A. Burak Uçar
| PhD, Chemical Engineering, 2013 North Carolina State University M. S., Chemical Engineering, 2009 North Carolina State University B. S., Chemical Engineering, 2007 Boğaziçi University, Istanbul, Turkey |  |
Research Focus: Materials with New Functionalities Imparted by Microfluidics
Microfluidic systems have been the focus of intense research and have provided advances in chemistry and biology by way of new systems for biosensing, microassays, detection and analysis of samples, and synthesis and fabrication of colloidal particles. The potential of microfluidics to revolutionize other fields has only begun to be explored, and one unique application of microfluidics has been fabriaction of materials with extraordinary adhesion, self-healing properties, or other functionalities. Our goal is to develop novel materials by incorporating micro-scale channels transporting fluids which enable enhanced functionalities.
Photocurable Microfluidic Endoskeleton
We have fabricated a class of novel microfluidic materials in the form of flexible sheets that can be solidified by light and so possess the unique ability to “memorize” and retain user-defined shapes upon illumination. Using soft lithography, we embedded the microfluidic channel network in a soft silicon sheet of polydimethylsiloxane (PDMS), and filled the channels with a liquid photocurable polymer, SU-8. When exposed to UV light, SU-8 solidifies and subsequently acts as endoskeleton within the PDMS layer by increasing the composite’s elastic modulus up to 40 times depending on the SU-8/PDMS ratio used in the sample. This approach also significantly improves the material’s resistance to bending. This process is quite simple and scalable, and the photopolymer could be replaced with any other shape-memory material. Some potential applications include instant containers, patches, or packaging for delicate and irregular shaped objects.
Figure 1: Schematic of the fabrication of photocurable microfluidic endoskeleton (left) and journal cover showing microfluidic materials having ~400µm wide channels filled with a photopolymer, SU-8, and exposed to UV-light to retain the pre-defined shapes (right)
Figure 2: Elastic and bending moduli of the new microfluidic material. As SU-8 content in the material increases, the resistance of the material to external forces increases, too.
“Chameleon” Microfluidic Sheets
We have demonstrated a class of simple and versatile soft microfluidic materials that can be made optically transparent or colored on demand. These materials are in the form of flexible sheets containing microchannel networks embedded in polydimethylsiloxane (PDMS). The channels are filled with liquids whose refractive index is matched with that of the PDMS matrix. By pumping dye solutions into the channel networks and consecutively replacing the medium, we can control the material’s color and light transmittance in the visible and near-infrared regions. The rate of “switching” of the color or transparency of such sheets is slower than the alternative electrochromic and other electrically actuated materials. However, the microfluidic material has the advantage of being inexpensive, flexible and it is able to maintain its color without continuous application of external voltage. One of the most intriguing and potentially useful functions that can be realized in such materials is the control of the IR transmittance. Microfluidic materials with switchable IR transmittance could be used in “smart” windows for radiative energy management and conservation that allow control of the passing near-infrared heat portion of the sunlight spectra.
Figure 3: Channel structure of the color changing elastomeric sheets: a) sample with a typical double-channel network, also illustrating a color change from red to blue. b) Side view schematic. c) Image of the flexible microfluidic network filled with Allura Red dye solution. d) Magnified view of the same material illustrating the double-channel network design. The channels are sealed onto each other orthogonally.
Figure 4: Digital photographs of the single-channel PDMS network material filled with: a) air, b & f) 61 wt% glycerol–water mixture to make the channels visually transparent, and c–e) Brilliant Green, Allura Red and Bromophenol Blue solutions in the refractive index matched liquid. The image in the inset illustrates a snapshot taken during the color change from green to red. Note that replacement took place without mixing due to the laminar flow in channels and one solution replaced another thoroughly in ~13 seconds. Experimentally measured and theoretically calculated absorbance spectra of g) Bromophenol Blue, Allura Red, Brilliant Green and h) N-IR dye solutions in a 61 wt% glycerol–water mixture introduced consecutively in the double-channel PDMS network.
Publications