Welcome to the Muller Group Website

Postdoctoral Researcher:

Dr. Boris Stoeber

Current Position: Assistant Professor, University of British Columbia, Vancouver, Canada

PhD Mechanical Engieering, University of California, Berkeley, 2002

MS Electrical Engineering, Technische Universität Darmstadt, Germany, 1998

MS General Engineering, Ecole Centrale de Lyon, France, 1998

Boris Stoeber, Assistant Professor
Department of Mechanical Engineering and
Department of Electrical and Computer Engineering
2054-6250 Applied Science Lane
University of British Columbia
Vancouver, B.C. V6T 1Z4, Canada
Phone: (604) 827-5907
Email: stoeber::at::mech.ubc.ca
Website: http://www.mech.ubc.ca/~stoeber/

Research project:

Microflow Control using Thermally Responsive Triblock Copolymer Solutions

Aqueous solutions of the triblock copolymer poloxamer form a gel at elevated temperatures and concentrations. This effect can be used for microflow management by actuating heaters integrated in flow channels, which induce reversible local gel formation and channel blockage when poloxamers are present in the working fluid. The rheology and fluid dynamics of different poloxamer solutions are investigated under various temperatures and flow conditions.

Figure 1: Concept of a microvalve using heat-induced gel formation of a thermally responsive polymer solution, that is present everywhere in the flow system.

Poloxamers are biocompatible symmetric triblock copolymers of poly (ethylene oxide)x - poly (propylene oxide)y - poly (ethylene oxide)x. They aggregate in aqueous solutions into micelles that form soft crystal structures at elevated temperatures and concentrations. The apparent viscosity of Pluronic® F127 (by BASF, chain lengths: x = 106, y = 70) was determined with a rotational cone and plate rheometer as a function of temperature. Figure 2 shows that as the solution reaches its gel formation temperature its apparent viscosity increases rapidly.

Figure 2: Viscosity of a 15 wt% Pluronic® F127 solution as a function of temperature from cone and plate viscometry at controlled shear stress (0.6 Pa).

Measurements of Pluronic solutions in the cone and plate viscometer at controlled shear rate reveal that these materials can be described as thermo-thickening and shear-thinning. These characteristics explain the observation of gel formation of Pluronic solutions flowing through a 10 µm deep flow channel shown in Figure 3, while simultaneously heating the channel substrate. Gel formation starts to occur in regions of low shear in the corners of the flow channel (Fig. 3a). Introducing an air bubble into the channel, which moves with the flow, generates regions of low shear along the stagnation streamlines of the bubble, where gel formation starts to occur (Fig. 3b).

Figure 3: A 15 wt% Pluronic F127 solution in a 10 µm deep flow channel is seeded with 0.7 µm large fluorescent particles for flow visualization. Heating of the channel substrate induces gel formation in the regions of low shear in the corners of the channel (a) and along the stagnation streamlines of an air bubble (b).

A microfluidic valve structure is shown in Figure 4. It has integrated resistive heaters on a silicon substrate and 10 µm deep flow channels in a Pyrex® substrate. All electrical components are located in shallow recesses in the silicon substrate so that it is possible to anodically bond the silicon substrate to the Pyrex® substrate to complete and seal the devices.

Figure 4: Top view of a microfluidic valve structure with two integrated heaters.

The operation of the microvalve in Figure 4 was demonstrated using digital particle image velocimetry (DPIV). In DPIV, the intensity fields of corresponding regions of consecutive images of the particle-seeded flow region are cross-correlated in order to generate the displacement field; dividing the displacements by the time between images yields the velocity field. A syringe pump provided a constant flow rate for a 15 wt% Pluronic® F127 solution through the microflow system. The microvalve was placed on the stage of an epifluorescence inverted stage microscope (Olympus iX70); illumination was provided by a mercury arc lamp. Images of particles flowing through the device were taken at a frame rate of 30 Hz with a CCD camera that was mounted to the microscope. Only the heater in the lower branch was activated by applying 100 ms long, 400 mW pulses to that heater. Each pulse produced sufficient heat to raise the temperature of the adjacent fluid enough to locally gel the fluid and stop the flow in the lower branch. This is demonstrated in Figure 5, which shows that the flow in the lower branch was stopped within 33 ms. The valve opened again within 33 ms.

Figure 5: DPIV measurements of the flow field in the valve structure from Figure 4 (a) before (b) after heater actuation.

If a Pluronic solution is driven through a microfluidic channel at ambient temperatures close to the gel formation temperature, viscous heating generated by the flow can produce sufficient heat for the Pluronic solution to gel on the channel walls. Depending on the flow conditions this can either lead to blockage of the entire channel, to a reduction of the channel diameter or to visco-thermal instabilities. This phenomenon is currently under investigation, and it could be used for environmentally controlled flow regulation.

