Recently, high performance flexible sensors are in great demand for improving the proprioceptive and exteroceptive capabilities of novel soft robots, for the health monitoring of complex structures such as morphing aircrafts and for the physiological-biomechanical monitoring with wearable devices. The project aims to develop a new sensing material for the real-time monitoring of small/large deformations and potentially interesting for those applications where lightness, relevant stretchability, compressibility and high sensitivity are required. In particular, the activity will be focused on the development of innovative piezoresistive sensors made of novel three-dimensional (3D) graphene nanoplatelets (MLG)/elastomer-based ordered open-cell foams, surpassing the limits of current graphene-based foams with disordered morphology that precludes the optimization of the material multifunctional properties. Basically, the cheap and easy-to-fabrication approach is constituted by a few steps. Firstly, either sacrificial acrilonitrile butadiene styrene (ABS) or poly(vinyl alcohol) (PVA) templates are 3D printed; successively they are infiltrated either with a neat ultra-soft silicone or with the same elastomer properly loaded with MLG; the ABS (PVA) is then leached in acetone (water) and the final structured polymer foam coated with a MLG film. Electrical, mechanical, thermal and piezoresistive tests together with morphological analyses will be used to develop a multi-physics model able to predict the resistance variation of the foams caused by cyclic applied stresses and attributable to the local modifications of the MLG percolative network. An ad-hoc electronic interface with excitation, conditioning circuits to boost the signal-to-noise ratio will be built and used to wirelessly transfer the output of the sensor module, after calibration, to a mobile phone thus verifying the feasibility to monitor remotely and in real-time different signals (i.e., pressure, strain).
The scientific relevance of the project is pointed out by the increasing number of publications in high-impact journals describing new strategies for the development of soft, high sensitive piezoresitive sensors using elastomer composites filled with micro-nano conducting particles, conductive polymers incorporated into elastomers, microstructured materials or foam-sponge like materials for health monitoring, medical diagnosis, electronic skins and soft robots.
However, to our knowledge, there is at the moment no publication or patent pending on graphene-based foam for sensor applications having the characteristics described in this project. The proposed piezoresistive foams represent a promising material that can provide high sensitivity for a very broad applied pressure/deformation range while maintaining excellent strength and flexibility, and manufacturable through a cost-effective, easy synthesis procedure. The research is innovative: hence, it is highly foreseeable the publication of several articles as well as the proposal of different patents.
In particular, the project aims to achieve both scientific and technological advancements over the state of the art.
Regarding the scientific goals, the project is aimed at:
a) Understanding the correlation between the ordered microarchitectural details of the elastomeric foam (shape, dimensions and degree of interconnection of pores, porosity) and morphological information of the graphene-based network (MLG sizes, MLG coverage) with the macroscopic physical and functional properties (electrical conductivity, thermal conductivity, stress-strain response, piezoresistivity) of the material produced through electrical, thermal, mechanical and electromechanical characterizations.
b) Understanding the effect of temperature on the electrical conductivity, stretchability, compressibility and electromechanical response of the produced foams.
c) Understanding the reasons and which one of the two types of proposed foams developed with the proposed fabrication routes provides better response in terms of sensitivity, limit-of-detection, sensing recoverability and reproducibility (that are also related to hysteresis and relaxation phenomena).
d) Developing a phenomenological multi-physics model able to describe the piezoresistive behavior of foams with ordered structure when subjected to quasi-static and cyclic stresses.
Regarding technological achievements, the project is aimed at:
a) Developing methods based on the use of additive manufacturing for the fabrication of two novel stretchable, ultra-soft foams with highly ordered structure and outstanding piezoresistive properties.
In particular, the investigated materials are:
- a 3D Ecoflex porous scaffold internally coated with a MLG film;
- a 3D porous nanocomposite made of a MLG-filled Ecoflex scaffold internally coated with a MLG film.
b) Exploiting the two types of foams to fabricate proof-of-concept reliable piezoresistive sensors with sensitivity and flexibility suitable, for example, for high-performance wearable applications. According to the achieved results the best type of foam will be directed towards use as pressure or/and deformation sensor. Moreover, considering their multifunctional characteristics, the produced Gr-based foam can also represent a possible interesting cost-effective and performant solution for different thermal management applications.
c) Validating the developed sensor modules for real time monitoring of physiological parameters and muscular movements.