(J-370) 3D printed skin-mountable strain sensor

Introduction:: Stretchable strain sensors have played an important role in wearable healthcare systems, soft robotics, biomedical field, and electronic skins. Many stretchable materials and fabrication methods have been studied to develop stable and sensitive sensors. However, further research is needed to obtain high sensitivity under large deformation of the strain sensors. Recently, auxetic structure can be applied to stretchable strain sensors for enhancing the sensitivity due to their negative Poisson’s ratio under tension. Herein, we have developed a 3D printed strain sensor designed in auxetic structure. Using finite element analysis (FEA), performance of the sensors with auxetic structure was evaluated to optimize the sensor design. A biocompatible conductive polymer was then 3D printed using extrusion-based 3D printing to fabricate a complex architecture of auxetic structure. Our 3D printed sensor was evaluated up to a maximum strain of 30%, the standard strain range associated with human body movements.


Materials and Methods:: Designing of auxetic structure was based on a myriad of kinds of auxetic structure in previous studies. Unit cells of each auxetic structure was modified in shapes and dimensions. Derived from widely used re-entrant auxetic structure, structure and dimension of each unit cells were modified. Deformation behavior and gauge factor was simulated using COMSOL Multiphysics 6.1 Structural Mechanics package. For computational simulation, properties of the material was adopted from previous studies. Conductive PEDOT:PSS solution (PH 1000, Ossila limited) was processed and mixed with deionized water-DMSO (Sigma-Aldrich) mixture (water:DMSO = 85:15 v/v) in concentration of 5wt% as 3D printable ink. Extrusion based 3D printer (Nordson EFD) was used to print silicone (Loctite SI 595) substrate and PEDOT:PSS ink. Printed auxetic structure PEDOT:PSS was connected to electrodes to measure sensitivity of real-time and deformation.


Results, Conclusions, and Discussions:: Modified auxetic structures was designed using computer aided design (CAD) program (Fig. 1a,b). Using COMSOL 6.1, deformation behaviors of two different auxetic structures were simulated (Fig 1c,d). These models were deformed with negative Poisson’s ratio where both horizontal and vertical direction are stretched (Fig 1c,d). Re-entrant structure exhibited lower negative Poisson’s ratio (v_xy=-0.52) compared to star honeycomb structure (v_xy=-0.24). When tensile stress is applied to the structure, their von Mises stress distribution showed different tendency. The re-entrant structure showed lower stress transferred to a unit cell far from the point of force applied (Fig. 1c). The star-honeycomb structure presented relatively uniform stress and deformation distribution, showing its structural stability under deformation (Fig. 1d). Extrusion-based 3D printer was used to fabricate the auxetic structure of the sensors using 5wt% PEDOT:PSS hydrogel ink (Fig. 1e-g). The 3D printed PEDOT:PSS ink showed a resistance of near 60Ω, and the thickness of the structure was around 0.1mm. The optimized sensor design exhibited increase in the resistance compared to most of the reported polymer-based strain sensors. Further, the 3D printed sensor was evaluated up to a maximum strain of 30%, the standard strain range associated with human body movements. Our 3D printed strain sensor with auxetic structure showed high sensitivity and strong adhesion to deforming skin, showing a great possibility to develop patient-specific sensors for more precise data acquisition.




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