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  1. 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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    Free, publicly-accessible full text available September 11, 2024
  2. The inflammation marker Interleukin 6 (IL-6) typically remains below 5 pg/mL in the serum of healthy individuals but can increase tenfold during inflammation in chronic conditions like COVID-19 and rheumatoid arthritis, as well as acute conditions like sepsis. This study is focused on the rapid detection of IL-6 to monitor both chronic and acute diseases. The novel sensor, designed with gold-coated micropyramids on the electrodes, was fabricated using the two-photon polymerization method, enabling low-volume sensing capabilities (2-3 μL). The micropyramids were surface functionalized with interleukin-6 antibodies towards developing an affinity biosensor specific to the physiological relevant range of IL-6 of 5.1 and 18.8 pg/mL in mild inflammation. Sensing was achieved by measuring impedance changes associated with IL-6 binding to the antibodies on the micropyramids interfaced using electrochemical impedance spectroscopy. It was observed that the signals from the lowest detection concentration was enhanced by 3 times at 1500 hz when the 532 nm green laser was incident on the micropyramids. This innovative approach can be expanded to the detection of cytokines not only in serum but also in respiratory samples. As a result, it opens up new avenues for monitoring local inflammation within the lungs and assessing systemic inflammation levels throughout the body. 
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    Free, publicly-accessible full text available March 2, 2025
  3. This study presents a novel design for a microlens coupler to transfer light from a straight waveguide to a single-mode fiber (SMF). Our design combines improved mode matching and enhanced alignment tolerance compared to edge coupling. An investigation of the alignment tolerance is done by assessing coupling efficiency under various degrees of manufacturing-induced misalignment. Singlet and diffractive lenses are incorporated into our design to focus the light into the fiber precisely. Comprehensive simulations demonstrate that the diffractive lens outperforms edge coupling and singlet lens in coupling efficiency. Fabrication methods such as additive manufacturing are discussed for future works. Our findings underscore the potential of innovative microlens coupler design. 
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    Free, publicly-accessible full text available January 21, 2025
  4. Recent developments in micro-scale additive manufacturing (AM) have opened new possibilities in state-of-the-art areas, including microelectromechanical systems (MEMS) with intrinsically soft and compliant components. While fabrication with soft materials further complicates micro-scale AM, a soft photocurable polydimethylsiloxane (PDMS) resin, IP-PDMS, has recently entered the market of two-photon polymerization (2PP) AM. To facilitate the development of microdevices with soft components through the application of 2PP technique and IP-PDMS material, this research paper presents a comprehensive material characterization of IP-PDMS. The significance of this study lies in the scarcity of existing research on this material and the thorough investigation of its properties, many of which are reported here for the first time. Particularly, for uncured IP-PDMS resin, this work evaluates a surface tension of 26.7 ± 4.2 mN/m, a contact angle with glass of 11.5 ± 0.6°, spin-coating behavior, a transmittance of more than 90% above 440 nm wavelength, and FTIR with all the properties reported for the first time. For cured IP-PDMS, novel characterizations include a small mechanical creep, a velocity-dependent friction coefficient with glass, a typical dielectric permittivity value of 2.63 ± 0.02, a high dielectric/breakdown strength for 3D-printed elastomers of up to 73.3 ± 13.3 V/µm and typical values for a spin coated elastomer of 85.7 ± 12.4 V/µm, while the measured contact angle with water of 103.7 ± 0.5°, Young’s modulus of 5.96 ± 0.2 MPa, and viscoelastic DMA mechanical characterization are compared with the previously reported values. Friction, permittivity, contact angle with water, and some of the breakdown strength measurements were performed with spin-coated cured IP-PDMS samples. Based on the performed characterization, IP-PDMS shows itself to be a promising material for micro-scale soft MEMS, including microfluidics, storage devices, and micro-scale smart material technologies.

