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1. Electrode filling using capillary action of 3D printed elastomer microchannels

   https://doi.org/10.1117/12.2658146  

   Stark, Taylor; Kim, Daewon (April 2023, SPIE)

   Madden, John D.; Anderson, Iain A.; Shea, Herbert R. (Ed.)

   Soft polymer actuators are in increasing demand due to their more fluid like motion and flexibility when actuated than compared with rigid actuators, which makes them valuable in diverse engineering applications. One of the main types of soft polymer actuators is the dielectric elastomer actuator, whose working principle is to apply a voltage potential difference between electrodes to reduce the thickness of the elastomeric material while expanding its area. This paper looks at manufacturing a micro soft polymer dielectric elastomer actuator utilizing two-photon polymerization 3D printing. The actuator contains micro channels that are filled with an electrode by using capillary action. A complex helical geometry is designed, printed, and tested for electrode filling capabilities. Quite a few obstacles are described in this paper including the use of a newly released two-photon polymerization resin which has limited supporting resources, as well as the complex helical geometry having a large compliance that vastly complicates its fabrication, post-processing, handling, electrode filling, electrode integration, and actuation testing. However, these challenges are overcome by using the standard printing recipes currently available for the resins, adding electrode isolation layers, and printing thicker elastomer zones for more structural support. The results found solidify the approach of filling microchannels with electrodes through capillary action and lead to further the focus and creation of multi-functional micro soft actuators. 

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2. Fully inkjet-printed multilayered graphene-based flexible electrodes for repeatable electrochemical response

   https://doi.org/10.1039/d0ra04786d  

   Pandhi, Twinkle; Cornwell, Casey; Fujimoto, Kiyo; Barnes, Pete; Cox, Jasmine; Xiong, Hui; Davis, Paul H.; Subbaraman, Harish; Koehne, Jessica E.; Estrada, David (October 2020, RSC Advances)

   null (Ed.)

   Graphene has proven to be useful in biosensing applications. However, one of the main hurdles with printed graphene-based electrodes is achieving repeatable electrochemical performance from one printed electrode to another. We have developed a consistent fabrication process to control the sheet resistance of inkjet-printed graphene electrodes, thereby accomplishing repeatable electrochemical performance. Herein, we investigated the electrochemical properties of multilayered graphene (MLG) electrodes fully inkjet-printed (IJP) on flexible Kapton substrates. The electrodes were fabricated by inkjet printing three materials – (1) a conductive silver ink for electrical contact, (2) an insulating dielectric ink, and (3) MLG ink as the sensing material. The selected materials and fabrication methods provided great control over the ink rheology and material deposition, which enabled stable and repeatable electrochemical response: bending tests revealed the electrochemical behavior of these sensors remained consistent over 1000 bend cycles. Due to the abundance of structural defects ( e.g. , edge defects) present in the exfoliated graphene platelets, cyclic voltammetry (CV) of the graphene electrodes showed good electron transfer ( k = 1.125 × 10 −2 cm s −1 ) with a detection limit (0.01 mM) for the ferric/ferrocyanide redox couple, [Fe(CN) 6 ] −3/−4 , which is comparable or superior to modified graphene or graphene oxide-based sensors. Additionally, the potentiometric response of the electrodes displayed good sensitivity over the pH range of 4–10. Moreover, a fully IJP three-electrode device (MLG, platinum, and Ag/AgCl) also showed quasi-reversibility compared to a single IJP MLG electrode device. These findings demonstrate significant promise for scalable fabrication of a flexible, low cost, and fully-IJP wearable sensor system needed for space, military, and commercial biosensing applications. 

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3. 3-D PRINTING OF DIELECTRIC ELECTROACTIVE POLYMER ACTUATORS AND CHARACTERIZATION OF DIELECTRIC FLEXIBLE MATERIALS

   Gonzalez, David; Newell, Brittany; Garcia, Jose; Noble, Lucas; Mamer, Trevor (October 2018, ASME Conference on Smart Materials, Adaptive Structures, and Intelligent Systems (SMASIS))

