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Multi-Layer Deformable Design for Prosthetic Hands
robotic hand completes most grasp types with high of deformability and compared the proposed structure
success rates (rates >80%). Furthermore, the performance with other methods.
of the proposed robotic hand is superior to that of the First, we considered two 3D printing materials for
InMoov hand and the Nadine’s hand V4 with all 33 grasp comparison, namely, the flexible material which is used
types. Note that the number of actuators of the proposed by the Nadine’s hand V4, and polylactic acid which is
robotic hand is equal to that of Nadine’s hand V4 (i.e., 6 used by InMoov hand. As demonstrated in Figure 7, the
actuators), indicating that the success of our robotic hand deformation curves of these two materials are far away
mainly comes from the deformable multi-layer design. from those of the human finger. As mentioned above,
the soft material the proposed robotic hand used is still
3.2. Ablation study of the multi-layer design harder than human tissues, and hence, merely using
To provide an insight into the effectiveness of the multi- this material with the solid structure cannot achieve the
layer design, we further conducted an ablation study desirable deformability. On the other hand, with the tube-
in which the robotic hands with different layers were based structure, the deformability of the tissue layer was
evaluated. Specifically, we performed the test of 33 improved greatly.
grasping gestures with 3 variants of our design, including More importantly, the experimental results suggest
a Bone-only design, a Bone + tissue design, and a Bone that the proposed design can explicitly control its
+ skin design. deformability through setting σ, the thickness of tubes. We
report the deformation curves of σ = [0.3 0.4 0.5 0.6 0.8]
Figure 6 demonstrates the results of the ablation
study. We observe that the bone-only design failed with in Figure 7. It can be observed that the deformability of
the proposed robotic hand is inversely proportional to σ.
multiple grasp types, such as prismatic finger, extension, With σ = 0.4, we can obtain the tissue layer which has the
and writing tripod. This again validates the opinion that deformation curves approximating those of the human
single layer/structure robotic hands are hard to fully finger.
imitate the ability of human hands. Nevertheless, with As emphasized previously, the deformability of
an extra layer of skin or tissue, the proposed design robotic hand is the key to grasping objects of various
obtained the significant performance gains, for example, textures and weights. To validate this, we also conducted
the success rate of the “prismatic 3 finger” type raised five experiments on grasping special objects, and include
from below 20% to higher than 80%. The design with all InMoov hand, Nadine Hand V3 and V4, and the proposed
three layers obtained the highest success rates. Based on robotic hand without the tissue layer for comparison.
these results, it can conclude that the multi-layer design is The experimental results of the five experiments are
a practicable solution for robotic hands. summarized in Table 2, and further descriptions are
3.3. Deformability of the tissue layer shown in the following:
Task A: Side pinch a fragile silken tofu block. This
The deformability of the proposed robotic hands task is introduced to demonstrate the benefits of soft and
was determined by the tissue layer. To demonstrate that deformable materials, since rigid robotic hands without
the proposed concentric tube structure for the tissue layer the tissue layer will crush the tofu block.
can effectively simulate the deformability of the human Task B: Side pinch smooth marbles with diameter of
hand, we adopted the deformation curve as the measure 16 mm and 24 mm. This task is designed to validate that

Table 2. Comparison of the deformability of various robotic hand designs
InMoov hand Nadine hand V3 Nadine Ours without tissue Ours
hand V4
Task A: Side pinch silken tofu x x ✓ x ✓
Task B: Side pinch marble x x ✓ x ✓
Task C: Side pinch water bottle (g) 250 200 50 200 450
Task D: Fingertip pinch screws x x ✓ x ✓
Task E: Press and pick up M1 screw x x x x ✓
cap
Maximum deformation length (x : y, 2 : 5 2 : 2 4.78 : 5.79 2 : 5 13.3 : 15.7
in mm)
Sliding resistance (N) 0.96 0.93 2.16 1.18 3.43
Plane rotation resistance (N) 0.25 0.25 1.18 0.15 1.77
A design fails in a task is marked with “x,” otherwise with “✓.”
16 International Journal of Bioprinting (2022)–Volume 8, Issue 1