CC BY
6DOI 10.24412/cl-37136-2023-1-71-75
3D PRINTABLE SOFT ARTIFICIAL OPTICAL SKIN FOR HEALTHCARE APPLICATION
ABHIJIT CHANDRA ROY1, NAVIN KUMAR1, SHREYAS B S1, ANANYA GUPTA1, ALOKE KUMAR2, AVEEK BID1, AND V. VENKATARAMAN1
department of Physics, Indian Institute of Science, Bangalore, Karnataka, India-560012
2Department of Mechanical Engineering, Indian Institute of Science, Bangalore, Karnataka, India-
560012
ABSTRACT
The human skin being the largest and most exposed organ in the body provides various essential information including touch, temperature, pressure, vibration, and humidity of the surrounding for smooth and safe functioning of our body. Similarly, artificial soft electronic skin, like human skin, perceives various environmental stimuli by transducing them into an electrical signal. Soft artificial optical skin capable of sensing touch and pressure is essential in many applications, including social robotics, healthcare, and augmented reality. However, several hurdles remain challenging, such as highly complex and expensive fabrication processes, instability in long-term use, and difficulty producing large areas and mass production. Here, we present a robust 3D printable large area soft artificial optical skin made of a soft and resilient polymer capable of detecting touch, load, and bending with extreme sensitivity to touch and load, 750 times higher than earlier work. The soft artificial optical skin shows excellent long-term stability and consistent performance up to almost a year. In addition, we describe a fabrication process capable of producing large areas, large numbers, yet cost-effective. The soft artificial optical skin consists of a uniquely designed optical waveguide and a layer of a soft membrane with an array of soft structures which work as passive sensing nodes. The use of a soft structure provides the freedom of stretching to the soft artificial optical skin without considering the disjoints among the sensing nodes. The soft artificial optical skin's operation has been shown using a variety of techniques. INTRODUCTION
Soft artificial skin similar to human natural skin perceives various surrounding stimuli by transducing them into an electrical signal through various methods. Soft artificial skin has earned overwhelming consideration in recent time due to the rise of diversified fields of science and technologies including the social interactive robots[1] internet of 'action' (Iowa)[2], modern health-monitoring technologies[3] prosthetics[4] and augmented reality, etc. In spite of several endeavor in developing soft artificial skin, there are still several considerable hurdles to make soft-ao-skin suitable for practical use and industrial-scale production. For instance, most of the earlier works on soft-ao-skin at least have one or the other inadequacies such as highly complex and expensive fabrication process, consume high power, low sensitivity, shows instability at long-term, high response time and short operational bandwidths etc.
Therefore, we have attempts to resolve those issues by exploiting a novel design of the soft optical waveguide with a strong promise of mass production and improved sensitivity i.e. possible to attain sensitivity more than 750 times higher than the sensitivity of earlier work based on optical waveguide strategy[5] and longer stability, less response time ~ 63 ms.
WORKING OF THE SOFT-AO-SKIN
Figure 1a and b depict the soft artificial optical skin cell (SAOS-Cell), where soft hemispherical lens (SHL) is without and with the contact of soft Dove prism (SDP) respectively. Clearly, with no contact as in Figure 1a, light from an LED in air medium (air RI ~1) parallel to the longitudinal axis of the prism enters one sloped side of the SDP, get total internal reflection (TIR) from the top surfaces and refract back to a photodetector (PT) without loss. On the other hand, in Figure 1b, a fraction of the light ray escapes from getting TIR from the top surface of the SDP where both lens and SDP are in physical contact. The contact between SHL and SDP creates a bridge of homogeneous circular area of same refractive index (RI) through which light passes and reaches to the edge of the SHL Figure 1b. Figure 1c shows the schematic representation of the arrangement
of array of light source and the array of array of photodetector to make a complete soft-ao-skin. Figure 1e shows image of a complete soft-ao-skin. The light sources and the photodetectors are attached to the soft waveguide by using 3D printed cages.
Figure 1. Schematic representation of the working principle of electronic skin (soft-ao-skin). (a) Schematic diagram of soft-ao-skin unit cell (SAOS-Cell), a soft hemispherical lens (SHL) before touching a Dove prism shaped soft slab kept between light source (LS) and the photodetector (PT). Light passes through the SAOS-Cell from the LS to the PT without loss due to total internal reflection of light ray (TIRL). (b) SHL touches the surface of the SAOS-Cell, creates a circular contact area, light otherwise totally reflected, passes through the contact area in the expanse of lowering output intensity. (c) Schematic representation of soft-ao-skin with light sources and the array of detectors. (d) 3D representation of SAOS-Cell. (e)Image of the soft-ao-skin. (SDP- soft dove prism, SAOS- soft artificial optical skin).
