Plasma-Assisted Deposition of Natural Polymers for Flexible Biosensor Applications

Flexible biosensors have gained increased attention in the scientific and medical community in the last decades. They provide an easy and fast way of monitoring the physiological condition of the human body, collecting and analyzing different types of data according to their location. Flexible biosensors for health monitoring can be distinguished into 2 categories: implantable biosensors and wearable devices. Temperature, contraction, elongation, pressure, and motion together with a wide range of physiological metabolites, including lactate, cortisol, and other tiny ions can be measured from sweat or tears using biosensors attached to the skin. On the other side, for the detection of (bio)chemical analytes within the body, such as metabolites, proteins, and biomarkers, the implantable biosensor should be in contact with the biological environment, such as the surface of organs, the endothelial walls of veins, the brain connections together with the internal physiological fluids (blood, saliva, or cerebrospinal fluid). Flexibility and stretchability are the key characteristics of flexible biosensors, which enable adaptability to the physical dynamics and non-rigid environment of the human body and enhance the interaction between analytes and sensing elements. Moreover, specific applications require implantable or wearable biosensors to provide a high level of biocompatibility and also biodegradability together with the essential requirements of biosensors, such as accuracy, selectivity, sensitivity, repeatability, and stability. Biocompatibility is essential for both types of flexible biosensors to prevent any kinds of inflammation or adverse reaction of the device, while biodegradability is necessary for implantable biosensors to avoid additional surgical intervention to remove the device from the body. To fulfill these requirements (flexibility, biodegradability, and biocompatibility), a diverse set of biocompatible and biodegradable polymers from synthetic and natural origins have been proposed for flexible biosensor production. Despite the advantages of synthetic polymers in terms of processing, stability, and mechanical properties, natural polymers are preferable for many applications due to their enhanced bioactivity, biodegradability, and biocompatibility. Among natural polymers, chitosan and silk fibroin have been widely investigated for biosensor production due to their remarkable properties, such as nontoxicity, and immunological activity. Different production approaches, from a thin coating deposition to the production of relatively big solid constructs, have been applied to natural polymers in order to achieve the necessary structure complexity for the realization of the biosensor structure. Generally, for flexible biosensors biopolymers have been proceeded into a film/layer/coating configuration as flexible substrates, electroactive (sensing) parts, or insulating layer. Despite the progress in the fabrication processes, the continuous evolution of flexible biosensors from 2-3 layered structures into multilayers structures requires the development of novel deposition/production methods for natural polymers in order to provide a high level of adhesion between layers, stability, and patterning capabilities. One of the possibilities is to use atmospheric plasma technologies, especially atmospheric pressure plasma jet (torch) (APPJ), that have been investigated for different types of biomedical applications. The increased attention got plasma processes related to surface cleaning, modification, and film formation. The combination of etching and deposition might allow the formation of the desired sensor structure. Moreover, cold plasma provides mild and technological processes at room temperature, with low changes in material properties. The core of the thesis is to develop novel deposition methods for the production of flexible biosensors. A special focus is put on the plasma- assisted deposition of silk fibroin and chitosan, representing 2 different natural polymers structures, proteins, and polysaccharides respectively, for a wide range of applications: bioactive coatings with enhanced adhesion and stability with no need for pre or post-treatment steps; layered biofilms with the ability to control and guide cell behavior; PEDOT:PSS electroactive coatings for the realization of biosensor layered structures.