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Development of biocompatible ionic electromechanically active polymer actuator/sensor

Periodic Reporting for period 1 - BIOACT (Development of biocompatible ionic electromechanically active polymer actuator/sensor)

Reporting period: 2018-09-01 to 2020-08-31

Bioinspired devices and soft robotics are of great interest in nowadays science. Technological development towards biomimetic systems requires replacement for traditional actuators (thermochemical motors, electromagnetic drivers and hydraulic or pneumatic machines). Classical engineering uses joined rigid parts, but biological structures are flexible and generate motion without rigid mechanical constituents. Moreover, in biological systems the materials often fulfil different roles at the same time, increasing efficiency.
Electroactive polymers (EAPs) possess several biomimetic characteristics. It is a large family of different materials that response to external electrical stimulation with change in size or shape. The current project focused on ionic electromechanically active polymers (IEAP). Potential applications for IEAPs include implantable or disposable biomedical devices, smart prosthesis, soft haptic devices and wearable electronics. The mentioned applications require biocompatibility from the materials, which has remained challenging during decades on research in EAPs. The aim of the current project was to develop biocompatible IEAPs functioning as actuators/sensors applicable in smart medical devices.
A typical IEAP is a soft thin laminate composed of a microporous ion-permeable polymer membrane placed between electrodes with a high specific surface area consisting of either metals, conductive polymers or carbon materials. The system is swollen by an appropriate electrolyte, for example ionic liquid (IL). IEAPs can work as actuators or as motion sensors: transducing between electric current and mechanical deformation.
To achieve biocompatible IEAPs, all the components need to be low toxicity and safe to use. There are suitable candidates for electrode and membrane materials but the key towards biocompatible IEAPs is non-toxic ILs working as electrolytes. The research carried out towards the aim had two objectives. First, to develop low toxicity ILs to be applied as electrolytes in the IEAPs. Choline ILs and their mixture were proposed as safer alternative to the rather toxic imidazolium ILs currently used. Secondly, to prepare and characterize biocompatible IEAPs consisting of biopolymer membrane, polypyrrole (PPy) electrodes and the developed biofriendly electrolytes.
According to the proposed objectives, the research carried out during the project focused first on the development of low toxicity choline ILs and after that the prepared ILs were applied to prepare biocompatible IEAPs. The electro-chemo-mechanical performance of the materials was evaluated and compared with the benchmarked IEAP actuators.
A series of choline based ionic liquids and ionic liquid mixtures was synthesized and characterized (physico-chemical properties, thermal properties, toxicity). The choline cation was chosen due to its proven biocompatibility: choline salts have shown low toxicity towards various phyla of living organisms. Moreover, the choline salts have many biological functions in the human body and are vital for building up cell membranes and neurotransmitters. All the synthesised salts contained carboxylate anion, but the alkyl chain lengths, branching and number of carboxylic groups were varied. Toxicological properties of the ILs were evaluated using different bacteria (Escherichia coli, Staphylococcus aureus, Shewanella oneidensis MR-1) and cell-lines (HeLa, C2C12). Although all the tested ILs were found to have low toxicity, there were remarkable differences depending on the number of carboxylic groups in the anions. The synthesised ILs were further used for studying the possibilities to form eutectic mixtures. This approach is useful for fine-tuning the properties of electrolytes, especially the melting point and viscosity. Phase diagrams for the mixtures were constructed. The results are important for widening the knowledge about behaviour of IL mixtures. Although the various mixtures of imidazolium based ILs have been evaluated before, the data concerning choline ILs was lacking before the current study. Moreover, a computational method predicting the melting points for ionic liquids was developed. This helps to design ILs with required properties and decrease the experimental burden.
Following the developed ILs were used for preparation of biocompatible IEAPs. As electrode material PPy was chosen due to its proven biocompatibility (suitable substrate for cell growth, implantable). For membrane electrospun gelatin was compared with the widely used and medically approved PVdF. Performance of the choline IL electrolytes was tested against 1-ethyl-3-methylimidazolium trifluromethanesulfonate commonly used in the IEAPs. The electro-chemo-mechnical performance of the developed actuators was evaluated using different driving signals varying in frequencies and shapes. Obtained strain differences describing bending of the actuators were comparable with the benchmarked IEAPs. Computational methods such as MD and DFT simulations were used to shed light on the differences in the performance of the IEAPs depending on the applied electrolyte. The results revealed that clustering of the choline cations in the ILs strongly influences the actuation and strain difference of the IEAPs.
Results of the project were published in 4 peer-reviewed journal articles and disseminated to scientific community in conference presentation and to the broader audience through special events (art exhibitions, public lecture) and media releases.
During the past 20 years the IEAPs have gone through remarkable development in terms of preparation methods and performance. Among the different type of the IEAPs the current project focus to conductive polymers as these show the highest potential for medical applications. The project progressed beyond state of the art by combining biopolymers and low toxicity ILs to prepare biocompatible IEAP actuators. The main result of the project was finding an optimal combination of materials for achieving safety without compromising the performance.
The EAP field is currently undergoing a transition from academia into commercialization meaning that the industrial companies are showing interest to use and invest in the technology. The project addressed to the timely need for biocompatible materials to bring the IEAPs closer to applications. Future prospect for the developed materials would be continuous research towards soft robotic devices. The quality of societies depends strongly on understanding and utilisation of the materials. The novel materials are fundamental for over 70% of all technology-based innovation. It is expected that the project outcomes will have positive societal impact through future research in medical engineering to develop novel multifunctional devices such as catheters and guidewires, cochlear implants and exoskeletons. New assistive and care-wearables will have major impact in medicine. Due to the increasing proportion of elderly people, the demand for skeletal muscle assistance units for gait control will grow in the coming decades. The new technologies using smart materials such as IEAPs are expected to reduce the clinical costs associated with the assistance and rehabilitation of people with neurological and age-related disorders.
The image describes working principle of a trilayer ionic electroactive polymer actuator.