The name artificial muscles has given dielectric elastomer actuators much attention in the field of medical technology. In healthcare applications, these compliant materials are particularly suitable for soft and lightweight actuators and in addition, they show many advantageous properties such as great extensibility for on-body sensors. However, the lack of industrial scaled production capabilities is still a limiting factor in the transfer of these promising materials into a broad application. To address this issue, we have modified an aerosol jet printer for automated stacking of electrodes and dielectric layers in one process device, which is described in earlier publications. As the performance is mainly dependent on the electrical and mechanical characteristics of the dielectric, a broad material variety is required to face different tasks in healthcare technology. Therefore, different medical grade silicones are investigated in this paper regarding the suitability for aerosol jet printing. In a first step, the printing parameters are derived from rheological testing of the uncured components that are used pure and in mixture with silicone oils to vary the specific Young’s module. Through heat application and dilution, the range for printable materials increases to silicones with initial viscosities up to 8200 mPas. Subsequently aerosol jet printing is used to manufacture stacked silicone dielectrics that are then investigated regarding their mechanical and electrical characteristics for dielectric elastomers. Finally, the collected parameters are compared and an overview of the feasible property combinations is given which can be used as a material guideline for different applications.
In healthcare products the proper fit of e.g. orthosis is crucial for its therapeutic success. To monitor movement and pressure, sensors with the ability to adapt their shape to the curvilinear form of the human body as well as a highly dynamic performance during motion are needed. Dielectric elastomer sensors (DES) present most of the required specifications and can be an excellent solution for body-near motion detection. For signal capturing either changes in resistance, capacitance or impedance can be measured. Therefore, different circuit configurations can be used that follow different methods e.g. frequency based or an impedance approach. In this paper, an impedance measurement bridge that enables the detection of stretch and pressure loads with a frequency of 200 Hz is modified. The novel architecture is conceptualized to work with a battery based energy supply and have small enough dimensions to function as a wearable with a wireless communication interface. Furthermore, our former presented additive manufacturing method for dielectric elastomers via aerosol jet printing allows an integrated tuning of the measurement bridge in the process. In addition, to the electrodes of the capacitor one electrode with a defined layer resistance is stacked on top of the DES. Hence, the additional electrode, which makes the architecture scalable for different sensor sizes and shapes, serves as an integrated adjustment of the measurement bridge. The data acquisition of multiple sensors is feasible by time division multiplexing, but is limited in its frequency due to the switching rate of the multiplexer.
The efficient manufacturing of dielectric layers and electrodes for dielectric elastomers is challenging especially for stacked configurations. In an earlier publication we describe a contactless programmable manufacturing approach for aerosol-jet-printing stacked silicone and reduced graphene oxide (rGO)-layers in one process device. One limitation of the setup presented there is the long production time of e.g. 45 minutes for a 1 cm2 electrode with a sheet resistance of 1 MΩ. This contradicts the goal of printing numerous layers for stacked systems and is related to the usage of an ultrasonic atomizer. Therefore we present a new hybrid atomizer, which significantly lowers the printing times of rGO-electrodes. This hybrid atomizer again uses an ultrasonic aerosol generation system. Hence, particle-inks without chemical stabilizers that would affect the curing process of printed adjacent silicone structures, can still be used in the system over hours without agglomeration. In addition, a pneumatic aerosol generation is integrated with the new hybrid atomizer. The combined material output of the pneumatic and the ultrasonic approach allows for the printing of rGO-electrodes in two minutes or less with characteristics comparable to those described above. Furthermore, a light-heating-system is integrated into the printing system which enables the fast evaporation of the solvents independently of the height of the previously stacked layers. For this new system, several printing strategies are evaluated by comparing different programmed trajectories of the print head. Printed rGO-electrodes with thicknesses from 3-20 μm and values for the sheet resistance from 1 MOhm to 200 kOhm are characterized regarding their ability to bear a maximal deformation up to 100 % and repeated cyclic stretching up to 25 %.
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