Shuying Wu

Strain Sensors with Adjustable Sensitivity by Tailoring the Microstructure of Graphene Aerogel/PDMS Nanocomposites

Strain sensors with high elastic limit and high sensitivity are required to meet the rising demand for wearable electronics. Here, we present the fabrication of highly sensitive strain sensors based on nanocomposites consisting of graphene aerogel (GA) and polydimethylsiloxane (PDMS), with the primary focus being to tune the sensitivity of the sensors by tailoring the cellular microstructure through controlling the manufacturing processes. The resultant nanocomposite sensors exhibit a high sensitivity with a gauge factor of up to approximately 61.3. Of significant importance is that the sensitivity of the strain sensors can be readily altered by changing the concentration of the precursor (i.e., an aqueous dispersion of graphene oxide) and the freezing temperature used to process the GA. The results reveal that these two parameters control the cell size and cell-wall thickness of the resultant GA, which may be correlated to the observed variations in the sensitivities of the strain sensors. The higher is the concentration of graphene oxide, then the lower is the sensitivity of the resultant nanocomposite strain sensor. Upon increasing the freezing temperature from -196 to -20 °C, the sensitivity increases and reaches a maximum value of 61.3 at -50 °C and then decreases with a further increase in freezing temperature to -20 °C. Furthermore, the strain sensors offer excellent durability and stability, with their piezoresistivities remaining virtually unchanged even after 10 000 cycles of high-strain loading-unloading. These novel findings pave the way to custom design strain sensors with a desirable piezoresistive behavior.

Novel Electrically Conductive Porous PDMS/Carbon Nanofiber Composites for Deformable Strain Sensors and Conductors

Highly flexible and deformable electrically conductive materials are vital for the emerging field of wearable electronics. To address the challenge of flexible materials with a relatively high electrical conductivity and a high elastic limit, we report a new and facile method to prepare porous polydimethylsiloxane/carbon nanofiber composites (denoted by p-PDMS/CNF). This method involves using sugar particles coated with carbon nanofibers (CNFs) as the templates. The resulting three-dimensional porous nanocomposites, with the CNFs embedded in the PDMS pore walls, exhibit a greatly increased failure strain (up to ∼94%) compared to that of the solid, neat PDMS (∼48%). The piezoresistive response observed under cyclic tension indicates that the unique microstructure provides the new nanocomposites with excellent durability. The electrical conductivity and the gauge factor of this new nanocomposite can be tuned by changing the content of the CNFs. The electrical conductivity increases, while the gauge factor decreases, upon increasing the content of CNFs. The gauge factor of the newly developed sensors can be adjusted from approximately 1.0 to 6.5, and the nanocomposites show stable piezoresistive performance with fast response time and good linearity in ln(R/R0) versus ln(L/L0) up to ∼70% strain. The tunable sensitivity and conductivity endow these highly stretchable nanocomposites with considerable potential for use as flexible strain sensors for monitoring the movement of human joints (where a relatively high gauge factor is needed) and also as flexible conductors for wearable electronics (where a relatively low gauge factor is required).

A new route to nanostructured thermosets with block ionomer complexes

We report a novel approach to prepare nanostructured thermosets using block ionomer complexes. Neither block copolymer polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) nor block ionomer sulfonated SEBS (SSEBS) is miscible with diglycidyl ether of bisphenol A (DGEBA) type epoxy resin. It is thus surprising that the block ionomer complex of SSEBS with a tertiary amine-terminated poly(e-caprolactone) (PCL), denoted as SSEBS-c-PCL, can be used to prepare nanostructured epoxy thermosets. The block ionomer complex SSEBS-c-PCL is synthesized via neutralization of SSEBS with 3-dimethylamino-propylamine-terminated PCL. Sulfonation of SEBS yields the block ionomer SSEBS which is immiscible with epoxy. But the block ionomer complex SSEBS-c-PCL can be easily mixed with DGEBA. When the curing agent 4,4′-methylenedianiline (MDA) is added and the epoxy cures, the system retains the nanostructure. In cured epoxy thermosets containing up to 30 wt% SSEBS-c-PCL, the exclusion of the poly(ethylene-ran-butylene) (EB) phase forms spherical micro-domains surrounded by separated sulfonated polystyrene phase while the PCL side-chains of SSEBS-c-PCL are dissolved in the cured epoxy matrix. The spherical micro-domains are highly aggregated in the epoxy thermosets containing 40 and 50 wt% SSEBS-c-PCL. The existence of epoxy-miscible PCL side-chains in the block ionomer complex SSEBS-c-PCL avoids macro-phase separation. Hence, the block ionomer complex can act as an efficient modifier to achieve nanostructured epoxy thermosets.

Multifunctional Polymer Nanocomposites Reinforced by Aligned Carbon Nanomaterials

Carbon nanomaterials such as carbon black (CB), carbon nanotubes (CNTs), and graphene have demonstrated significant potential as fillers to improve the electrical, thermal, and mechanical properties of polymers and their fiber-reinforced polymer composites. The level of improvement has been found to depend significantly on the degree of alignment of carbon nanomaterials. Due to the very small scale and complex interactions of carbon nanomaterials with polymers and structural fibers, alignment in a given direction has been a major challenge. Over the past decade, considerable effort has been devoted to developing effective strategies to align carbon nanomaterials in polymer matrices. However, significant technological challenges remain, and there is still a lack of understanding of the alignment mechanisms and their effects on the properties of polymers and composites. This paper reviews in situ alignment techniques including shear deformation, mechanical stretching, electrospinning, and application of an external magnetic or electric field, and ex situ techniques including using vertically grown CNTs or graphene. This review particularly focuses on physical mechanisms underpinning the magnetic or electric field-induced alignment and theoretical analyses that describe the different motions occurring and the major parameters controlling alignment. Moreover, this review highlights the recent research findings of the effects of alignment on the properties of polymer nanocomposites. The outlook towards the challenges and opportunities in this field are also discussed in this review.

Biotechnologies toward Mitigating, Curing, and Ultimately Preventing Edema through Compression Therapy

For a century-old problem, edema and its treatment have gone remarkably unnoticed by the biomedical community. Given the prevalence of lymphedema and its debilitating repercussions, there is an acute need for both efficacy-based measures and clinical standards to guide compression garment design and therapeutic application. This review outlines the current state of the art in compression treatment and suggests an integrated biomedical engineering approach going forward. Characterizing the pressure gradient profiles of commercial compression sleeves is necessary to better understand the role of compression treatment in the mitigation of swelling. Integration of pressure sensor technologies with advanced materials design and manufacture provides a critical path not only to elucidate the mechanisms of but also to improve on current compression-based therapies and associated therapeutic devices.