Three-dimensional
flexible porous conductors have significantly
advanced wearable sensors and stretchable devices because of their
specific high surface area. Dip coating of porous polymers with graphene
is a facile, low cost, and scalable approach to integrate conductive
layers with the flexible polymer substrate platforms; however, the
products often suffer from nanoparticle delamination and overtime
decay. Here, a fabrication scheme based on accessible methods and
safe materials is introduced to surface-dope porous silicone sensors
with graphene nanoplatelets. The sensors are internally shaped with
ordered, interconnected, and tortuous internal geometries (i.e., triply
periodic minimal surfaces) using fused deposition modeling (FDM) 3D-printed
sacrificial molds. The molds were dip coated to transfer-embed graphene
onto the silicone rubber (SR) surface. The presented procedure exhibited
a stable coating on the porous silicone samples with long-term electrical
resistance durability over ∼12 months period and high resistance
against harsh conditions (exposure to organic solvents). Besides,
the sensors retained conductivity upon severe compressive deformations
(over 75% compressive strain) with high strain-recoverability and
behaved robustly in response to cyclic deformations (over 400 cycles),
temperature, and humidity. The sensors exhibited a gauge factor as
high as 10 within the compressive strain range of 2–10%. Given
the tunable sensitivity, the engineered biocompatible and flexible
devices captured movements as rigorous as walking and running to the
small deformations resulted by human pulse.
This study presents a low‐cost, tunable, and stretchable sensor fabricated based on spandex (SpX) yarns coated with graphene nanoplatelets (GnP) through a dip‐coating process. The SpX/GnP is wrapped into a stretchable silicone rubber (SR) sheath to protect the conductive layer against harsh conditions, which allows for fabricating washable wearable sensors. Dip‐coating parameters are optimized to obtain the maximum GnP coating rate. The covering sheath is tailored to achieve high stretchability beyond the sensing limit of 104% for SpX/GnP/SR sensors. Adjustable sensitivity is attained by manipulating SpX immersion times broadening its application for a wide range of strains: Gauge factors as high as two orders of magnitude are achieved at tensile strains greater than ≈40%. The fabricated sensors are tested for two applications: First, the SpX/GnP sensors are integrated into composite fabrics (with no negative impact on the structural integrity of the part) for screening the yarn displacements, resin flow, solidification during the hot press forming process, and structural health monitoring under mechanical loads with minimal cross‐sensitivity to temperature/humidity. Second, the capability of SpX/GnP/SP sensors in detection of a wide range of bodily motions (from the joint motion to arterial blood pressure) is demonstrated.
Laser additive manufacturing has led to a paradigm shift in the design of next-generation customized porous implants aiming to integrate better with the surrounding bone. However, conflicting design criteria have limited the development of fully functional porous implants; increasing porosity improves body fluid/cell-laden prepolymer permeability at the expense of compromising mechanical stability. Here, functionally gradient porosity implants and scaffolds designed based on interconnected triply periodic minimal surfaces (TPMS) are demonstrated. High local porosity is defined at the implant/tissue interface aiming to improve the biological response. Gradually decreasing porosity from the surface to the center of the porous constructs provides mechanical strength in selective laser melted Ti−6Al−4V implants. The effect of unit cell size is studied to discover the printability limit where the specific surface area is maximized. Furthermore, mechanical studies on the unit cell topology effects suggest that the bending-dominated architectures can provide significantly enhanced strength and deformability, compared to stretching-dominated architectures. A finite element (FE) model developed also showed great predictability (within ∼13%) of the mechanical responses of implants to physical activities. Finally, in vitro biocompatibility studies were conducted for two-dimensional (2D) and threedimensional (3D) cases. The results of the 2D in conjunction with surface roughness show favored physical cell attachment on the implant surface. Also, the results of the 3D biocompatibility study for the scaffolds incorporated with a cell-laden gelatin methacryloyl (GelMA) hydrogel show excellent viability. The design procedure proposed here provides new insights into the development of porous hip implants with simultaneous high mechanical and biological responses.
X-ray computed tomography provides qualitative and quantitative structural and compositional information for a broad range of materials. Yet, its contribution to the field of advanced composites such as carbon fiber reinforced polymers is still limited by factors such as low imaging contrast, due to scarce X-ray attenuation features. This article, through a review of the state of the art, followed by an example case study on Micro-computed tomography (CT) analysis of low X-ray absorptive dry and prepreg carbon woven fabric composites, aims to highlight and address some challenges as well as best practices on performing scans that can capture key features of the material. In the case study, utilizing an Xradia Micro-CT-400, important aspects such as obtaining sufficient contrast, an examination of thin samples, sample size/resolution issues, and image-based modeling are discussed. The outcome of an optimized workflow in Micro-CT of composite fabrics can assist in further research efforts such as the generation of surface or volume meshes for the numerical modeling of underlying deformation mechanisms during their manufacturing processes.
Temperature and humidity
measurements in electrochemical energy
devices are essential for maximizing their overall performance under
different operating conditions and avoiding hazardous consequences
that may arise from the malfunction of these systems. Using sensors
for in situ measurements of temperature and relative humidity (RH)
is a promising approach for continuous monitoring and management of
electrochemical power sources. Here, we report on the feasibility
of using thread-based sensors for in situ measurements of temperature
and RH in proton exchange membrane fuel cells (PEMFCs) as an example
of electrochemical energy devices. Commodity threads are low-cost
and flexible materials that hold great promise for the creation of
complex three-dimensional (3D) circuits using well-established textile
methods such as weaving, braiding, and embroidering. Ex situ and in
situ characterization show that threads can be introduced in the gas
diffusion layer (GDL) structure to inscribe water highways within
the GDL with minimal impact on the GDL microstructure and transport
properties. Fluorinated ethylene propylene (FEP) is coated on thread-based
sensors to decouple the response to temperature and humidity; the
resulting threads achieve a linear change of resistance with temperature
(−0.31%/°C), while RH is monitored with a second thread
coated with poly(dimethylsiloxane) (PDMS). The combination of both
threads allows for minimally invasive and dynamically responsive monitoring
of local temperature and RH within the electrode of PEMFCs.
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