Dae Kun Hwang

Microarchitecture for a three-dimensional wrinkled surface platform.

Dr. M. Li, N. Hakimi, Dr. R. Perez, Prof. S. Waldman, Prof. D. K. Hwang Department of Chemical Engineering Ryerson University 350 Victoria Street , Toronto , Ontario M5B 2K3 , Canada E-mail: dkhwang@ryerson.ca Dr. R. Perez Department of Nanobiomedical Science Dankook University Cheonan 330–714 , South Korea Prof. S. Waldman Li Ka Shing Knowledge Institute St. Michael’s Hospital 30 Bond Street , Toronto , Ontario M5B 1W8 , Canada Prof. J. A. Kozinski Lassonde School of Engineering York University 4700 Keele Street , Toronto , Ontario M3J 1P3 , Canada

Functional polymer sheet patterning using microfluidics.

Poly(dimethylsiloxane) (PDMS)-based microfluidics provide a novel approach to advanced material synthesis. While PDMS has been successfully used in a wide range of industrial applications, due to the weak mechanical property channels generally possess low aspect ratios (AR) and thus produce microparticles with similarly low ARs. By increasing the channel width to nearly 1 cm, AR to 267, and implementing flow lithography, we were able to establish the slit-channel lithography. Not only does this allow us to synthesize sheet materials bearing multiscale features and tunable chemical anisotropy but it also allows us to fabricate functional layered sheet structures in a one-step, high-throughput fashion. We showcased the techniques potential role in various applications, such as the synthesis of planar material with micro- and nanoscale features, surface morphologies, construction of tubular and 3D layered hydrogel tissue scaffolds, and one-step formation of radio frequency identification (RFID) tags. The method introduced offers a novel route to functional sheet material synthesis and sheet system fabrication.

Wrinkling Non-Spherical Particles and Its Application in Cell Attachment Promotion.

Surface wrinkled particles are ubiquitous in nature and present in different sizes and shapes, such as plant pollens and peppercorn seeds. These natural wrinkles provide the particles with advanced functions to survive and thrive in nature. In this work, by combining flow lithography and plasma treatment, we have developed a simple method that can rapidly create wrinkled non-spherical particles, mimicking the surface textures in nature. Due to the oxygen inhibition in flow lithography, the non-spherical particles synthesized in a microfluidic channel are covered by a partially cured polymer (PCP) layer. When exposed to plasma treatment, this PCP layer rapidly buckles, forming surface-wrinkled particles. We designed and fabricated various particles with desired shapes and sizes. The surfaces of these shapes were tuned to created wrinkle morphologies by controlling UV exposure time and the washing process. We further demonstrated that wrinkles on the particles significantly promoted cell attachment without any chemical modification, potentially providing a new route for cell attachment for various biomedical applications.

Fabrication of Highly Porous Nonspherical Particles Using Stop-Flow Lithography and the Study of Their Optical Properties

A microfluidic flow lithography approach was investigated to synthesize highly porous nonspherical particles and Janus particles in a one-step and high-throughput fashion. In this study, using common solvents as porogens, we were able to synthesize highly porous particles with different shapes using ultraviolet (UV) polymerization-induced phase separation in a microfluidic channel. We also studied the pore-forming process using operating parameters such as porogen type, porogen concentration, and UV intensity to tune the pore size and increase the pore size to submicron levels. By simply coflowing multiple streams in the microfluidic channel, we were able to create porous Janus particles; we showed that their anisotropic swelling/deswelling exhibit a unique optical shifting. The distinctive optical properties and the enlarged surface area of the highly porous particles can improve their performance in various applications such as optical sensors and drug loading.

MODELING MECHANICAL CELL DAMAGE IN THE BIOPRINTING PROCESS EMPLOYING A CONICAL NEEDLE

Biofabrication technologies involve the incorporation of living cells into various bioproducts by employing different cell manipulation techniques. Among them, bioprinting, delivering cell suspension through a fine needle under pressurized air, has been widely used because of its capability of precise process control. In the cell-printing process of bioprinting, cells are exposed to fluid stresses due to the velocity gradient in the fine needle. If the stresses exceed a certain level, the cell membrane may be overstretched, leading to membrane failure and thus causing mechanical cell damage. Modeling the mechanical cell damage in the bioprinting process is a challenging task due to the complex fluid flow and cell deformation involved. This paper introduces a novel method based on computational fluid dynamics (CFD) to represent the mechanical cell damage in the bioprinting process using a conical needle. Specifically, the cell deformation and movement during the cell-fluid interaction processes were represented by the immersed boundary method (IBM). A strain energy density (SED)-based cell damage criterion was developed and used to determine cell damage. Experiments were performed by using 3T3 fibroblasts and the results agree well with the proposed model.

A microfluidic approach for the synthesis and assembly of multi-scale porous membranes

A microfluidic approach is used to synthesize porous membranes with various advanced features such as multiscale pores, heterogeneous chemistry and customizable geometries. In the synthesizing process, while photomasks define microscale regular pores, polymerization-induced phase-separation forms nanoscale irregular pores. Thus, this combination offers the ability to generate multiscale pores on a membrane in a single step. The resulting membranes exhibit heterogeneous chemical properties by co-flowing prepolymer solutions with different chemical components and have designed geometries defined by photomasks. Furthermore, complex layered structures with chemical and physical anisotropies in the cross-membrane direction can be fabricated through magnet assisted self-assembly by encapsulating magnetic particles into the membranes.

Direct cell-cell communication with three-dimensional cell morphology on wrinkled microposts

Cell-cell communication plays a critical role in a myriad of processes, such as homeostasis, angiogenesis, and carcinogenesis, in multi-cellular organisms. Monolayer cell models have notably improved our understanding of cellular interactions. However, the cultured cells on the planar surfaces adopt a two-dimensional morphology, which poorly imitates cellular organization in vivo, providing physiologically-irrelevant cell responses. Non-planar surfaces comprising various patterns have demonstrated great abilities in directing cellular growth and producing different cell morphologies. In recent years, a few topographical substrates have provided valuable information about cell-cell signalling, however, none of these studies have reported a three-dimensional (3D) cell morphology. Here, we introduce a structurally tunable topographical platform that can maintain cell coupling while inducing a true 3D cell morphology. Optical imaging and fluorescence recovery after photobleaching are used to illustrate these capabilities. Our analyses suggest that the intercellular signalling on the present platform, which we propose is mainly through gap junctions, is comparable to that in natural tissue.nnnSTATEMENT OF SIGNIFICANCEnA better understanding of direct cellular communication can help treating neurological diseases and cancers, which may be caused by dysfunctional intercellular signaling. To investigate cell-cell contact, cells are conventionally plated onto planar surfaces, where they flatten and adopt a two-dimensional cell morphology. These unrealistic models are physiologically-irrelevant since cells exhibit a three-dimensional (3D) shape in the body. Therefore, porous scaffolds and topographical surfaces, capable of inducing various cell morphologies, have been introduced, in which the latter is more desirable for sample imaging and screening. However, the few non-planar substrates used to study cell coupling have not produced a 3D cell shape. Here, we present a tunable culture platform that can control direct cell-cell communication while maintaining true 3D cell morphologies.