Dynamic Observation of Retinal Response to Pressure Elevation in a Microfluidic Chamber.

Dynamic observation of cell and tissue responses to elevated pressure could help our understanding of important physiological and pathological processes related to pressure-induced injury. Here, we report on a microfluidic platform capable of maintaining a wide range of stable operating pressures (30 to 200 mmHg) while using a low flowrate (2-14 µL/h) to limit shear stress. This is achieved by forcing flow through a porous resistance matrix composed of agarose gel downstream of a microfluidic chamber. The flow characteristics were investigated and the permeabilities of the agarose with four different concentrations were extracted, agreeing well with results found in the literature. To demonstrate the capability of the device, we measured the change in intracellular Ca2+ levels of retinal ganglion cells in whole mouse retina in response to pressure. The onset of enhanced pressure results in, on average, an immediate 119.16% increase in the intracellular Ca2+ levels of retinal ganglion cells. The demonstrated microfluidic platform could be widely used to probe cell and tissue responses to elevated pressure.

Rapid prototyping of cell culture microdevices using parylene-coated 3D prints.

Fabrication of microfluidic devices by photolithography generally requires specialized training and access to a cleanroom. As an alternative, 3D printing enables cost-effective fabrication of microdevices with complex features that would be suitable for many biomedical applications. However, commonly used resins are cytotoxic and unsuitable for devices involving cells. Furthermore, 3D prints are generally refractory to elastomer polymerization such that they cannot be used as master molds for fabricating devices from polymers (e.g. polydimethylsiloxane, or PDMS). Different post-print treatment strategies, such as heat curing, ultraviolet light exposure, and coating with silanes, have been explored to overcome these obstacles, but none have proven universally effective. Here, we show that deposition of a thin layer of parylene, a polymer commonly used for medical device applications, renders 3D prints biocompatible and allows them to be used as master molds for elastomeric device fabrication. When placed in culture dishes containing human neurons, regardless of resin type, uncoated 3D prints leached toxic material to yield complete cell death within 48 hours, whereas cells exhibited uniform viability and healthy morphology out to 21 days if the prints were coated with parylene. Diverse PDMS devices of different shapes and sizes were easily cast from parylene-coated 3D printed molds without any visible defects. As a proof-of-concept, we rapid prototyped and tested different types of PDMS devices, including triple chamber perfusion chips, droplet generators, and microwells. Overall, we suggest that the simplicity and reproducibility of this technique will make it attractive for fabricating traditional microdevices and rapid prototyping new designs. In particular, by minimizing user intervention on the fabrication and post-print treatment steps, our strategy could help make microfluidics more accessible to the biomedical research community.

Bidirectional Modulation of Contact Thermal Resistance between Boron Nitride Nanotubes from a Polymer Interlayer.

Enhancing the thermal conductivity of polymer composites could improve their performance in applications requiring fast heat dissipation. While significant progress has been made, a long-standing issue is the contact thermal resistance between the nanofillers, which could play a critical role in the composite thermal properties. Through systematic studies of contact thermal resistance between individual boron nitride nanotubes (BNNTs) of different diameters, with and without a poly(vinylpyrrolidone) (PVP) interlayer, we show that the contact thermal resistance between bare BNNTs is largely determined by reflection of ballistic phonons. Interestingly, it is found that a PVP interlayer can either enhance or reduce the contact thermal resistance, as a result of converting the ballistic phonon dominated transport into diffusion through the PVP layer. These results disclose a previously unrecognized physical picture of thermal transport at the contact between BNNTs, which provides insights into the design of high thermal conductivity BNNT-polymer composites.

Contact Thermal Resistance between Silver Nanowires with Poly(vinylpyrrolidone) Interlayers.

Various nanofillers have been adopted to enhance the thermal conductivity of polymer nanocomposites. While it is widely believed that the contact thermal resistance between adjacent nanofillers can play an important role in limiting thermal conductivity enhancement of composite materials, lack of direct experimental data poses a significant challenge to perceiving the effects of these contacts. This study reports on direct measurements of thermal transport through contacts between silver nanowires (AgNWs) with a poly(vinylpyrrolidone) (PVP) interlayer. The results indicate that a PVP layer as thin as 4 nm can increase the total thermal resistance of the contact by up to an order of magnitude, when compared to bare AgNWs, even with a larger contact area. On the other hand, the thermal boundary resistance for PVP/silver interfaces could be significantly lower than that between polymer-carbon nanotubes (CNTs). Analyses based on these understandings further show why AgNWs could be more effective nanofillers than CNTs.

Electrical and Thermal Transport through Silver Nanowires and Their Contacts: Effects of Elastic Stiffening.

Silver nanowires have been widely adopted as nanofillers in composite materials used for various applications. Electrical and thermal properties of these composites are critical for proper device operation, and highly depend on transport through the nanowires and their contacts, yet studies on silver nanowires have been limited to one or two samples and no solid data have been reported for individual contacts. Through systematic measurements of silver nanowires of different sizes, we show that the Lorenz number increases with decreasing wire diameter and has a higher value at wire contacts. Examination of the corresponding electrical and thermal conductivities indicates that these changes are due to contributions of phonons that become more important as a result of elastic stiffening. The derived contact thermal conductance per unit area between silver nanowires is ∼10 times that between carbon nanotubes. This helps to explain the more significant thermal conductivity enhancement of silver nanowires-based composites.

The relationship between the Young's modulus and dry etching rate of polydimethylsiloxane (PDMS).

Polydimethylsiloxane (PDMS) has been the pivotal materials for microfluidic technologies with tremendous amount of lab-on-a-chip devices made of PDMS microchannels. While molding-based soft-lithography approach has been extremely successful in preparing various PDMS constructs, some complex features have to been achieved through more complicated microfabrication techniques that involve dry etching of PDMS. Several recipes have been reported for reactive ion etching (RIE) of PDMS; however, the etch rates present large variations, even for the same etching recipe, which poses challenges in adopting this process for device fabrication. Through systematic characterization of the Young's modulus of PDMS films and RIE etch rate, we show that the etch rate is closely related to the polymer cross-link density in the PDMS with a higher etch rate for a lower PDMS Young's modulus. Our results could provide guidance to the fabrication of microfluidic devices involving dry etching of PDMS.

Thermal transport in electrospun vinyl polymer nanofibers: effects of molecular weight and side groups.

Understanding and enhancing thermal transport in polymers is of great importance, and is necessary to enable next-generation flexible electronics, heat exchangers, and energy storage devices. Over the past several decades, significant enhancement of the thermal conductivity of polymeric materials has been achieved, but several key questions related to the effects of molecular structure on thermal transport still remain. By studying a series of electrospun vinyl polymer nanofibers, we investigate the relationship between thermal conductivity and both molecular chain length and side group composition. For polyethylene nanofibers with different molecular weights, the measured thermal conductivity increases monotonically with molecular chain length, as energy transport along molecular chains is more efficient than between chains. The observed trend is also consistent with structural characterization by Raman spectroscopy, which shows enhanced crystallinity as molecular weight increases. Further, by comparing the measured thermal conductivity of vinyl polymer nanofibers with different side groups, we found that phonons travel along polymer chains more effectively when the side groups are either lighter or more symmetric. These experimental results help reveal the underlying correlation between the molecular structure and thermal conductivity of polymer nanofibers, providing valuable insights into the design of polymeric materials with enhanced thermal conductivity.