RESULTS
Figure 2a shows the schematic representation of a custom-designed instrument to calibrate the SAOS-Cell in terms of load and voltage. The PDMS soft hemispherical lens made using a hemispherical mold (with a diameter of 3 mm) is attached to a computer-controlled micro-stage to exert load on the PDMS SDP. Optical images were recorded from the top during the application of loads 0, 0.13, 0.53,1.17, and 2 N on the SDP through a soft hemispherical lens (Figure 2c-g). Figure 2c show that no light appears at the edge of the soft hemispherical lens at zero applied load, i.e., light rays travel through the SDP without any loss (barring negligible losses due to absorption by the PDMS) and reach the photodetector. As a result, the PT shows maximum intensity at the output. However, with increasing applied load, the intensity at the edge of the soft hemispherical lens increases (Figure 2c-g), and that at the output decreases proportionally. The contact formation between the soft hemispherical lens and the SDP is depicted by capturing the images at various loads by replacing PT with an optical camera (Figure 2h-l). These optical images also confirm that the contact area between the SDP and soft hemispherical lens increases with increased load.
Figure 3a shows a plot of the output voltage versus the applied load for SAOS-Cell made of various soft hemispherical lenses like PDMS, Ecoflex, and PDMS oligomer-filled hemispherical lens (OFHL). In all cases, for small, applied loads, a linear relationship is observed between the applied load through the soft hemispherical lens on the SDP and the cubic power of the output intensity (V^3). This can be explained through the solid-solid contact deformation model introduced by Johnson, Kendall, and Roberts in 1971, popularly known as the JKR model [6]. Briefly, the relation among the contact radius (a), work of adhesion (W), elastic modulus (E), and applied load (P) is as follows:
a3 = —{P + 3 nWR + [6 nWRP + (3nWR)2]05} (1)
K
Here,
1 . fl-nf l-n%)
1 - 1 + 1 (3)
R Rt R2
P is an external load, nx and n2 are Poisson ratios of the materials, R is the radius of curvature of the soft hemispherical lens. Note that the result shows a linear relation between Vn and a, (SI, Figure S4a, measured for PDMS soft hemispherical lens), implying a linear relationship between F and V„ up to a specific load.
(a)
Motorized linear micro-stage —
CL _i
SHL-
Up /\
V7
Down
Lens
Side view
Photo detector
Condenser lens(CL)
1 Optical
bench
a
Soft Dove prism (SDP) ■■■ Light source(LS)
a
Lens
F(N)
0.13
0.53
1.17
Light at HL edge
Contact spot
Figure 2: Characterization of the soft-ao-skin. (a) Schematic diagram from the side view of a custom-made characterization tool for a soft-ao-skin. (b) Top view schematic of the experimental setup. (c-g) Images captured from the top of the soft hemispherical lens (SHL) during the application of loads of 0, 0.13, 0.53,1.17, and 2 N, respectively, on the SHL. (h-l) Optical images corresponding to the loads 0, 0.13, 0.53,1.17, and 2 N, respectively, on SHL when viewed from
the side (a camera replaces PT). The scale bar is 1 mm.
PDMS SHL of diameter 3 mm shows excellent linear relation of applied load with V„ up to ~1.2 N, however, beyond that load it follows nonlinearity. Within the linear region, the SAOS-units with SHL diameter 3mm show sensitivity as high as ~4.1, 13, and 21.2 kPa_1for PDMS, Ecoflex and OFHL soft hemispherical lens respectively. The results show the sensitivity of the soft-ao-skin can be improved more than 3, and 5 folds when the PDMS SHL is replaced by material of lower modulus Ecoflex SHL and OFHL respectively. Figure 3b shows the voltage output with respect to various applied step loads. Figure 2c insert image shows the graph for finding the response time of the soft-ao-skin. The measurement shows the response time of SAOS is ~63 ms. Figure 3c the calibration estimation of the SAOS at various time to shows the robustness and durability of the SAOS. The experiment from 2 to 5 depict the calibration of the SAOS after 10 months and experiment 1 represent the calibration of the SAOS before 10 months. The results show the SAOS exhibits excellent reproducibility in terms of calibration with an accuracy of more than 99.56%. This result shows a superior long-term stability of the SAOS compared to many soft artificial skins reported earlier[7].
0.9
0.7
Vn (V)
0.5
0.3
0.1
-0.1
- 1.0 \ 0.8 j \ (a)
- \ In 0.6 \ (V) f
- \ 0.4 0.2 0.0 'jT2 3
\ C .1 0.3 0.7 F(N) 1.1 1.5
\ 1A 1 1 2 3
■S.PDMS Ecoflex ■w Oligomer
Vn (V)
-0.5 0.5
1.5
2.5
F(N)
3.5 4.5
5.5
(c)
0 Exp 1
# Exp 2
# Exp 3
# Exp 4 O Exp 5
0 1 2 3 4 5
t (sec)
10 15
t (sec)
0.4 0.6 F(N)
0.8
1.0 1.2
Figure 3. Characterization of the soft-ao-skin unit node. (a) The plot shows the calibration between distance and load for various hemispherical lens of similar diameter 3mm. The curves 1, 2, and 3 depict SHL of 3 mm diameter made of PDMS, Ecoflex and PDMS membrane SHL filled with PDMS oligomer. (b) Step response in terms of normalized voltage at various applied loads and unloads. (c) Long term stability test in terms of soft-ao-skin calibration and response time (insert
image). PDMS- Polydimethylsiloxane.
