@article{chang_ucar_swindlehurst_bradley_renk_velev_2009, title={Materials of Controlled Shape and Stiffness with Photocurable Microfluidic Endoskeleton}, volume={21}, ISSN={["1521-4095"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-67651108709&partnerID=MN8TOARS}, DOI={10.1002/adma.200803638}, abstractNote={Microfluidic systems have been the focus of intense research and development as they promise a multitude of advantages in the chemical and biological laboratory practice, such as rapid analysis of samples of small sizes, high resolution and sensitivity, easy automation, and direct integration with sample pretreatment and detection systems on a single chip. Microfluidics has already revolutionized some aspects of biosensing and microassays, chemical microsynthesis, and fabrication of special colloidal particles. The potential of microfluidics in other areas of technology has only recently begun to be realized and investigated. A few examples of unconventional use of microfluidic technology include fabricating optofluidic devices based on the large refractive-index contrast of liquid/liquid or liquid/air cladding through microchannels, performing digital coding/decoding and logic operations by control of streams of droplets and fluorescent molecules, design of electronic paper based on electrowetting, and, notably, fabrication of 3D metallic microstructures by solidifying liquid solder into microchannel molds, called ‘‘microsolidics.’’ One promising, yet largely unexplored, area is the fabrication of materials with embedded microchannel networks, where the flow, pressure, color, and other properties of the liquid inside the channels impart a certain function to the matrix material in which the network is embedded. Microchannels in thin elastomeric polydimethylsiloxane (PDMS) sheets filled with viscous liquid have been used as a means to produce reusable strongly adhesive materials without sticky layers. The large increase in the adhesion forces results from the crack-arresting properties of microchannels and the surface stresses caused by capillary forces. Toohey et al. have demonstrated the use of 3D microvascular networks in a self-healing material, which is inspired by the functionality of natural skin derived from its vesicular blood network. Their self-healing composite is capable of multiple healing cycles by capillary wicking of liquid polymerizable epoxy through the cracks, reaching the 3D microvascular network embedded in the substrate. We describe here a microfluidic material in the form of flexible sheets that can be solidified on demand by light to acquire specific shapes. The matrix of the material is thin sheets of PDMS. The microfluidic-channel networks embedded in the elastomer are filled with liquid photocurable polymer. The materials formed in this way possess the unique ability to ‘‘memorize’’ and retain a certain user-defined shape upon illumination. When the microchannel networks are deformed and exposed to UV light, the photoresist inside the channels is solidified and subsequently acts as the endoskeleton within the PDMS layer, locking in the programmed shape. The bending and stretching moduli of the materials with solidified endoskeleton increase drastically. Even if the resulting sculptured sheets are deformed, the ‘‘memorized’’ shapes are recovered after the external force causing the deformation is removed. The procedure for fabrication of shape-controlled microchannel materials is schematically illustrated in Figure 1. The microfluidic channels inside PDMS were fabricated using soft lithography. Two PDMS sheets with arrays of channels facing each other in perpendicular directions were sealed irreversibly by air-plasma treatment (Fig. 2). Liquid SU-8 (photocurable epoxy resin) was injected into the microchannel network using a syringe. The filling was done on a hot plate to lower the viscosity and improve the SU-8 wetting of PDMS. The elastomer sheets with microchannel networks filled with liquid SU-8 prepolymer are transparent, soft, and easily bent, similarly to the original silicone rubber (Fig. 3a). The transparent PDMS host can transmit incident light in the near-UV region (350–400 nm), where SU-8 photopolymer is light-sensitive. The soft material filled with liquid SU-8 could be deformed into a variety of shapes, such as wave, spiral, saddle, and pocket, and then solidified by exposure to UV light for 15min. The resulting PDMS slabs with solidified internal networks after the UV exposure retained the defined deformation, while still having soft and rubber-like surfaces (Fig. 3b–d). The PDMS sheets with photocured network could be stretched, bent, or twisted manually with high recoverable strain. The SU-8 photoresist has high mechanical stability, which enables its application in reliable replication and reinforcement of sophisticated microstructures. The elastic modulus of PDMS is 0.75 MPa and the one of SU-8 after complete crosslinking is 4400 MPa. Thus, solidification of the SU-8 prepolymer in the PDMS microchannels should increase drastically the elastic modulus of the composites. We prepared samples with various volume fractions of SU-8 in the PDMS}, number={27}, journal={ADVANCED MATERIALS}, author={Chang, Suk Tai and Ucar, Ahmet Burak and Swindlehurst, Garrett R. and Bradley, Robert O., IV and Renk, Frederick J. and Velev, Orlin D.}, year={2009}, month={Jul}, pages={2803-+} }
