Controlling the local composition of hydrogels using microfluidics traps

Project

Jan. 2018 – July 2020: Scientist, Marie Curie Fellow.
Collaboration with Michael Kessler, Isabelle Heimgartner, Soujanya Madasu, Professor Kenneth A. Brakke, Professor François Gallaire and Professor Esther Amstad at EPFL, Lausanne, Switzerland.
Keywords: Microfluidics, capillary traps, droplet-based microstructures, hydrogel synthesis.

Many natural materials display locally varying compositions that impart unique mechanical properties to them which are still unmatched by manmade counterparts. Synthetic materials often possess structures that are well-defined on the molecular level, but poorly defined on the microscale.
A fundamental difference that leads to this dissimilarity between natural and synthetic materials is their processing. Many natural materials are assembled from compartmentalized reagents that are released in well-defined and spatially confined regions, resulting in locally varying compositions. By contrast, synthetic materials are typically processed in bulk. Inspired by nature, we introduce a drop-based technique that enables the design of microstructured hydrogel sheets possessing tuneable locally varying compositions.
This control in the spatial compositon and microstructure is achieved with a microfluidic Hele-Shaw cell that possesses traps with varying trapping strengths to selectively immobilize different types of drops. This modular platform is not limited to the fabrication of hydrogels but can be employed for any material that can be processed into drops and solidified within them. It likely opens up new possibilities for the design of structured, load-bearing hydrogels, as well as for the next generation of soft actuators and sensors.

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Figure 1- Microfluidic traps with different trapping forces. a) Schematic illustration of a microfluidic device, containing two types of traps to selectively immobilize different batches of drops. b) Sketch of a microfluidic device used to measure the critical flow rate of traps. c) Side view of a trap occupied by a drop that is simulated with Surface Evolver. The relevant dimensions are defined. d) Time-lapse optical microscopy images of a trapped drop teared off at a critical flow rate Qc.

Importantly, our method enables tuneable, abrupt changes in material composition on the 100 μm length scale, in stark contrast to most additive manufacturing techniques that rely on the continuous deposition of filaments. Importantly, this method is not limited to the fabrication of hydrogels but can be extended to many other types of materials. Its facile and passive approach facilitates upscaling such that this platform opens new possibilities to study fundamentals of fracture and crack propagation in soft, structured materials or, by adding functionality to well-defined regions, to design the next generation of sensors and actuators.

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Figure 2- (a)-(f) Fabrication protocol of a hydrogel sheet with locally varying compositions. (g) Fluorescence micrograph of an integral hydrogel sheet encompassing two different types of microgels. (h) Free-standing macroscopic hydrogel sheet.

Acknowledgements

We acknowledge funding from the Marie Sklodowska-Curie Actions Fellowship, project “El_CapiTun” n°750802.


Related publications:

  1. Everything in its right place: controlling the local composition of hydrogels using microfluidic traps

    M. Kessler, H. Elettro, I. Heimgartner, S. Madasu, K. A. Brakke, F. Gallaire & E. Amstad

    Lab on a Chip, In Press (2020)