OPTICALLY RECONFIGURABLE MICROFLUIDICS

M. Krishnan and D. Erickson*

Cornell University, Ithaca, NY, USA

ABSTRACT

Reconfigurable systems change their properties on-command or autonomously in response to stimuli and are ubiqui-

tous in nature. Many engineering realizations exist including self-healing polymers [1], self-reproducing robots [2] and in

computing [3], offering advantages of cost, adaptability, robustness. Despite these potential advantages, reconfiguration

in microfluidics remains poorly developed. We present a fast, low power optical approach to channel based reconfigur-

able microfluidics, using a thermorheological solution that forms a gel on heating. By selectively heating this solution

within a microfluidic device, using dynamic photomasking and an absorbing substrate, we created regions of high yield

stress that behave as tunable channel walls.

KEYWORDS: Reconfigurable systems, optofluidics, thermorheological, sol-gel, dynamic photomasking

INTRODUCTION

Most microfluidic applications are channel based, where flow networks and valves need to be designed, fabricated

and placed on chip a priori, and the reconfiguration of flow networks cannot be done on-the-fly. Since flow paths are

fixed, it is difficult to change the analysis performed based on earlier results and to manufacture versatile chips that can

perform diverse applications. Previous work on optical reconfiguration of flow has required the use of high power lasers

and/or high switching times [4, 5]. This paper describes the use of light to trigger reversible structural changes within a

microfluidic device thus enabling large scale reconfiguration of microscale environments at timescales on the order of

seconds. Our technique offers a fast and low power approach for the creation of reconfigurable channel structures in

situ.

THEORY

As shown in Figure 1, this effect uses two coupled processes (1) Photothermal conversion, where energy from an

optical image is converted into a local thermal field using an absorbing substrate and (2) Thermorheological conversion,

where the thermal field triggers a reversible change in the rheology of a polymer solution flowing within the microfluidic

device, resulting in the creation of local gelled regions of high yield strength that behave as wall-like structures. The re-

gions of the fluidic chamber that are unheated act as microfluidic flow pathways, while the heated gelled regions of the

chip resist flow and thus form wall networks within the larger microfluidic chamber. On cooling, the gelled regions re-

turn to a solution state, and the original flow resumes through the chamber. Since the optical image illuminating the chip

can be controlled using dynamic photomasking, channel networks can be created and reconfigured on-the-fly by manipu-

lation of the optical image.

Figure 1: Optically reconfigurable microfluidics. Schematic demonstrating the use of light to create reconfigurable

channels in a microfluidic chip using dynamic photomasking and an absorbing substrate.

EXPERIMENTAL

The thermorheological fluid used here is an aqueous solution (14.5 - 15% w/w) of Pluronic F127 (BASF, USA), a

triblock copolymer made of poly(ethylene oxide)106-poly(propylene oxide)70-poly(ethylene oxide)106. A 15% solution of

this polymer undergoes a sol-gel transition at about 30 °C. We have previously demonstrated the ability to create recon-

figurable valves with this solution [6]. Here we demonstrate the creation and reconfiguration of entire channel struc-

tures. The substrate of the device used for our experiments consists of a glass slide coated with poly(dimethylsiloxane)

or PDMS, followed by a thin layer of carbon black and PDMS. The latter acts as an absorbing substrate to convert opti-

978-0-9798064-3-8/µTAS 2010/$20©2010 CBMS 611 14th International Conference onMiniaturized Systems for Chemistry and Life Sciences

3 - 7 October 2010, Groningen, The Netherlands

cal to thermal energy. The upper layer of the device consists of a 25 µm tall fluidic chamber made of PDMS. The proc-

ess of photothermal conversion is carried out by illuminating the microfluidic chip with an optical image using a stan-

dard projector system fitted with a focusing lens to project images onto the chip, as shown in Figure 2(a). Fluid is

pumped into the chip using pressure manifolds to provide flow at a constant pressure to the chip.

RESULTS AND DISCUSSION

Figures 2(b) and (c) show experiments where channels are created on-the-fly within a microfluidic chamber. The

polymer solution flowing within the fluidic chamber was seeded with 3 µm fluorescent polystyrene particles to image the

flow. In the first case, shown in Figure 2(b), a bifurcating channel was created within a larger fluidic chamber causing

flow around the center region of the chip. In the second case, shown in Figure 2(c), a narrow straight channel running

through the center of the chip was created, with channel walls along the side. The time scale of channel creation and re-

configuration was on the order of seconds. In both cases, the original flow pattern was obtained on turning the heat off,

demonstrating the ability to reconfigure channel networks within the microfluidic device on-the-fly.

Figure 2: Channel creation and reconfiguration. (a) Experimental setup. (b) A bifurcating channel created within a

larger microfluidic chamber by selective heating. (c) A narrow straight channel created within the chamber.