DISCUSSION
The results show that the shape, i.e., the curvature (k), height (h), conic constant (k) of the hemispherical structure above the soft optical waveguide (SOW) plays an important role in determining the sensitivity (S) and the pressure detection range of the soft-ao-skin. The modulus of the soft hemispherical lens (considering other constant parameters) is also crucial in determining the sensitivity and pressure detection range of the soft-ao-skin. Another important finding from this work reveals that the waveguide design plays an important role in detecting the bending and spatial resolution of the soft-ao-skin. Results show that the sensitivity of the soft-ao-skin made of Ecoflex (Ecoflex-00-30) SHL with a mold-based technique and having a modulus ~ 0.07MPa is three times higher than soft-ao-skin made of PDMS SHL (PDMS: Crosslinking ratio; 10:1) with modulus ~ 2MPa. Further, for a soft hemispherical lens made of the oligomer-filled hemispherical lens (OFHL, membrane thickness ~20 ^m), a fivefold increment of sensitivity of the soft-ao-skin has been observed. However, this was accompanied by a reduction of pressure detection range from 350 kPa to 26 kPa. In the case of a 3D printed soft hemispherical lens, a sensitivity of 30kPa_1 is achieved, which in the case of oligomer filled SHL will lead to the maximum sensitivity of 150kPa_1 with a limit of detection (LoD) ~0.056 kPa(SI for LoD, Figure S10). In real-world applications, the sensor with ultra-high sensitivity at lower load regime and low sensitivity at higher load is desirable. The size of the sensing nodes will also play a role in the detection limit of the soft-e-skin. The smaller the sensing node's size, the lesser the detection limits would be. The waveguide design is important in determining the sensitivity, pressure detection range, the sensor nodes' spatial density, and the bendability of the soft-ao-skin. Using the LASER diode as a light source can give the advantage of not using a condenser lens in constructing soft-ao-skin. The result shows that if the soft-ao-skin is transparent, stray light may affect the performance of the soft-ao-skin depending upon the presence of environmental/stray light. Therefore, to overcome such a problem, an approach consists of a completely flexible opaque top membrane to obstruct the stray light can be deployed. Another attractive prospect of our soft-ao-skin is the use of SHLs as passive sensing elements rather than electrically connecting active/passive elements 6,29,31, which is usually vulnerable to disjointing. Hence, using the passive soft hemispherical lens gives the freedom to stretch the soft-ao-skin until the material fails. Considering the overall fabrication process of the soft-ao-skin, both the mold-based technique and 3D printing technique have their advantages and limitations; however, the 3D printing technique gives more freedom in tuning parameters of SHLs, and that gives the liberty to fine-tuning of soft-ao-skin parameters within a size limit of SHL. Also, the 3D printing technique is cost-effective compared to making new molds. One important issue with this soft-ao-skin is maintaining the position of the light source and photodetector unaltered after multiple uses by using 3D printed external holders. However, this issue can be addressed by introducing organic light-emitting diode (OLEDs) and organic photodetectors attached directly to the soft skin rather than attaching 3D printed holders.
In summary, many previous works have been reported on soft-ao-skin, however, most of them partially satisfy the criteria to be a complete realistically useable product for real-world use. For example, many previously reported soft-ao-skins show superior sensitivity but show long response time and instability in long term, etc. On the contrary, this work focuses on the realization of a practical, robust, suitable for industrial-scale production and easily scalable (3D printable) large-area soft artificial skin without compromising quality i.e. highly sensitive (750 times more sensitive), quick response time, bendability, stretchability etc. This reliable soft artificial skin is made by using a specially designed soft optical waveguide and soft hemispherical structure as a sensing node placed on the top of the waveguide. In addition, we have achieved significantly improved i.e. possible to attain sensitivity as high as 150 kPa"1 with a quick response of 63ms. The multifold increase in sensitivity is the outcome of a special design of the soft optical waveguide, and the unique aspherical soft hemispherical structures and their material property e.g. elastic modulus of the material. Moreover, we have observed the consistent performance of the soft-ao-skin up to almost a year, an indication of better reliability of the soft-ao-skin. Also, we have demonstrated a very convenient and cost-effective process of making the soft electronic skin by involving ultrafast liquid 3D printing technique (specially designed for this purpose) capable of printing large area with an ability of mass production. ACKNOWLEDGEMENTS
The authors acknowledge funding from the Department of Science and Technology, DST, India and IISc Bangalore, India.
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