PROSTHETICS
New possibilities for prosthetics
A new way of harnessing the sun’s rays to power ‘synthetic skin’ could help to create advanced prosthetic limbs capable of returning the sense of touch to amputees. Here, Dr Ravinder Dahiya, from the University of Glasgow’s School of Engineering, discusses the project.
The five basis senses in the human body form the interface with the environment. Among these, the sense of touch is unique as it is not merely receptive, but it is crucial for guiding motor behaviour. Tactile sensing plays a fundamental role in providing action-related information such as sticking and slipping; vital control parameters for manipulation/control tasks such as grasping; and estimation of contact parameters such as contact force, soft contact, hardness, texture, temperature, etc. We interact with the environment to acquire tactile information, and use that knowledge to modify the world. Touch is essential for guiding the behaviour of our hands. Yet the articifial limbs today are unable to provide the amputee a tactile sensory feeling, even if they are able to generate complex movements. This is because they do not have artificial skin such as the one on the surface of the human body. Clearly, there is a need to develop artificial tactile skin to allow amputees to detect the strength and location of subtle pressure or temperature changes. The tactile skin conforming to surgical tools could also revolutionise surgical practices such as minimally invasive surgery.
Developing electronic skin
The skin is a huge sheet of unevenly distributed tactile receptors with capability to partially process the tactile data. The development of an artificial tactile skin or electronic skin (e-skin) is challenging, as it involves a large number of different types of unevenly distributed sensors, flexibility, reliable and repeatable performance, large- area manufacturability, and large data handling. Further, energy needed to operate a large number of sensors is a major bottleneck. A number of attempts made to harvest ambient energy (e.g. from vibration, thermal, etc) have often failed as the energy generated is insufficient. The energy from sun harvested through photovoltaic (PV) cells could be sufficient. But the PVs require direct exposure to sunlight, which is difficult
The above diagram shows how the sun’s rays can be used to power skin sensors and actuators.
as the current non-transparent artificial tactile skin solutions form the top layer. The issue can be overcome by developing a transparent tactile skin. We recently explored this novel approach and developed a layered skin structure consisting of a photovoltaic cell attached to the back plane of a transparent tactile skin. The transparent skin allows light to pass through to allow PVs to generate energy needed for the skin’s operation and therefore opening a new line for energy-autonomous devices. The tactile skin is made of graphene, which is a single atomic layer thick and allows about 98% of light to pass thorugh. Graphene also has a good combination of stiffness (≈1000 GPa) and tensile strength (≈100 GPa). Together with its sunlight blindness and good electrical conductivity, graphene has also emerged as a viable candidate for various flexible, transparent electronic and optoelectronic devices.
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Our work entitled “Energy-Autonomous, Flexible, and Transparent Tactile Skin” recently published in Advanced Functional Materials journal shows a novel structure, consisting of a transparent tactile sensitive layer based on single-layer graphene and a photovoltaic cell underneath as a building block for an energy- autonomous, flexible and tactile sensitive skin. Transparency of the touch sensitive layer is considered a key feature to allow the photovoltaic cell to effectively harvest light. Moreover, ultralow power consumed by the sensitive layer (20 nW cm−2) further reduces the photovoltaic area required to drive the tactile skin. In addition to its energy autonomy, the fabricated skin is sensitive to touch, mainly because a transparent polymeric protective layer, spin-coated on the sensor’s active area, makes the graphene based coplanar capacitor sensitive to touch, detecting minimum pressures of 0.11 kPa with a uniform sensitivity of 4.3 Pa−1 along a broad pressure range. The demonstrated touch selectivity of the sensors increases electronic-skin functionality and would allow the spatial detection of objects with different compositions in contact with the electronic-skin.
Benefits of graphene
Durability and stability of this electronic-skin was probed by bending the skin through hundreds of cycles and applying strains up to two per cent, respectively, showing changes in the electrical performance of the skin below one per cent. This result is mainly because of the intrinsic mechanical robustness of graphene, which preserves electrical properties after transfer to either at or nonplanar surfaces. In addition, there is good conformal contact formed between graphene and the flexible substrate during the transfer procedure, making the device architecture more robust and very stable, even under the stresses experienced during bending. Regarding the fabrication method, in this work we present a novel dry method that introduces features such as reliability, amenability, upward scalability, and
SEPTEMBER 2017
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