Abstract—We develop highly flexible and compressible 3D interconnected porous PDMS structures by using sugar cubes as templates. The porous structures can be compressed to more than 80% without any side-wall buckling comparable with bulk polymers. Force sensitive resistors were fabricated by filtration of the CNT solution inside the porous structure of PDMS. We found that sufficient acid treatment can increase the adhesion and bonding between CNTs and PDMS. Force sensitive resistors respond the applied pressure and compressive strains by high linearity (R 2 >0.97) and sensitivity (GFs>2) with a reliable manner. Finally, as an application of our force sensitive resistors, a flexible foot pad containing force sensitive resistors arrays is developed for the foot sole distributed force detection. I. INTRODUCTION Development of flexible, stretchable and wearable electronic devices has recently drawn tremendous attention due to their numerous applications in biomedicine and robotics [1-4]. For example, flexible, stretchable and wearable strain sensors were utilized for the human motion detection [1, 5]; artificial electronic skins were developed for the pressure sensing and visualization [2, 6]; ultra-thin flexible sensors were used for continues temperature monitoring of the human skin [7] or flexible and transparent batteries were employed for the endoscopy application [4]. Pressure sensors transduce the pressure to the electrical changes such as resistance and capacitance. Capacitive type pressure sensors are made by a dielectric layer laminated between two electrodes. These types of pressure sensors exhibit a good linearity with low hysteresis. However, low sensitivity, limited pressure measurement ranges and low compressibility are main drawbacks of the capacitive type pressure sensors [8]. Furthermore, piezoresistive type pressure sensors respond to external stimuli by the change of resistance. Even though they might have high sensitivity and wide pressure measurement ranges, pressure sensors have suffered from nonlinearity and high hysteresis [9]. In particular, high performance pressure sensors are need for foot plantar pressure measurements because of their potential applications in rehabilitation and personal health monitoring, sport performance and injury prevention, footwear design and diagnosing disease [9]. High linearity, low hysteresis, temperature independency, wide pressure measurement ranges (i.e. 0-2 MPa for typical applications and up to 3 MPa for extreme conditions), reliability and low cost of fabrication *Corressponding author: I. Park, Tel: +82-42-350-3240; inkyu@kaist.ac.kr are among minimum requirements of pressure sensors for the foot plantar pressure measurement devices [9]. Herein, we describe force sensitive resistor (FSR) sensors based on the CNTs-porous PDMS composites. To overcome the bulk mechanism of compressibility for polymers, we proposed the porous polymer structure which achieves high flexibility, stretchability and compressibility (up to 80%) without any side-wall buckling. The porous PDMS structures were fabricated by using sugar cubes as templates. FSR sensors were made by the filtration of CNTs inside the 3D interconnected porous structure of PDMS. We found that acrylic acid treatment can remarkably improve the interfacial adhesion and bonding between CNTs and polymer. FSR sensors respond to 70% of the compressive strain with a linear manner (R 2 >0.97), high sensitivity (GFs>2) and good reliability. As an application of our FSR sensors, we fabricated a foot plantar measurement device for the foot sole distributed force detection. II. MATERIALS AND SAMPLE FABRICATION Multi-walled carbon nanotubes (MWCNTS) with an average length (10-30 µm) and diameter (20-30 nm) were purchased from Cheap Tubes Inc. 0.5 wt.% of CNTs in isopropyl alcohol (IPA) solution was sonicated for an hour in sonication bath. The solution was further stirred for an hour and well-suspended CNT solution was stored for further experiments. Fabrication processes of the porous PDMS are illustrated in Figure 1. A very simple and low cost porous structured PDMS was fabricated by using sugar cubes as templates [10, 11]. The liquid PDMS with 7.5 wt.% of curing agent was cast on a sugar cube. The liquid PDMS penetrated into the 3D interconnected porous structure of sugar cube owing to the low viscosity and low surface energy of the liquid PDMS [1]. The sugar cube casted with liquid PDMS was then put into a vacuum chamber for an hour to facilitate the penetration. After complete penetration of the liquid PDMS into the porous structure of the sugar cube, the liquid PDMS was cured at 70 °C for 2 hours forming a composite of the sugar powders and PDMS. Then, excess PDMS was trimmed away and the sugar powder-PDMS composite was immersed into the hot water at 80 °C for 2 hours to completely dissolve the sugar powders (Figure 1). Highly 3D interconnected porous structured PDMS was obtained by washing and drying the samples. Even though the structure is highly porous, it is very robust and stretchable. Unlike the bulk polymers, the porous PDMS is very compressible (compressibility> 80%) without side-wall buckling. Therefore, the porous PDMS structures would be quite useful for the pressure sensing applications Flexible and Sensitive Foot Pad for Sole Distributed Force Detection Morteza Amjadi 1, 2 Min Seong Kim 2 and Inkyu Park 1, 2 * 1 Department of Mechanical Engineering, KAIST, South Korea 2 Mobile Sensor and IT Convergence (MOSAIC) Center, KAIST Institutes (KI), KAIST, South Korea Proceedings of the 14th IEEE International Conference on Nanotechnology Toronto, Canada, August 18-21, 2014 978-1-4799-4082-0/$31.00 ©2014 IEEE 764