Flexible PDMS Foams Decorated with Multi–Walled CNTs for Unprecedented Detection of Ultralow Strain and Pressure Coupled with Large Working Range
Authors: Iglio Rossella , Mariani Stefano , Robbiano Valentina , Strambini Lucanos , Barillaro Giuseppe 
Published: February 2020

Over the last years a huge effort has been paid to the development of flexible and wearable strain and pressure sensors for, though not limited to, health care applications. In spite of the tremendous effort paid over the last years, the analysis of the literature on this field points out that it was still unmet the challenge to achieve high performance piezoresistive sensors with simultaneous ultralow limit of detection and large working range both for pressure and strain, able to address both subtle (1Pa-1 kPa) to medium (up to 100 kPa) pressures and ultrasmall (≤0.1%) to large (>50%) strains.
In this work, we demonstrate unprecedented simultaneous detection of ultrasmall strain (0.1%) and subtle pressure (20 Pa) coupled with a large working range (up to 60% for strain and 50 kPa for pressure)using piezoelectric, flexible three-dimensional (3D) macroporous PDMS (pPDMS) foams with pores of 500µm decorated with a thin network of pristine multi-walled CNTs. Limit of detection of our sensors are 0.03% for strain (i.e) and 6 Pa for pressure, which corresponds to a minimum detectable displacement of 3 µmover a length of 10 mm and to a minimum detectable force of 0.6 mN over an area of 1 cm2, given that the pPDMS/CNTs foams of this work are cubes of 1 cm in size. Adding to these excellent features, the foams can reliably support deformation up to 6 mm (60% strain) and measure a maximum pressure of about 50 kPa (i.e. a force of 5 N over an area of 1 cm2).
In a few words, our foams are able to detect both displacement and pressure produced by a fly (about 70 mg of weight), at the same time being able to measure displacement and pressure produced by an Apple iPad (about 500 g).

Remarkably, preparation of our 3D pPDMS/CNTs foams is low-cost and up-scalable. In fact, it is based on simple replica molding of sacrificial sugar templates and subsequent drop-casting of CNT ink drops. We show in the manuscript that the preparation is highly robust in terms of both static and dynamic piezoelectric performance, also for long cycling. Further, by simply changing geometry and size of the sacrificial sugar templates, the foams can be arbitrarily shaped with different areas, ranging from <1 cm2 up to 10 cm x 10 cm, and thicknesses, ranging from 1 mm to 10 mm, and geometry tunable from simple element to complex meshes.
We envisage the use our flexible, piezoresistive strain/pressure sensors for high-precision personalized healthcare and, more broadly, for wearable applications, though applications to other fields where displacement and pressure need to be monitored with extremely high resolution can be also envisaged.
[1] Flexible Polydimethylsiloxane Foams Decorated with Multiwalled Carbon Nanotubes Enable Unprecedented Detection of Ultralow Strain and Pressure Coupled with a Large Working Range, R Iglio, S Mariani, V Robbiano, L Strambini, G Barillaro, ACS Applied Materials & Interfaces 10 (16), 13877-13885 (2018).