Publications:

1. B. Stoeber, C.-M. Hu, D. Liepmann, S.J. Muller, “Thermally responsive PEO-PPO-PEO Triblock Copolymers for Flow Control in Microdevices”, in preparation to be submitted to the journal Physics of Fluids.

2. K. Wolf, B. Stoeber, T. Sattel, “Power flow in asymmetric piezoelectric bending actuators”, in preparation to be submitted to the journal Smart Materials and Structures.

3. B. Stoeber, D. Liepmann, “Arrays of Hollow out-of-Plane Microneedles for Drug Delivery”, submitted to the Journal of Microelectromechenical Systems.

4. B. Stoeber, Z. Yang, D. Liepmann, S.J. Muller, “Flow Control in Microdevices using Thermally-Responsive Triblock Copolymers”, accepted for publication in the Journal of Microelectromechenical Systems.

5. R.K. Sivamani, B. Stoeber, G.C. Wu, H. Zhai, D. Liepmann, H. Maibach, “Clinical microneedle injection of methyl nicotinate: stratum corneum penetration”, accepted for publication in the journal Skin Research and Technology.

6. B. Stoeber, D. Liepmann, S.J. Muller, “Microvalve concepts based on thermally-responsive triblock copolymers”, Proceedings of the XIVth International Congress on Rheology (ICR2004), Seoul, Korea, August 22 – 27 2004.

7. B. Stoeber, D. Liepmann, S.J. Muller, “Microflow Control using Thermally Responsive Triblock Copolymers”, Proceedings of the Seventh International Symposium on Micro Total Analysis Systems, Squaw Valley, California, U.S.A., October 5-9 2003, pp. 183-186.

8. B. Stoeber, D. Liepmann, “An Integrated MEMS Syringe for Advanced Drug Delivery”, Proceedings of the Annual Fall Meeting of the Biomedical Engineering Society 2003, Nashville, Tennessee, U.S.A., October 1-4 2003, p. 77.

9. S. Zimmermann, B. Stoeber, D. Fienbork, D. Liepmann, “Diabetes Under Control: A Microneedle-Based Continuous Glucose Monitor”, Proceedings of the Annual Fall Meeting of the Biomedical Engineering Society 2003, Nashville, Tennessee, U.S.A., October 1-4 2003, pp. 63, 64.

10. S. Zimmermann, D. Fienbork, B. Stoeber, A.W. Flounders, D. Liepmann, “A Microneedle-Based Glucose Monitor: Fabricated on a Wafer-Level Using In-Device Enzyme Immobilization”, Technical Digest of the 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Boston, Massachusetts, U.S.A., June 8-12 2003, vol. 2, pp. 99-102.

11. B. Stoeber, S. Zimmermann, “Diabetes Under Control: A Microneedle-based Continuous Glucose Monitor”, Extended Abstracts of the 1st Annual Berkeley-Stanford Innovators’ Challenge Competition 2003, Stanford, California, U.S.A., April 19 2003, pp. 34, 35.

12. B. Stoeber, “An Integrated MEMS Syringe for Advanced Drug Delivery: Design, Fabrication and Fluid Mechanics of Suspension Flow through Microneedle Arrays”, Ph.D. dissertation, University of California at Berkeley, College of Engineering, Mechanical Engineering, December 2002.

13. B. Stoeber, D. Liepmann, “Design, Fabrication and Testing of a MEMS Syringe”, Technical Digest of the 2002 Solid-State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, U.S.A., June 2-6 2002, pp. 77-80.

14. B. Stoeber, E. Español, D. Liepmann, “Operational Limits of Suspension Flow through Sudden Contractions”, Proceedings of the 2001 ASME International Mechanical Engineering Congress and Exposition, New York, New York, U.S.A., November 11-16 2001, disc set vol. 2, IMECE2001/MEMS-23878.

15. B. Stoeber, D. Liepmann, “Fluid Injection Through Out-Of-Plane Microneedles”, Proceedings of the 1st Annual International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine and Biology, Lyon, France, October 12-14 2000, pp. 224-228.

16. B. Stoeber, D. Liepmann, “Two-Dimensional Arrays of Out-of-Plane Needles”, Proceedings of the 2000 ASME International Mechanical Engineering Congress and Exposition, Micro-Electro-Mechanical Systems (MEMS), Orlando, Florida, U.S.A., November 5-10 2000, pp. 355-359.

Patent: B. Stoeber, D. Liepmann, “Method of Forming Vertical, Hollow Needles within a Semiconductor Substrate, and Needles Formed thereby”, United States Patent No. US 6,406,638 B1, June 18, 2002.

This page last updated 5/25/2004 by Wes Marner.

http://www.cchem.berkeley.edu/~sjmgrp/people/boris/boris.htm