The vision of wearable biomedical systems capable of monitoring, healing, or even replacing damaged functionalities in the human body is gradually turning into foreseeable future technology, supported by the advancements in miniaturized and flexible electronics. Among these, electronic skin (e-skin) is one of the most interesting concepts, aiming to mimic the tactile and sensory properties of natural skin, in applications as prosthetics, robotics, and vital sign monitoring.
A central requirement of such a device is biocompatibility. For this aim, we have proposed to develop PepZoSkin (peptide-based piezoelectric skin), an ultra-thin, flexible, self-powered e-skin device that combines innovative piezoelectric materials, microelectronics, and sensors for wearable and implantable applications. Piezoelectricity describes the capacity of certain materials to generate an electric potential when subjected to mechanical stress and vice-versa. In the frame of the ERC- BISON-694426 project, our team has successfully synthesized a family of novel piezoelectric peptide-based materials. These new materials are composed of di- or tri-peptides (e.g. hydroxyproline-phenylalanine-phenylalanine (Hyp-Phe-Phe)), that self-assemble into 3D structures, which can be arranged as a thin layer. Our vision is to turn these materials into a key component of next-generation compact and self-powered wearable and implantable systems, targeting e-skin applications as a market entry point.
The project was composed of three main levels: 1. The material level- choosing the best peptides for the task and developing a deposition method for fabricating the optimized thin layer; 2. Developing an in-house system for piezoelectrical measurements; and 3. Testing the samples and optimizing the results.
At the first level, the deposition method developed herein was based on the study of peptide self-assembly and growth under various conditions to produce the optimized alignment. The effect of various factors such as humidity, peptide concentration, and temperature, on structure formation during the deposition process was examined. The formed structures were evaluated under scanning electron microscope and the best architectures were selected for further processing and analysis. These included the selection of the most fitting electrodes and optimization of their contact points.
At the second level, we first used a commercial piezometer which is considered to be a useful instrument to determine the piezoelectric performance of materials. However, we faced several challenges with these tests, including the inability to obtain reproducible and valid results. We therefore decided to design a measurement system for piezoelectric performance characterization in our lab rather than to use a commercial piezometer. Our costume-made piezoelectric testing lab for piezoelectric performance evaluation was designed to measure the direct piezoelectric effect, i.e. measure the voltage generated by a piezoelectric sample in response to force application. We successfully established a reliable, costume-made, piezoelectric measurement system.
At the third level, using our newly-designed measurement system, the piezoelectric response and coefficients of the aligned peptide layer were analyzed. Our results emphasize the challenge of realizing the full potential of such materials at the macroscale, thereby fabricating a functional piezoelectric unit. The piezoelectric performance can be further improved by applying an electric field during self-assembly. We have recently purchased a high-voltage power supply and we are currently working on the integration of electric field application during the deposition.
