24th International Colloquium Tribology - January 2024 213 Soft and Highly Sensitive Contact Pressure Sensors Based on Randomly Rough Surfaces Luciana Algieri 1,2 , Luigi Portaluri 1,2 , Marco Bruno 1,2 , Massimo De Vittorio 1,2 , Michele Scaraggi 1,2* 1 Center for Biomolecular Nanotechnologies, Istituto Italiano di Tecnologia, Arnesano (LE), 73010, Italy 2 Department of Innovation Engineering, University of Salento, Lecce, 73100, Italy * e-mail: michele.scaraggi@unisalento.it 1. Introduction (TAE_Heading) Soft contacts are widely used in macro, micro, and nano-electromechanical systems, including flexible electronics for applications in translational medicine, energy, and the automotive sector [1]. Contact mechanics properties such as friction, adhesion, thermal and electrical contact resistance, etc., do strictly depend on the roughness and rheological properties of the mating surfaces. Nowadays, they can be predicted by recurring to mean-field formulations of the contact mechanics [2]. The availability of such mean field theories makes it possible to construct bio-interfaces and -sensors based on arbitrarily rough surfaces, which are often characterized by a facile microfabrication process. In this work, we adopted randomly rough surfaces to develop capacitance-based soft contact pressure sensors. Due to its inherent characteristics, which include high sensitivity, temperature stability, low consumption, and simplicity, the capacitive transductive approach was chosen. Capacitive sensing can make use of the dielectric permittivity change, the overlapping area, or the distance between the plates. Changing the separation and the corresponding capacitance variation is the most widely used method of pressure sensing. Indeed, due to the applied pressure, the dielectric material between the parallel plate electrodes is compressed, resulting in a change in capacity. [3]. For this reason, an ideal dielectric layer should be as soft as possible to be highly sensitive to several pressure ranges. An effective approach toward realizing high sensitivity is to employ dielectric layer micropatternig. In the following, we established that the sensitivity of capacitive soft contact pressure sensors based on dielectric having random roughness is higher compared with the capacitive soft contact pressure sensors based on dielectric with deterministic topography. 2. Experimental Details Chemicals and materials PDMS used in the work was prepared by thermal curing a silicone elastomer (Sylgard ® 184) with a curing agent in a 10: 1 ratio. Nanoscribe IP-S photoresist was used in DLW configuration to obtain deterministic mold. The conductive glass slides used have a dimension of 25 x 75 mm with 25 nm of conductive Indium Tin Oxide (ITO) (Xin Yan Technology LTD). Poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrene sulfonate) (PEDOT: PSS, Sigma-Aldrich, 3-4 wt%) was used to cover the surface of PDMS domes. Soft Surface Fabrication Random deterministically-patterned roughness was fabricated in PDMS with dimensions of 8 mm × 8 mm by soft lithography and subsequently PDMS double casting method. The mold used in soft lithography is made to employ two different fabrication strategies. A pressurized water steam system was optimized to obtain the random roughness while a two-photon polymerization by Nanoscribe Photonic Professional GT system (Nanoscribe, Karlsruhe, Germany) was used to obtain a deterministic array. Surface Characterization The surface topographies were characterized with AFM (AFM Nano-Observer CSI ) and profilometer (Dektak XT Bruker) to extract the surface power spectral densities - needed in the contact mechanics theory. 3. Results and Discussion Experimentally, we have unraveled the role of roughness randomicity on the generation of capacitance thanks to the development of a home-made opto-electro-mechanical triboscope (Fig.1a). The contact interface is made by an ITO-coated, optically smooth microscope slide in contact with either a i) randomly rough self-affine or ii) deterministically-patterned PDMS layer with back electrode, the latter in adhesive contact on the top of a PDMS dome (Figure 1b). Both dome and layer share the same rheological properties. The surfaces have a root mean square roughness of ≈15 µm for both random and deterministic patterns. The power spectral density of the random roughness is reported in the inset of Figure 1b, whereas the deterministic pattern is constituted by a square array, with a lattice distance of 400 µm, of hemispheres with a radius of 100 µm. The domes are approached with a constant speed to the conductive glass with a load range of 0.1N to 1.6N. Changing the separation gap induced by the different loads applied allows for recording a variation in capacitance and therefore capacitive pressure sensing. The capacitive measurements were performed using the Digilent Analog Discovery 2, connected to the sample and the conductive glass while the triboscope allowed to record the image during the true contact. The contact area and the capacitance are reported as a function of the applied normal load (Fig.1c), where the black (blue) line is for the deterministic (random) topography. We observe that the capacitance range (sensitivity) is superior for the random roughness compared to the deterministic pattern, due to the multiscale nature of the random topography. In Figure 1 e) and f) we show the optically acquired true contact domains for the random and deterministic roughness cases, respectively. Different colors identify different simply connected contact patches. We note that a larger contact area is coupled with a smaller average 214 24th International Colloquium Tribology - January 2024 Soft and Highly Sensitive Contact Pressure Sensors Based on Randomly Rough Surfaces interface separation [2], thus an enhanced capacitance range, in agreement with our experimental findings. 4. Conclusion In summary, we built a highly sensitive soft capacitive pressure sensor based on random roughness using a novel and low-cost fabrication method. These remarkable characteristics show that the developed sensor can be implemented as a fully flexible sensing device in different applications, such as for robotics hands, artificial skin, and human health monitoring. References [1] R. B. Mishra et al., Advanced Materials Technologies 6, (2021),2001023. [2] M. Scaraggi et al., Journal of Chemical Physics 143 (2015), 224111. [3] Mishra, Rishabh B., et al. Advanced materials technologies-6.4 (2021): 2001023. Figure 1: (a) Schematic of the opto-electro-mechanical tribometer adopted in this study. (b) Contact interface, as made by an ITO-coated, optically smooth microscope slide in contact with a randomly (left) or deterministically (right) rough PDMS layer with back electrode. (c) Contact area (right) and the capacitance (left) as a function of the applied normal load. (e) and f) optically acquired contact domains for the random and deterministic case, respectively (at different loads).