Design and Manufacturing

Though many devices used external actuation, we found many that did not and were completely passive, usually drawing inputs from the wrist, through the use of pulley mechanisms attached to a glove. We were very drawn to the idea of having a passive device in our mechanism, but we did not want to have the input at the wrist. Instead we decided consider the idea of utilizing the palm as the driving input. Additionally, we wanted to scale our first model to a larger size than the finished product would be so that we could get a better understanding of our device beyond calculations and the manufacturing that would be necessary. With this knowledge we developed an initial design of our device, as seen below, which used six linkages and was a single degree of freedom (DOF).

 Figure 5: Initial 2D finger prototype

Though this was a very interesting initial concept, we soon realized this would not be feasible within the timeframe given due to the complexity of the slots for the sliding links; we would have had to make these very large to actually work. We proceeded to create a device consisting of two four bar linkages in series, with the initial input still at the palm. After careful consideration, we determined that we could manage our tolerances and produce a very nice finish using 3D printing. Below is the 2D prototype for this model.

Figure 6: 2D final prototype with four bar and motion labels

The long input arm presses against the flat plat – meant to act as the palm – and drives the first four bar linkage which has an output that drives the second. We utilized a rubber band as a return mechanism for when the finger is outstretched. Notice both the four bar mechanisms are in the crossed configuration. The two four bars emulate the natural curling out the finger as they move together, though the second curls quicker which makes the motion more realistic. We did not have to tap any of the links of our mechanism because the screws were easily press fit into the plastic used in 3D printing.

After creating our model in 2D we proceeded to create a final model in 3D, meaning it would be ready for use by the patient, and we had to scale it down to normal size. To achieve this, we added a socket to the 2D design. We had to develop our 3D model to have the mechanism acting on either side of the amputation. To add comfort, ease of use, and a good fit, we developed a tightening system similar to a shoe laces. After finalizing our model, we proceeded to 3D print this. The resulting parts can be seen below.

Figure 7: 3D printed parts for first iteration of 3D prototype

We placed three pairs of extruded plates on top of the socket with holes in them that could be tied closer together to grip the patient’s amputation, pointed to in Figure 7.

Due to the small parts and tolerances required, the print did not come out well. We decided to reprint the parts using a fused deposition modeling (FDM) 3D printer to achieve more desirable results. Our final model is seen below.

Figure 8: Final printout of 3D prosthetic finger

In this model, we used two springs to work as our return mechanism. Other than the screws, our entire device was 3D printed.