 @article{chang_beaumont_petsev_velev_2008, title={Remotely powered distributed microfluidic pumps and mixers based on miniature diodes}, volume={8}, ISSN={["1473-0189"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37349032809&partnerID=MN8TOARS}, DOI={10.1039/b712108c}, abstractNote={We demonstrate new principles of microfluidic pumping and mixing by electronic components integrated into a microfluidic chip. The miniature diodes embedded into the microchannel walls rectify the voltage induced between their electrodes from an external alternating electric field. The resulting electroosmotic flows, developed in the vicinity of the diode surfaces, were utilized for pumping or mixing of the fluid in the microfluidic channel. The flow velocity of liquid pumped by the diodes facing in the same direction linearly increased with the magnitude of the applied voltage and the pumping direction could be controlled by the pH of the solutions. The transverse flow driven by the localized electroosmotic flux between diodes oriented oppositely on the microchannel was used in microfluidic mixers. The experimental results were interpreted by numerical simulations of the electrohydrodynamic flows. The techniques may be used in novel actively controlled microfluidic-electronic chips.}, number={1}, journal={LAB ON A CHIP}, author={Chang, Suk Tai and Beaumont, Erin and Petsev, Dimiter N. and Velev, Orlin D.}, year={2008}, pages={117–124} }
 @article{cayre_chang_velev_2007, title={Polyelectrolyte diode: Nonlinear current response of a junction between aqueous ionic gels}, volume={129}, ISSN={["1520-5126"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-35948967966&partnerID=MN8TOARS}, DOI={10.1021/ja072449z}, abstractNote={We demonstrate that a fixed junction between two aqueous gels containing oppositely charged polyelectrolytes could rectify electric current. The agarose-based gels were "doped" with sodium poly(styrene sulfonic acid) and poly(diallyl dimethylammonium chloride). The unidirectional current response of the interface between the cationic and anionic gels originates directly from anisotropy in the mobile ionic charges in the gels. The current depends on the concentration of polyelectrolyte, the background ionic concentration, and the distance traveled by the ions. The I-V curves from the devices demonstrated a combination of transient and stationary rectification effects. The current densities achieved were comparable to or higher than those obtained with previously reported organic semiconductor diodes. The diodes had good long-term stability in both DC and AC conduction modes. The materials and the process of preparation of these devices are simple, inexpensive, and scalable. They could be used in flexible and biocompatible electronic circuits.}, number={35}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Cayre, Olivier J. and Chang, Suk Tai and Velev, Orlin D.}, year={2007}, month={Sep}, pages={10801–10806} }
 @article{chang_paunov_petsev_velev_2007, title={Remotely powered self-propelling particles and micropumps based on miniature diodes}, volume={6}, ISSN={["1476-4660"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847678889&partnerID=MN8TOARS}, DOI={10.1038/nmat1843}, abstractNote={Microsensors and micromachines that are capable of self-propulsion through fluids could revolutionize many aspects of technology. Few principles to propel such devices and supply them with energy are known. Here, we show that various types of miniature semiconductor diodes floating in water act as self-propelling particles when powered by an external alternating electric field. The millimetre-sized diodes rectify the voltage induced between their electrodes. The resulting particle-localized electro-osmotic flow propels them in the direction of either the cathode or the anode, depending on their surface charge. These rudimentary self-propelling devices can emit light or respond to light and could be controlled by internal logic. Diodes embedded in the walls of microfluidic channels provide locally distributed pumping or mixing functions powered by a global external field. The combined application of a.c. and d.c. fields in such devices allows decoupling of the velocity of the particles and the liquid and could be used for on-chip separations.}, number={3}, journal={NATURE MATERIALS}, author={Chang, Suk Tai and Paunov, Vesselin N. and Petsev, Dimiter N. and Velev, Orlin D.}, year={2007}, month={Mar}, pages={235–240} }
 @article{chang_velev_2006, title={Evaporation-induced particle microseparations inside droplets floating on a chip}, volume={22}, ISSN={["0743-7463"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33644552476&partnerID=MN8TOARS}, DOI={10.1021/la052695t}, abstractNote={We describe phenomena of colloidal particle transport and separation inside single microdroplets of water floating on the surface of dense fluorinated oil. The experiments were performed on microfluidic chips, where single droplets were manipulated with alternating electric fields applied to arrays of electrodes below the oil. The particles suspended in the droplets were collected in their top region during the evaporation process. Experimental results and numerical simulations show that this microsepration occurs as a result of a series of processes driven by mass and heat transfer. An interfacial tension gradient develops on the surface of the droplet as a result of the nonuniform temperature distribution during the evaporation. This gradient generates an internal convective Marangoni flow. The colloidal particles transported by the flow are collected in the top of the droplets by the hydrodynamic flux, compensating for evaporation through the exposed top surface. The internal flow pattern and temperature distribution within evaporating droplets were simulated using finite element calculations. The results of the simulation were consistent with experiments using tracer particles. Such microseparation processes can be used for on-chip synthesis of advanced particles and innovative microbioassays.}, number={4}, journal={LANGMUIR}, author={Chang, ST and Velev, OD}, year={2006}, month={Feb}, pages={1459–1468} }