allow

it to react while in contact with the human body. This would cause a safety hazard and could

harm the user. Additionally, it needs to be designed for freestyle swimming. Freestyle is the most

commonly used swimming style, so users would be able to swim in a familiar style to what they

are used to. Finally, the manufacturing cost of the prosthetic should be less than one hundred

dollars. This would allow a minimal barrier to entry for new users.

Criteria

We came up with 5 criteria for the design. First, the design should be easy to mount with

one hand. If someone comes to pool alone, he/she should be able to do the preparation easily and

in a short amount of time. Second, our design should focus on maintaining efficient motion

through the water which requires the device to move with minimal excess drag. The design will

follow the laws of hydrodynamics and aerodynamics, and the prosthetic hand should be bent to a

certain angle that will produce the most efficient structure. Next, The design should contain the

fewest number of parts possible. It should also take a short period of time to produce. Then, we

want to use materials that are friendly to skin for the parts that are in contact with the skin to

reduce chafing on the amputated section. Finally, the design should allow the user to easily

output the same amount of force on the body with each hand. Having the hands output different

amounts of force could lead to discomfort and cause the swimmer to be unable to keep a straight

4

line. Additionally it should not obstruct any movement of the fully functional arm.

Potential Design Solutions

Throughout the design process we came up with three major designs. The one that

ultimately became our final design, Design 1, is a curved paddle with slits cut into the top. It is

very similar in appearance to a human hand. The curve maximizes the surface area for the

beginning of each stroke, maximizing the possible output force. It also minimizes the top surface

area allowing the part to easily come out of the water. The slits also help the surface area to

remain as high as possible while still allowing the water to pass through when the hand is being

raised out of the water. This is all due to the nature of the drag equation. This design mounts to

the user’s arm using a rounded socket and a strap that attaches to that point and then wraps

around the forearm. The mounting component is a separate component that attaches to the paddle

section using a threaded extrusion. The paddle contains a complementary hole and set of

threading which allows them to connect easily. Drawings and models of the design and two

components can be found in Appendix A.

Design 2 is a dynamically functioning prosthetic. It uses three flaps that are all connected

to a bar reaching all the way to just above the user’s elbow. When the user extends their arm the

flaps close allowing the user to get the maximum pull on the water as they start the stroke. When

the arm is bent and the user wants to pull it out of the water the flaps open allowing the water to

pass between the flaps reducing the amount of force required. This device comprises of more

than five individual components. It mounts to the user’s forearm with straps similar to Design 1

and connects with a velcro strap just above the elbow. A CAD drawing of this design is

contained in Appendix B.

Design 3 comprises of a paddle similar to that used in rowing. It is flat and mounts to the

arm in a similar manner to Design 1. It is capable of pushing more water and thus getting a

stronger push out of each stroke. Unfortunately it is also very unwieldy and hard to use.

Additionally, it may out push the user’s fully functional hand and thus would cause problems

when trying to swim in a straight line. A sketch of this design can be found in Appendix B.

Design Evaluation

For the pairwise comparison chart in the appendix B, five criteria are selected, and they

are installation maneuverability, efficiency, manufacturing ability, degrees of comfort, and

balance of hands. Installation maneuverability means how long it takes the users to put the

5

prosthetic on. Efficiency can be interpreted as “speed”, or how fast the amputee swimmers can

swim in the pool water using the prosthetic. Manufacturing ability stands for how easy the

prosthetic hand can be made. Then, degrees of comfort indicates whether or not the prosthetic

causes chafing or other minor harm. Last, the balance of hands can be defined as whether the

prosthetic hand gives the same push as the user’s other hand does, so that his or her body is

balanced when swimming. By comparison, Degrees of comfort and efficiency are the two most

important factors among the five criteria, and this fact can be deduced with logic. If the

prosthetic hands do give users chafing, and the salt water aggravates the pain, users would find

the product annoying and stop using it. If the prosthetic hands fail to give a decent amount of

push and force to move users forward, then the product does not succeed to give a boost on

swimmers’ swimming speed. Next, the installation maneuverability is scaled from 15 seconds,

which is slow for users to put the prosthetics on, up to 5 seconds, which is fast for users to put

the prosthetics on. The efficiency is scaled from 1 m/s up to 2.3 m/s, which is the fastest speed

that a person can swim in the pool water. Since manufacturing ability, degrees of comfort, and

balance of hands do not have units, they are scaled from 1 to 10. Finally, by using the

normalization equation, all five criteria are normalized for all three designs.

For the decision matrix in appendix B, three designs all fall under the five constraints.

Multiplying weights by the normalized scores, weighted scores are determined. By adding all

five criteria weighted scores together, the total scores are determined for three designs. Design 1

gets the highest score on degrees of comfort and efficiency, the two criteria that weigh the most.

Consequently the total score of Design 1 is higher than that of Designs 2 and 3. This results in

Design 1 being the most favorable one.

Explanation of the Final Decision

Design 1 placed higher in the criteria for several measurable reasons. Compared to

Design 2, Design 1 has less total components reducing the manufacturing cost. Because of the

moving parts, Design 2 has more chances of breaking down as opposed to Design 1 that is made

up of two solid blocks. Also, as Design 2 has many sharp edges, it can harm the user when it

comes in contact with the skin. Design 1 has only curved edges and is safer as a result. It takes a

6

lot of time for users to take off and put on Design 2 so attachment time will go up. However,

Design 1 can be easily put on or taken off.

