Flexible pressure sensors via engineering microstructures for wearable human-machine interaction and health monitoring applications

Flexible pressure sensors capable of transducing pressure stimuli into electrical signals have drawn extensive attention owing to their potential applications for human-machine interaction and healthcare monitoring. To meet these application demands, engineering microstructures in the pressure sensors are an efficient way to improve key sensing performances, such as sensitivity, linear sensing range, response time, hysteresis, and durability. In this review, we provide an overview of the recent advances in the fabrication and application of high-performance flexible pressure sensors via engineering microstructures. The implementation mechanisms and fabrication strategies of microstructures including micropatterned, porous, fiber-network, and multiple microstructures are systematically presented. The applications of flexible pressure sensors with microstructures in the fields of wearable human-machine interaction, and ex vivo and in vivo healthcare monitoring are comprehensively discussed. Finally, the outlook and challenges in the future improvement of flexible pressure sensors toward practical applications are presented

Study on interfacial properties of unidirectional flax/vinyl ester composites: resin manipulation on vinyl ester system

Flax fibers are widely used as reinforcements in bio-based polymer matrix composites. This study investigated the hydrophilic nature and surface purity of flax fiber that affects fiber/matrix adhesion in combination with hydrophobic structural polymers via matrix modification and the utilization of fiber treatment, specifically in a flax/vinyl ester (VE) composite. A new method to manipulate the vinyl ester system with acrylic resin (AR) was developed to produce flax reinforced. On the other hand, different types of chemical and physical treatments were applied on the flax fiber. FTIR was applied to evaluate the effects of surface treatments. Dynamic mechanical analysis (DMA) was used to analyze the unmodified and modified VE resin system. The surface of untreated and treated flax fibers and their composites were analyzed by scanning electronic microscopy (SEM). Sodium ethoxide-treated flax/ VE with 1% (wt) AR caused the best mechanical performance among all the flax/VE composites evaluated

Phase changes under efflorescence in alkali activated materials with mixed activators

Efflorescence in alkali-activated materials is a strong function of precursor and activator composition, which dictates their engineering properties and durability. In this study, the efflorescence of naturally cured NaOH/Na2SiO3 alkali-activated fly ash and alkali-activated fly ash-slag blended binder mixes was assessed with alkali concentration of 9 wt% Na2O, and 10 to 30 wt% of slag, and compared with binder mixes with 9 wt% Na2O, and 10 to 30 wt% of slag along with 2 wt% Na2CO3. The effects of efflorescence were assessed using visual and leaching inspections, and the compressive and split tensile strengths were determined post activation. Atomic absorption spectrometry was used to quantify free alkalis in the leachate, and X-ray diffraction, and Fourier transform infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance and thermo-gravimetric analysis were performed to analyse the phase changes in binder pastes after efflorescence. The increased slag content facilitated the formation of C-A-S-H gel and enhanced both chemical and mechanical properties of binder pastes. Furthermore, the inclusion of slag content also led to the reduction of the open porosity and efflorescence formation. Subsequent exposure of binder specimens to efflorescence conditions aided the formation of carbonate products, degradation of N-A-S-H and N-(C)-A-S-H gel, and a decrease in split tensile strength in the binder paste specimens

Acrylonitrile butadiene styrene (ABS)/lignocellulosic fiber biocomposite: Effect of artificial weathering on impact properties

Impact properties of four ABS grades have been investigated as a function of artificial weathering under ultraviolet (UV)/condensation conditioning. Natural-colored, carbon-black-filled, and two lignocellulosic biocomposites filled with sunflower hull (SFH) and distillers' dried grains with solubles (DDGS) were used in this study. The neat ABS and filled grades were extruded and injection molded. Notched and unnotched Izod impact testing was performed to determine the impact resistance at 0 h and 168 h of UV/condensation conditioning. Chromatometry was performed to monitor color change and Fourier-transform infrared spectroscopy (FTIR) was used to assess molecular changes because of photo-oxidation. Scanning electron microscopy (SEM) was used for fractography of UV/condensation-exposed and impact fracture surfaces. The impact resistance of the lignocellulosic-filled ABS grades showed higher property retention at exposed condition in comparison to neat ABS. The analyses were supported by electron microscopy and FTIR spectroscopy

Polylactide/hemp hurd biocomposites as sustainable 3D printing feedstock

Industrial hemp hurd (HH) is emerging as a bio-based filler in thermoplastic biocomposites. In this paper, HH/polylactide (PLA) biocomposites were developed as fused deposition modelling (FDM) feedstock through parametric analysis of the effects of HH loading with respect to melt flow, rheology, physical, thermo mechanical, and mechanical properties of the biocomposites. Poly (butylene adipate-co-terephthalate) (PBAT) and ethylene-methyl acrylate-glycidyl methacrylate terpolymer (EGMA) were used as toughening and compatibilisation agents respectively in melt-compounding and extrusion to produce FDM filament. The FDM-printed standard samples were compared against corresponding injection-moulded biocomposites. The FDM filament exhibited a diameter tolerance within ±0.02 mm, and roundness variability below 0.03 mm, and the FDM-printed parts with HH loading under 30 phr showed higher impact toughness than the commercial PLA filament control. In addition, the FDM-printed samples exhibited greater dimensional accuracy with increasing HH loading

Evaluating the heat resistance of thermal insulated sandwich composites subjected to a turbulent fire