   Dielectric electroactive polymers are materials capable of mechanically adjusting their volume in response to an electrical stimulus. However, currently these materials require multi-step manufacturing processes which are not additive. This paper presents a novel 3D printed flexible dielectric material and characterizes its use as a dielectric electroactive polymer (DEAP) actuator. The 3D printed material was characterized electrically and mechanically and its functionality as a dielectric electroactive polymer actuator was demonstrated. The flexible 3-D printed material demonstrated a high dielectric constant and ideal stress-strain performance in tensile testing making the 3-D printed material ideal for use as a DEAP actuator. The tensile stress- strain properties were measured on samples printed under three different conditions (three printing angles 0°, 45° and 90°). The results demonstrated the flexible material presents different responses depending on the printing angle. Based on these results, it was possible to determine that the active structure needs low pre-strain to perform a visible contractive displacement when voltage is applied to the electrodes. The actuator produced an area expansion of 5.48% in response to a 4.3 kV applied voltage, with an initial pre-strain of 63.21% applied to the dielectric material. 

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4. A printed highly stretchable supercapacitor by a combination of carbon ink and polymer network

   https://doi.org/10.1016/j.eml.2021.101459  

   Song, Chiho; Chen, Baohong; Hwang, Jeonguk; Lee, Sujin; Suo, Zhigang; Ahn, Heejoon (November 2021, Extreme Mechanics Letters)

   A supercapacitor requires two electronic conductors with large surface areas, separated by an ionic conductor. Here we demonstrate a method to print a highly stretchable supercapacitor. We formulate an ink by mixing graphene flakes and carbon nanotubes with an organic solvent, and use the ink to print two interdigitated electronic conductors on the surface of a dielectric elastomer. We then submerge the printed electronic conductors in an aqueous solution of monomer, photoinitiator, crosslinker, and salt. The organic solvent and water form a binary solvent in which the ions are mobile. Upon UV irradiation, a polymer network forms. In each printed electrode, the graphene flakes and carbon nanotubes form a percolating network, which interpenetrates the polymer network. The electronic and ionic conductors form large interfacial areas. When the supercapacitor is stretched, the graphene flakes and carbon nanotubes slide relative to one another, and the polymer network deforms by entropic elasticity. The polymer network traps individual graphene flakes and carbon nanotubes, so that repeated stretch neither breaks the percolating network nor shorts the two electrodes. The supercapacitor maintains 88% the initial capacitance after 1600 cycles of stretch to five times its initial dimension. The interpenetration of a covalent network of elastic polymer chains and a percolating network of conductive particles is generally applicable for making stretchable ionotronic devices. 

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5. Additive Manufacturing of Photocurable PVDF-Based Capacitive Sensor

   https://doi.org/10.1115/SMASIS2023-111151  

   Srinivasaraghavan Govindarajan, Rishikesh; Ren, Zefu; Madiyar, Foram; Kim, Daewon (September 2023, American Society of Mechanical Engineers)

   Abstract The demand for the capacitive sensor has attracted substantial attention in monitoring pressure due to its distinctive design and passive nature with versatile sensing capability. The effectiveness of the capacitive sensor primarily relies on the variation in thickness of the dielectric layer sandwiched between conductive electrodes. Additive manufacturing (AM), a set of advanced fabrication techniques, enables the production of functional electronic devices in a single-step process. Particularly, the 3D printing approach based on photocuring is a tailorable process in which the resin consists of multiple components that deliver essential mechanical qualities with enhanced sensitivity towards targeted measurements. However, the availability of photocurable resin exhibiting essential flexibility and dielectric properties for the UV-curing production process is limited. The necessity of a highly stable and sensitive capacitive sensor demands a photocurable polymer resin with a higher dielectric constant and conductive electrodes. The primary purpose of this study is to design and fabricate a capacitive device composed of novel photocurable Polyvinylidene fluoride (PVDF) resin utilizing an LCD process exhibiting higher resolution with electrodes embedded inside the substrate. The embedded electrode channels in PVDF substrate are filled with conductive silver paste by an injection process. The additively manufactured sensor provides pressure information by means of a change in capacitance of the dielectric material between the electrodes. X-Ray based micro CT-Scan ex-situ analysis is performed to visualize the capacitance based sensor filled with conductive electrodes. The sensor is tested to measure capacitance response with changes in pressure as a function of time that are utilized for sensitivity analysis. This work represents a significant achievement of AM integration in developing efficient and robust capacitive sensors for pressure monitoring or wearable electronic applications. 

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