Compared to Design 3, Design 1 has a slits so that it can put the same force on the water

while minimizing the lift drag because of the flat nature of Design 3. There are slits in Design 1

so there will be less force exerted on the hand when it is coming out of the water, whereas

Design 3 has no such slits so there will be more force exerted on the arm. As a result, that arm

would become more tired than the other arm and we want both arms to feel the same way after

swimming. Also, Design 3 is pointed and has sharp edges whereas Design 1 has smooth edges,

thus it is safer.

Testing

To test the design we determined three systems that would allow us to determine whether

or not the design meets the criteria and constraints as much as reasonably possible. One test is to

see if the designed prosthetic results in the same speed as the fully functional hand. In this test

we have the person perform one stroke with the prosthetic hand and determine their final speed

after the stroke. Then they would perform one stroke with their fully functional hand. We could

run this test several times and find what the average resulting speed of each stroke was. Then we

could compare the two and determine what the difference between the two designs was.

Another test would enable us to determine the total time required for a person to put on

and take off the prosthetic. We want this to take the minimum possible time to reduce frustration

for the user. First we would demonstrate how to attach the prosthetic to the amputated point on

the forearm and then have them attempt to put it on with and without help. Once they became

comfortable with how the design works we can test how long it takes for them to put on the

prosthetic. This would be done in multiple trials. This would help us determine if the design is

truly able to be quickly mounted to the amputated area.

The third test we developed would determine the buoyancy of the product in water. This

would require the materials to be the same as the final product as the density of the different

materials would determine the overall buoyancy. We would then put the prosthetic into water

and find the distance below the water it sinks. A normal human hand is roughly equivalent in

density to water and thus would float just below the surface thereof. We can test whether this is

the case for our design and determine if it is the same density as water.

7

Future Plan and Limitations

There are still places for improvement for Design 1. We cannot make the entire final

product out of ABS plastic like the prototype. There should be a thin metal rod through the

center and the main substance should be made of a strong plastic to improve the strength of the

product. In addition, the outer layer can be silicone to make it have a more human-skin-like feel.

We also have to put into consideration that the silicone skin cover can be damaged through wear

and tear. It will raise the manufacturing cost, so thought must be given as to how much of each

material we must use to make the total cost of manufacture of the product less than $100.

Moreover, we can make the prototype full size for experiments designed for it.

Since our product is only based on freestyle swimming, we should try to improve the

product so that other styles can be done as easily as freestyle in the final product. Also we should

try to improve the mechanism that connects Part 1 and Part 2 of Design 1. By threading the

connection point, the person without their dominant hand may have to take time to screw the

components together. We would rather have an automatic mechanism where the user can just

place the hand component onto a given position and lock it in with the push of a button. This

would reduce the attachment time and makes attaching the components much faster than if the

user had to screw it into position with one hand. Also through using the threaded connector, the

forces exerted from the water can unscrew the two parts.

As we are mass producing the prosthetic, we cannot customize it for each user according

their individual weight, so we are making the prosthetic for an average athletic person as they

will be most likely to use it. We also need to perform the experiments we planned on the full size

prosthetic to see if there are any other changes that we can pursue that will improve the design

further. Also, as we made the chosen prototype into 2 parts, we can expand the use of the

prototype into replacing part 1 with other prosthetics that can do things other than swimming.

Conclusion

In conclusion, our hand shaped plastic prosthetic can provide a low-cost, efficient

solution for people with hand and forearm amputations who have the passion to swim. Our

product will help them enjoy swimming to the same degree as people who have both functional

hands and can help them gain confidence and happiness to live a better life.

8

References

[1] “Limb loss a grim, growing global crisis,” NBC News, 20-Mar-2010. [Online]. Available:

http://haitiamputees.nbcnews.com/_news/2010/03/19/4040341-limb-loss-a-grim-growing-global-

crisis. [Accessed: 10-Aug-2016].

[2] M. Garcia, L. Gonzales, M. Kim, M. Kulley, Y. Oh, and D. Ruan, “Bio 108 - Organ

Replacement - Hand Prosthetics - Statistics,” Bio 108 - Organ Replacement - Hand Prosthetics -

Statistics. [Online]. Available:

http://biomed.brown.edu/courses/bi108/bi108_2003_groups/hand_prosthetics/stats.html.

[Accessed: 09-Aug-2016].

[3]“Result Filters,” National Center for Biotechnology Information. [Online]. Available:

http://www.ncbi.nlm.nih.gov/pubmed/25562689. [Accessed: 26-Jul-2016].

[4]“Prosthetic Flipper Turns Amputees Into Mermen,” Fast Company, 2010. [Online]. Available:

http://www.fastcompany.com/1662946/prosthetic-flipper-turns-amputees-mermen. [Accessed:

26-Jul-2016].

[5] “Body Segment Data,” Body Segment Data. [Online]. Available:

http://www.exrx.net/kinesiology/segments.html. [Accessed: 27-Jul-2016].

9

Appendix A-Chosen Prototype

Isometric view

Orthographic multiview

Isometric of Part 1: The Paddle Component

10

Isometric of Part 2: The Mounting Component

11

Appendix B-Design Alternatives & Other Graph

Design 2 - the first design alternative

Design 3 - the second design alternative

Pairwise Comparison Chart

12

Decision Matrix

Appendix C - Billable Hours and Project Cost

13

Rate per hour=$125

Names Total Hours

Sam 22.50

Frank 23.25

Tim 16.75

Rajarshi 17.25

Team Total Hours 79.75

Project Cost $9,968.75

14