The fire structural response of sandwich composite laminates incorporating bio-derived constituents subjected to a turbulent flaming fire was investigated. Fire structural tests were conducted on thermal insulated sandwich composites incorporating a thin surface-bonded non-woven glass fibre tissue impregnated with char-forming fire retardant, ammonium polyphosphate. The sandwich composite laminates were loaded in compression at 10%, 15% or 20% of the ultimate compressive strength while simultaneously subjected to turbulent flames imposing an incident heat flux of 35 kW/m2. Generally, the failure time increased with the reduced applied compressive load. The thermal insulated sandwich composite laminates had considerably improved fire resistance in comparison to their unmodified counterparts. The unmodified composites failed 96 s earlier than the thermal insulated specimens when the compression load was 10% of the ultimate compressive strength. The presence of ammonium polyphosphate at the heat-exposed surface promoted the formation of a consolidated char layer, which slowed down heat conduction into composite laminate substrate. The fire reaction parameters measured via the cone calorimeter provided insights into the thermal response hence fire structural survivability of sandwich composite laminates

Effects of wood fiber size on the performance of biodegradable foam

Biodegradable foam for cushion packing materials was prepared with wood fiber and starch through mold foaming. This study investigated the effect of wood fiber size and content on the mechanical properties of the foam. The results showed the size and content of the wood fiber bearing significant influences on the density, compressive strength, and tensile strength of the resultant foams. Lower size of wood fiber aided in better foaming, and a 40 wt% of 125–180 μm wood fiber yielded the best mechanical properties among the blends investigated. The behavior of the foaming agent was a function of the foaming temperature, and 150 °C was deemed as the optimum temperature for foaming. The compressive strength increased with an increase in wood fiber fraction, whereas the tensile strength decreased with increased wood fiber fraction. Overall, physical and mechanical properties of the biodegradable foams developed herein showed potential as cushion packing materials

A review of extending performance of epoxy resins using carbon nanomaterials

Carbon nanomaterials are receiving worldwide attention because of their multi-faceted superiority in thermal conductivity, flame retardancy, mechanical stability, electrical conductivity, and biocompatibility. In this review, a survey of the literature on extending performance of epoxy resins based on carbon nanomaterials is presented. The structure-performance relationships for different carbon nanomaterials modified epoxy are closely analyzed. The performance extension in mechanical, electrical, thermal conductivity, flame retardancy, antidegradation, and tribological properties of epoxy are reviewed in detail. Other application areas including biocompatibility, biodegradability, gas barrier properties, shape memory, and electromagnetic interference shielding are touched. The challenges and opportunities in carbon nanomaterials functionalized epoxy composites are also discussed

Synergistic flame retardancy effect of graphene nanosheets and traditional retardants on epoxy resin

Novel non-toxic halogen-free flame retardants are replacing traditional flame retardants in polymer and polymer matrix composite structures. In this study, graphene nanosheet (GNS) is investigatedin combination with traditional layered double hydroxide (LDH), layered rare-earth hydroxide (LRH), and phosphorus-based flame retardant (DOPO) to enhance the flame retardancy of epoxy resin. A synergistic flame retardancy effect is achieved in GNS/LDH and GNS/DOPO systems where combined GNS and LDH increased the viscosity of the epoxy melt, and limited the flame propagation through inhibition of dripping. The limiting oxygen index of epoxy increased from 15.9 to 23.6 with addition of 0.5 wt. % each of GNS and LDH. With the addition of 2.5 wt. % of both GNS and LDH, the total heat release of epoxy resin also reduced from 33.4 MJ/m2 to 24.6 MJ/m2. The synergistic effect of GNS and DOPO adopted a different mechanism. The addition of 2.5 wt. % of GNS and DOPO reduced the peak heat release rate from 1194 kW/m2 to 396 kW/m2, and the total heat release rate from 72.5 MJ/m2 to 48.1 MJ/m2. The synergistic mechanisms of the flame retardants were closely analyzed and correlated with the flame retardant properties

Enhanced toughness of PLLA/PCL blends using poly(d-lactide)-poly(ε-caprolactone)-poly(d-lactide) as compatibilizer

Poly(l-lactide) (PLLA)/poly(ε-caprolactone) (PCL) blends traditionally show low ductility because of the immiscibility between PLLA and PCL. In this study, this ductility challenge was addressed by modifying the compatibility between PLLA and PCL using poly(d-lactide)-poly(ε-caprolactone)-poly(d-lactide) (PDLA-PCL-PDLA or PCDL) tri-block copolymer. PLLA/PCL and PLLA/PCL blends with 0.7 phr and 3.5 phr PCDL were prepared by melt-compounding and extrusion and analyzed. The compatibilized PLLA/PCL blend with 3.5 phr of PCDL exhibited an elongation-at-break of 43%, compared to 18% in uncompatibilized PLLA/PCL, although PLLA/PCL/PCDL3.5 showed the higher crystallinity of 10.0% compared to 3.1% in PLLA baseline. The stereocomplexation effect between PLLA and PDLA was confirmed with a melting peak of a stereocomplex crystallite at 212 °C through differential scanning calorimetry. PCDL compatibilization improved miscibility between PLLA and PCL as evidenced through the interfacial morphology analysis, and supported by the rheological analysis, which elucidated the enhanced melting viscosity and interfacial adhesion of PLLA/PCL. Overall, the compatibilization of PLLA/PCL blends with PCDL was effective in achieving an enhanced interfacial morphology and adhesion, and improved elongation-at-break