We also demonstrated the ability to carry out directed fluidic assembly using this technique. As shown in Figure 3,

our assembly chamber contained a “mock” tile made of PDMS that was fixed within the chamber. A 500 µm “mobile”

silicon tile was introduced into the chamber to assemble with the mock tile using fluid flow. As can be seen from Figure

3(a), we were able to successfully assemble the silicon tile at its target location (on top of the mock PDMS tile) by selec-

tively heating parts of the assembly chamber to create a direct pathway for the movement of the silicon tile. On the other

hand, we found that no such docking occurred with the use of just the flow field, as shown in Figure 3(b). Hence the use

of our technique of optical flow reconfiguration allowed us to successfully carry out directed fluidic assembly.

Figure 3: Directed assembly using reconfigurable microfluidics (a) A mobile tile was successfully docked on top of a

“mock” tile structure by dynamically creating a channel within the assembly chamber. (b) With no thermal field, no

such docking occurred at the target position.

We also carried out simulations using COMSOL to investigate the temperature rise in the fluid as a function of heat

flux, heated area and absorber thickness. Figure 4(a) shows the variation in temperature for an applied heat flux of 105

W/m2, across a patterned heated region. We found that the region of elevated temperature does not exactly match the re-

gion of applied heat flux due to thermal diffusion. Thus it is important to model the heat transfer characteristics of this

system to properly design heat patterns for a desired channel configuration. We also investigated the effect of absorber

thickness on temperature. From the Beer-Lambert law, for light of intensity I0 incident on an absorbing layer of thick-

ness x, having an absorption coefficient α, the volumetric heat flux generated through the absorber, q is given by

αx

0eαIq−

= (1)

Tile dockedMobile tile

Fixed tile

500 µm

(a)

Flow

Heated region No heating

No docking(b)

FlowTile dockedMobile tile

Fixed tile

500 µm

(a)

Flow

Heated region No heating

No docking(b)

Flow

Mirror

Illumination objective

Projector

Gelled region Regular flow resumes

Heat on for 35s Heat off(c)

Flow

100 µm(b)(a) Heat on for 35s

Gelled regionChip

Lens system

100 µm

Flow

Regular flow resumes

Heat off

Mirror

Illumination objective

Projector

Gelled region Regular flow resumes

Heat on for 35s Heat off(c)

Flow

100 µm(b)(a) Heat on for 35s

Gelled regionChip

Lens system

100 µm

Flow

Regular flow resumes

Heat off

612

We modeled the heat transfer due to this volumetric heating for absorber thicknesses varying from 25 - 200 µm. The

absorber material for all cases allowed 1 % transmission through a 50 µm layer of the absorber. The region of heating

was a rectangle of dimensions 200 µm X 100 µm, in a chip similar to that in Figure 4(a). Figures 4(b) and (c) show the

variation of temperature across the height and width of the chamber respectively. The legend in these plots indicates the

thickness of the absorber. Reducing the thickness of the absorber was found to increase the peak channel temperature

since the heat flux incident on the chamber is increased. It can be seen from Figure 4(b) that the temperature remains

fairly constant across the height of the chamber. On the other hand, we see from Figure 4(c) that thermal diffusion re-

sults in elevated temperatures across the width of the chamber, away from the actual region of heating itself.

Figure 4: Numerical simulations using COMSOL. (a) Temperature rise across a patterned heated region for an ap-

plied heat flux of 105 W/m

2. (b) (c) Variation of temperature across chamber height for different thicknesses of absorb-

ers. (d)Variation of temperature across chamber width.

CONCLUSION

This paper describes our work on optically reconfigurable microfluidics. We have demonstrated the ability to carry

out channel reconfiguration using a fast, low power optical approach. An application of this technique to directed fluidic

assembly has also been presented along with numerical simulations to model the heat transfer.

ACKNOWLEDGEMENTS

This work was supported by the Defense Advanced Research Projects Agency (DARPA), Defense Sciences Office

under the Programmable Matter program.

REFERENCES

[1] A. C. Balazs, T. Emrick and T. P. Russell, Nanoparticle polymer composites: Where two small worlds meet, Sci-

ence, 314, pp. 1107-1110, (2006).

[2] V. Zykov, E. Mytilinaios, B. Adams and H. Lipson, Robotics: Self-reproducing machines, Nature, 435, pp. 163-

164, (2005).

[3] K. Compton and S. Hauck, Reconfigurable computing: A survey of systems and software, ACM Computing Sur-

veys, 34, pp. 171-210, (2002).

[4] S. Sugiura et al., On-demand microfluidic control by micropatterned light irradiation of a photoresponsive hydrogel

sheet, Lab on a Chip, 9, pp. 196-198, (2009).

[5] Y. Shirasaki et al., On-chip cell sorting system using laser-induced heating of a thermoreversible gelation polymer

to control flow, Analytical Chemistry, 78, pp. 695-701, (2006).

[6] M. Krishnan, J. Park and D. Erickson, Optothermorheological flow manipulation, Optics Letters, 34, pp. 1976-

1978, (2009).

CONTACT

*D. Erickson, tel: +1-607-2554861; de54@cornell.edu

500 µm

PDMS

Glass

PDMSMicrofluidic

chamber

K

(b)

(c)

(a)

Heat source

Source,

100 µm

region at

center

Boxes show

heated region

500 µm

PDMS

Glass

PDMSMicrofluidic

chamber

K

(b)

(c)

(a)

Heat source

Source,

100 µm

region at

center

Boxes show

heated region

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