     
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    Free, publicly-accessible full text available November 1, 2024
  5. 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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    Free, publicly-accessible full text available April 28, 2024
  6. Su, Zhongqing ; Limongelli, Maria Pina ; Glisic, Branko (Ed.)
    This paper aims to investigate the performance of piezoelectric sensors with different shapes of 3D-printed microstructures. Based on the numerical analysis in the time-frequency domain, the microstructures are printed directly on the PVDF transparent film exhibiting higher piezoelectric coefficients using a high-resolution two-photon polymerization method. Bi-directional gold IDTs are fabricated by sputtering gold onto the substrate surface using a 3D-printed stencil. The mechanical properties of the film and surface morphology of printed microstructures are examined using a nanoindenter and a 3D profilometer. The change in frequency response due to the microstructure is measured using a network analyzer. This study will be a reference for developing an efficient wave-based gas sensor with enhanced sensitivity. 
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    Free, publicly-accessible full text available April 18, 2024
  7. Significant progress into the development and use of stretchable sensors for structural health monitoring (SHM) has been made in the last several years. The fusion of stretchable, adaptable sensing materials with highly specialized additive manufacturing techniques allows for the development of highly adaptive, customizable, and easily accessible sensing solutions. However, a significant portion of these works explore SHM topics at a macro level, and with a reduced focus on implementation. As such, little application or experimentation into practical sensing elements, especially those at the micro scale, have followed the advances in sensing technology. In this work, we demonstrate the application of recent developments in stretchable electronics, alongside multiple advanced additive manufacturing processes, to develop a novel flexible microscale sensor. A complex sensor is designed and printed utilizing Digital Light Processing (DLP) to directly fabricate the structure. The printed sensor is then filled with a piezoresistive sensing element of either PEDOT:PSS or carbon-based PDMS (cPDMS), which provided strain readings via resistance change. After being filled with a sensing mixture, the sensor is shown to operate as desired under large deformations. Additionally, the sensor is shown to work effectively when embedded into a separate additively manufactured part. A flexible test coupon is manufactured using the DLP AM process, and a microsensor is embedded inside the coupon structure. This sensing systems is tested in both tension and bending. These results show the feasibility of implementing both modern day AM processes and into current structural health monitoring developments into practical applications. 
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  8. The demand for acoustic wave-based devices has been rapidly increasing in the aerospace, chemical, and biological fields due to their versatility towards sensing measurands. This paper explores the characteristics and effectiveness of acoustic wave-based two-port sensors designed with bidirectional IDT electrodes placed in different configurations, such as surface mounted or embedded inside the substrate, through numerical and experimental analysis. The numerical study involves 3D modeling of the sensor design to investigate wave characteristics by utilizing time-domain, i.e., time delay and wave patterns, and frequency-domain analysis, i.e., scattering parameter study. The sensor made of polyvinylidene fluoride polymer is modeled to ensure the concordance between the theoretical and numerical results as well as a preliminary experimental result obtained from transparent piezoelectric films. The coupling of modes theoretical model is used to obtain the device’s frequency response by a transmission matrix cascading technique. These investigated results will stand as guidance and facilitate defining an approach that can predict the behavior of the sensor with a specific design under different operating environments and expand its viability towards multi-functional devices that are reliable and sensitive to intended measurands. 
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  9. Zonta, Daniele ; Su, Zhongqing ; Glisic, Branko (Ed.)
  10. Space debris has been a growing concern in the space exploration sector. To reduce this issue, biomimicry is utilized to create gecko’s feet microstructures that can be attached to a gripper or robotic arm of the service spacecraft. However, the production of such microstructures is difficult and expensive which hinders their implementation. The objective of this research is to develop an advanced fabrication process to mass produce gecko’s feet microstructure with soft polymer mold. The possibility of using different coating methods using various coating materials are studied. The process of fabricating mold and replicating mold are studied and improved. The method of mass-producing microstructures is verified and the limitations of the method are also studied. 
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