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Multi-Layer Deformable Design for Prosthetic Hands
kinematics and dynamics of the human hand, while soft A fast template matching approach is proposed to obtain
robotic hands are deformable and inherently safer. the 3D models of bones, as well as a concentric tube-
However, despite the recent notable process in soft based structure as the 3D model of tissues. Besides, an
actuators and materials, object grasping and manipulations, underactuated system, which simplifies previous complex
such as the dexterous movement of objects within the hand, motion control systems [14,22] , is introduced in this paper.
remain challenging . The lack of actuators equivalent in Table 1 summarizes the differences between the
[17]
size to human hands, might be an issue to applications, proposed design and other robotic hands. To the best of our
such as social robotics and prosthetic hand. Besides, the knowledge, this paper proposes the first customizable 3D
customization of existing robotic hands is not easy, which printing multi-layer design for robotic hands. The proposed
means that many of them are built with predefined shapes design allows us to fully take the advantage of soft robotic
and sizes, instead of a target given on-the-fly. hands in deformation. We conducted extensive experiments,
To tackle the above issues, we propose to combine the including the standard Feix object grasping test [26] and the
advantages of both rigid and soft robotic hands, especially the trajectory test, to demonstrate that our design can effectively
human-like mechanism of rigid hands and the deformability simulate the functionality of the human hand. Due to the
of soft hands. We consider human hand as the template deformability of our design, special objects, including
because its kinematics and dynamics are well-studied, while extremely fragile silken tofu, smooth marbles, and tiny
the deformability provides more reliable contacts for object screw caps, can be grasped by our fabricated hand.
grasping and manipulation. We notice that most of the In summary, the contributions of this paper are
existing methods assume that the functionalities of human threefold:
hand can be realized by a single structure (e.g. Shadow i. We propose a multi-layer robotic hand design, which
hand has a single rigid structure representing phalanges). combines the stability of rigid hands (via the bone
Such an assumption actually increases the complexity of layer) and the deformability of soft hands (via the
the design process and limits the candidate materials for tissue layer).
fabrication, as various mechanical structures and actuators ii. We introduce a fast template matching method to
need to be compatible and integrated. Therefore, instead generate the customizable 3D models of bones and
of relying on a single structure, we propose a novel multi- tissues, which reduces the cost and complexity of
layer design, which highly replicates the anatomy of the robotic hand fabrication. A simplified underactuated
human hand. Our design is composed of three layers, system of 15 degrees of freedom (DOFs) and 6
namely, bone, tissue, and skin, and each of them is devised degrees of actuation (DOAs) is proposed as well.
for a particular purpose: the bone layer serves as the rigid, iii. The proposed tissue layer provides our robotic hand
supportive structure and is attached to actuators to enable with the notable deformability, which is validated via
object grasping; the tissue layer is made of soft materials to the contact models of our robotic hand and extensive
guarantee the deformability of the hand; and the skin layer experiments. Compared with existing prosthetic
provides the human-like appearance. robotic hand solutions, our design provides a rapid
To ensure that the proposed design can be and low-cost way to fabricate a customized robotic
fabricated efficiently, we exploit the advantages of three- hand with deformable tissues.
dimensional (3D) scanning and 3D printing in rapid and
adaptable fabrication [18-21] . There are a few available 2. Materials and methods
3D printable materials for prosthetic hand fabrication, 2.1. The multi-layer deformable design
such as Formlabs TM Clear and Durable that have similar
hardness as bones, and Formlabs TM Flexible and Elastic The goals of our robotic hands are dual: first, the
that is similar to human tissue in terms of flexibility. modeling process of our robotic hands should be fast and

Table 1. Comparison of the proposed design with other robotic hands
Model Skin Tissue Bone Actuator Joint type
Shadow hand [3] N.A. Rigid structures E-motor+tendon Rigid
EXMART hand [23] N.A. Rigid structures E-motor+tendon Dislocatable
RBO hand V2 [5] N.A. Soft materials Pneumatic motor Soft continuous
Soft robotic hand N.A. Soft materials Shape memory alloy Soft continuous
Biomimetic hand [13] N.A. N.A. Rigid materials E-motor+tendon Flexible
ACB hand [24] N.A. N.A. PolyJet, resin E-motor+tendon Flexible
Nadine hand V4 [25] Silicone rubber Flexible materials E-motor+tendon Rigid
Ours Silicone rubber Elastic materials Rigid materials E-motor+tendon Flexible

10 International Journal of Bioprinting (2022)–Volume 8, Issue 1