Session: 03-16-01: Manufacturing: General

Paper Number: 113267

113267 - Fabrication of Ultra-High Aspect Ratio Array Structures Using Spontaneous Evolution in Multiport Lifted Hele-Shaw Cell 

High aspect ratio (HAR) micro/meso-well array structures are useful in biomedical devices used in applications such as spheroid formation, drug-releasing patches, and cell-trapping. Nature-inspired honeycomb structures with high aspect ratio walls have the potential for such applications (Here, aspect ratio of wall refers to the ratio of the wall thickness and the wall height of the microwell.) This paper demonstrates lithography-less, spontaneous, scalable fabrication of an ultra-high aspect ratio structure in polymer by disciplining Saffman-Taylor fluid-fluid interface instability (Also known as viscous fingering). The process begins with squeezing a thin film of a highly viscous and volatile polymer solution between strategically placed ports in the cell plates of a Multiport Lifted Hele-Shaw Cell (MLHSC). Upon reaching a specified gap during squeezing the ports of MLHSC are unsealed before separating the plates at predetermined velocity. While solvent evaporation happens continuously during process, structures are evolved between the cell plates following the following key steps: (i) rearrangement of fluid in the hexagonal array during the initial stage of the plate separation due to evolution of air fingers without splitting because of controlled Saffman-Taylor instability, (ii) subsequent pinning of the fluid boundary due to evaporation of the solvent from the polymer solution (phenomena attributed to coffee stain effect observed in evaporating polymer solution), (iii) stretching of fluid due to continuous increment in the viscosity of the volatile polymer solution, and finally (iv) solidification due to volatility of the fluid.

Systematic tuning of the experimental parameters like fluid film thickness and the separation velocity is required to avoid extra parasitic branches in the final evolved hexagonal arrayed pattern in step (i). Similarly, the selection of fluid properties like viscosity and volatility is essential for out-of-plane stretching and retention of the structures, respectively (steps (ii)-(iv)). Parameters and fluid properties are chosen based on recent literature characterizing fluid interface evolution in Uniport Lifted Hele-Shaw Cell (ULHSC). The "Evaporation shielding" effect observed on account of limited volume of evolving air/vapor pockets needs to be considered, in addition, to fabricate the structures successfully. With the proposed method we successfully demonstrated fabrication of high aspect ratio (in the range of 200-300) structures with less than 10-micron wall thickness.

Experimental details: Fluid was prepared by dissolving 120% w/v polystyrene (Mw 192000) in the chloroform. The polymer solution is highly volatile due to the presence of chloroform. Experiments are carried out in controlled temperature environment with temperature maintained at 25^oC. Digitally controlled Hele-Shaw apparatus for the purpose is custom built using precision positioning stages, and arrangement for ensuring parallelity of the plates. dSPACE 1104 data acquisition system is used to interface motor and encoder and write control programs for precise squeezing at separation with slow speeds of around 10 microns per sec.

The proposed method of fabricating the HAR structure is scalable, cost-effective, and time efficient compared to conventional lithography-based processes and micro 3D printing techniques.

Presenting Author: Prasanna Gandhi Indian Institute of Technology Bombay

Presenting Author Biography: Prof. Prasanna Gandhi received a Ph.D. degree from Rice University, Houston, TX, USA, in 2001. He is currently a Professor at the Indian Institute of Technology Bombay, Mumbai, India. His research interests include robotics, mechatronics, multi-scale manufacturing using fluid instabilities, 3D micro printing, and dynamic systems and controls.

Authors:

Makrand Rakshe Indian Institute of Technology Bombay
Prasanna Gandhi Indian Institute of Technology Bombay