Rapid Microwave Polymerization of Porous Nanocomposites with Piezoresistive Sensing Function

Polymer Processing under Microwaves

Over the last decades, microwave heating has experienced a great development and reached various domains of application, especially in material processing. In the field of polymers, this unusual source of energy showed important advantages arising from the direct microwave/matter interaction. Indeed, microwave heating allows regio-, chemio-, and stereo-selectivity, faster chemical reactions, and higher yields even in solvent-free processes. Thus, this heating mode provides a good alternative to the conventional heating by reducing time and energy consumption, hence reducing the costs and ecological impact of polymer chemistry and processing. This review states some achievements in the use of microwaves as energy source during the synthesis and transformation of polymers. Both in-solution and free-solvent processes are described at different scales, with comparison between microwave and conventional heating.

Recent Progress in Conducting Polymer Composite/Nanofiber-Based Strain and Pressure Sensors

The Conducting of polymers belongs to the class of polymers exhibiting excellence in electrical performances because of their intrinsic delocalized π- electrons and their tunability ranges from semi-conductive to metallic conductive regime. Conducting polymers and their composites serve greater functionality in the application of strain and pressure sensors, especially in yielding a better figure of merits, such as improved sensitivity, sensing range, durability, and mechanical robustness. The electrospinning process allows the formation of micro to nano-dimensional fibers with solution-processing attributes and offers an exciting aspect ratio by forming ultra-long fibrous structures. This review comprehensively covers the fundamentals of conducting polymers, sensor fabrication, working modes, and recent trends in achieving the sensitivity, wide-sensing range, reduced hysteresis, and durability of thin film, porous, and nanofibrous sensors. Furthermore, nanofiber and textile-based sensory device importance and its growth towards futuristic wearable electronics in a technological era was systematically reviewed to overcome the existing challenges.

Radio frequency heating and material processing using carbon susceptors

Carbon nanomaterials have been shown to rapidly evolve heat in response to electromagnetic fields. Initial studies focused on the use of microwaves, but more recently, it was discovered that carbon nanomaterial systems heat in response to electric fields in the radio frequency range (RF, 1-200 MHz). This is an exciting development because this range of radio frequencies is safe and versatile compared to microwaves. Additional RF susceptor materials include other carbonaceous materials such as carbon black, graphite, graphene oxide, laser-induced graphene, and carbon fibers. Such conductive fillers can be dispersed in matrices such as polymer or ceramics; these composites heat rapidly when stimulated by electromagnetic waves. These findings are valuable for materials processing, where volumetric and/or targeted heating are needed, such as curing composites, bonding multi-material surfaces, additive manufacturing, chemical reactions, actuation, and medical ablation. By changing the loading of these conductive RF susceptors in the embedding medium, material properties can be customized to achieve different heating rates, with possible other benefits in thermo-mechanical properties. Compared to traditional heating and processing methods, RF heating provides faster heating rates with lower infrastructure requirements and better energy efficiency; non-contact RF applicators or capacitors can be used for out-of-oven processing, allowing for distributed manufacturing.

Conductive and injectable hyaluronic acid/gelatin/gold nanorod hydrogels for enhanced surgical translation and bioprinting

There is growing evidence indicating the need to combine the rehabilitation and regenerative medicine fields to maximize functional recovery after spinal cord injury (SCI), but there are limited methods to synergistically combine the fields. Conductive biomaterials may enable synergistic combination of biomaterials with electric stimulation (ES), which may enable direct ES of neurons to enhance axon regeneration and reorganization for better functional recovery; however, there are three major challenges in developing conductive biomaterials: (1) low conductivity of conductive composites, (2) many conductive components are cytotoxic, and (3) many conductive biomaterials are pre-formed scaffolds and are not injectable. Pre-formed, noninjectable scaffolds may hinder clinical translation in a surgical context for the most common contusion-type of SCI. Alternatively, an injectable biomaterial, inspired by lessons from bioinks in the bioprinting field, may be more translational for contusion SCIs. Therefore, in the current study, a conductive hydrogel was developed by incorporating high aspect ratio citrate-gold nanorods (GNRs) into a hyaluronic acid and gelatin hydrogel. To fabricate nontoxic citrate-GNRs, a robust synthesis for high aspect ratio GNRs was combined with an indirect ligand exchange to exchange a cytotoxic surfactant for nontoxic citrate. For enhanced surgical placement, the hydrogel precursor solution (i.e., before crosslinking) was paste-like, injectable/bioprintable, and fast-crosslinking (i.e., 4 min). Finally, the crosslinked hydrogel supported the adhesion/viability of seeded rat neural stem cells in vitro. The current study developed and characterized a GNR conductive hydrogel/bioink that provided a refinable and translational platform for future synergistic combination with ES to improve functional recovery after SCI.

Toward Enhanced Humidity Stability of Triboelectric Mechanical Sensors via Atomic Layer Deposition

Humid conditions can disrupt the triboelectric signal generation and reduce the accuracy of triboelectric mechanical sensors. This study demonstrates a novel design approach using atomic layer deposition (ALD) to enhance the humidity resistance of triboelectric mechanical sensors. Titanium oxide (TiO_x) was deposited on polytetrafluoroethylene (PTFE) film as a moisture passivation layer. To determine the effective ALD process cycle, the TiO_x layer was deposited with 100 to 2000 process cycles. The triboelectric behavior and surface chemical bonding states were analyzed before and after moisture exposure. The ALD-TiO_x-deposited PTFE showed three times greater humidity stability than pristine PTFE film. Based on the characterization of TiO_x on PTFE film, the passivation mechanism was proposed, and it was related to the role of the oxygen-deficient sites in the TiO_x layer. This study could provide a novel way to design stable triboelectric mechanical sensors in highly humid environments.

PDMS Sponges with Embedded Carbon Nanotubes as Piezoresistive Sensors for Human Motion Detection

Porous piezoresistive sensors offer promising flexible sensing functionality, such as human joint motion detection and gesture identification. Herein, a facile fabrication method is developed using a microwave-based rapid porogen removal technique for the manufacturing of porous nanocomposite sponges consisting of polydimethylsiloxane (PDMS) and well-dispersed carbon nanotubes (CNTs). The porogen amounts and CNT loadings are varied to tailor the porosity and electrical properties of the porous sensors. The sponges are characterized by a scanning electron microscope (SEM) to compare their microstructures, validate the high-quality CNT dispersion, and confirm the successful nanofiller embedding within the elastomeric matrix. Sponges with a 3 wt% CNT loading demonstrate the highest piezoresistive sensitivity. Experimental characterization shows that the sponges with low porosity have long durability and minimal strain rate dependence. Additionally, the developed sponges with 3 wt% CNTs are employed for the human motion detection using piezoresistive method. One experiment includes fingertip compression measurements on a prosthetic hand. Moreover, the sensors are attached to the chest, elbow, and knee of a user to detect breathing, running, walking, joint bending, and throwing motions.