Zhenwen Ding

Laser-treated hydrophobic paper: an inexpensive microfluidic platform

We report a method for fabricating inexpensive microfluidic platforms on paper using laser treatment. Any paper with a hydrophobic surface coating (e.g., parchment paper, wax paper, palette paper) can be used for this purpose. We were able to selectively modify the surface structure and property (hydrophobic to hydrophilic) of several such papers using a CO(2) laser. We created patterns down to a minimum feature size of 62±1 µm. The modified surface exhibited a highly porous structure which helped to trap/localize chemical and biological aqueous reagents for analysis. The treated surfaces were stable over time and were used to self-assemble arrays of aqueous droplets. Furthermore, we selectively deposited silica microparticles on patterned areas to allow lateral diffusion from one end of a channel to the other. Finally, we demonstrated the applicability of this platform to perform chemical reactions using luminol-based hemoglobin detection.

Selective Nanofiber Deposition through Field-Enhanced Electrospinning

In this paper, we demonstrate a nanofiber patterning technique using field-enhanced electrospinning. Polyethylene oxide (PEO) nanofibers were electrospun on an elastomeric substrate with gold-coated pyramidal protrusions with the majority of fibers being deposited at the tips. The deposited nanofiber spots ranged from 8 x 8 microm(2) to 60 x 60 microm(2) in size, uniformly covering an area of 5 x 10 mm(2). Our experiments also indicate that nanofiber pattern selectivity is highly dependent on the separation/size ratio of the pyramidal protrusions with a ratio of <1, resulting in a superior selectivity.

Ferrofluid-Impregnated Paper Actuators

In this paper, we report on an inexpensive method of fabricating miniature magnetic actuators using ferrofluid impregnated paper. Different types of papers (including soft tissue paper, cleanroom paper, Whatman-1 filter paper, printer paper, and newspaper) were loaded with oil-based ferrofluid, microma-chined by a CO2 laser and coated with a thin layer of parylene-C. The soaking capability of the different papers was investigated, with the soft tissue paper having the highest loading capacity, being able to absorb ferrofluid by as much as six times its original weight. Cantilever actuators fabricated from cleanroom and filter papers were able to generate the largest force (>; 40-mg equiva lent force), whereas the soft-tissue-paper cantilevers provided the greatest deflection (40° tip angle).

A pH-tunable hydrogel microlens array with temperature-actuated light-switching capability

In this letter, we demonstrate a two step casting process to fabricate a bifunctional hydrogel-based microlens array, which responds to both temperature (becomes opaque above certain temperature) and pH (changes its focal length at different pH levels), and can be operated in air for an extended period of time. Each lens in the array is 1 mm in diameter and its focal length changes from 4.5 to 55 mm when the environmental pH is varied between 2.0 and 5.0. The light-switching capability is measured to be ∼92% when temperature increases from 25 to 35 °C.

Aqueous microdrop manipulation and mixing using ferrofluid dynamics

In this letter, the authors present a simple method to manipulate free microdroplets using ferrofluid dynamics. For droplet transport, a set of periodic lines of ferrofluid on top of a silicon wafer is created by a single strip magnet and dynamically changed by the rotation of a magnetic stirrer underneath it. It is demonstrated that the speed of droplet movement depends on the rotation speed of the magnetic stirrer as well as the size of the droplet. For better droplet mixing efficiency, a discontinuous pattern at the mixing spots is created by adding a smaller strip magnet to the above setup.

Reflective Diffraction Gratings From Hydrogels as Biochemical Sensors

We report reflective diffraction gratings made from smart hydrogels for ultrasensitive biochemical detection. As an example for a stimuli-responsive hydrogel, we chose a pH-sensitive hydrogel to construct diffraction gratings that swell/shrink reversibly due to changes in pH. Interferometric analysis of the grating enabled detection of the hydrogels motions with nanoscale precision and resulted in a resolution of 6 × 10-4 pH units. The developed system is remarkably simple both to fabricate and operate, and yet extremely sensitive. Moreover, the concept of the reflective hydrogel grating is generic and can be applied detection of a wide range of other stimuli.

Stretchable microelectrode array using room-temperature liquid alloy interconnects

In this paper, we present a stretchable microelectrode array for studying cell behavior under mechanical strain. The electrode array consists of gold-plated nail-head pins (250 µm tip diameter) or tungsten micro-wires (25.4 µm in diameter) inserted into a polydimethylsiloxane (PDMS) platform (25.4 × 25.4 mm2). Stretchable interconnects to the outside were provided by fusible indium-alloy-filled microchannels. The alloy is liquid at room temperature, thus providing the necessary stretchability and electrical conductivity. The electrode platform can withstand strains of up to 40% and repeated (100 times) strains of up to 35% did not cause any failure in the electrodes or the PDMS substrate. We confirmed biocompatibility of short-term culture, and using the gold pin device, we demonstrated electric field pacing of adult murine heart cells. Further, using the tungsten microelectrode device, we successfully measured depolarizations of differentiated murine heart cells from embryoid body clusters.

Diffractometric biochemical sensing with smart hydrogels

We report reflective diffraction gratings made from smart hydrogels for ultrasensitive biochemical detection. As an example for a biochemically responsive hydrogel, we chose a pH-sensitive hydrogel to construct diffraction gratings that swell or shrink due to changes in pH. Interferometric analysis of the grating enabled detection of the hydrogels swelling/shrinking with nanoscale precision with a resolution of 6×10−4 pH units. The developed system is remarkably simple both to fabricate and operate. Moreover, the concept of the hydrogel grating is generic and can be widely applied to hydrogels responsive to other stimuli.

A Stretchable Cell Culture Platform with Embedded Electrode Array

In this paper, we present a stretchable electrode array for studying cell behavior subjected to mechanical strain. The electrode array consists of four gold nail-head pins (250¿m tip diameter and 1.75mm spacing) inserted into a polydimethylsiloxane (PDMS) platform (25.4×25.4mm2). Fusible indium alloy (liquid at room temperature) filled microchannels are used to connect the electrodes to the outside, thus providing the required stretchability. The electrode platform is biocompatible and can withstand strains of up to 40%. We tested these electrodes by repeatedly (100 times) subjecting them to 35% strain and did not notice any failure. We also successfully cultured mice cardiomyocytes onto the platform and performed electrical pacing.

Squeeze-film hydrogel deposition and dry micropatterning.

In this technical note, we demonstrate a squeeze-film based spacer-free method for creating controllable submicrometer hydrogel films on planar substrates that can be used to photolithographically fabricate hydrogel microstructures. This new technique improves the photolithographic resolution and yield by providing a uniform and low-defect hydrogel film. The optimum polymerization initiation time for achieving such a layer was determined to be around 1 min. For patterning, the dried hydrogel film was coated with a parylene-C masking layer. Subsequent etching in oxygen plasma was used to transfer selected patterns of hydrogel to the substrate in a batch scale.