MEMS-Based Mechanical Characterization of Micrometer-Sized Biomaterials

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Abstract

The mechanical properties of biomaterials play important roles in performing their specialized functions: synthesizing, storing, and transporting biomolecules; maintaining internal structures; and responding to external environments. Besides biological cells, there are also many other biomaterials that are highly deformable and have a diameter between 1µm and 100µm, comparable to that of most biological cells. For example, many polymeric microcapsules for drug delivery use are spherical particles of micrometers size. In order to characterize the mechanical properties of individual micrometer-sized biomaterials, the capability of capturing high-resolution and low-magnitude force feedback is required.

This research focuses on the development of micro devices and experimental techniques for quantifying the mechanical properties of alginate-chitosan microcapsules. The micro devices include microelectromechanical systems (MEMS) capacitive force sensors and force-feedback microgrippers, capable of measuring sub-µN forces. Employing the MEMS devices, systems were constructed to perform the micro-scale compression testing of microcapsules.

The force sensors are capable of resolving forces up to 110µN with a resolution of 33.2nN along two independent axes. The monolithic force sensors were applied to characterizing the mechanical properties of soft hydrogel microparticles without assembling additional endeffectors. Protein-loaded alginate-chitosan microcapsules of ∼20µm in diameter were prepared by an emulsion-internal gelation-polyelectrolyte coating method. The microcapsules were imii mobilized by a PDMS holding device and compressed between the sensor probe and the holding device. Young’s modulus values of individual microcapsules with 1%, 2%, and 3% chitosan coating were determined through the micro-scale compression testing in both distilled deionized (DDI) water and pH 7.4 phosphate buffered saline (PBS). The Young’s modulus values were also correlated to protein release rates.

Instead of compressing the microcapsule against the wall of the holding device using the force sensor, a force-feedback MEMS microgripper with the capability of directly compressing the microcapsule between two gripping arms has been used for characterizing both the elastic and viscoelastic properties of the microcapsules during micromanipulation. The single-chip microgripper integrates an electrothermal microactuator and two capacitive force sensors, one for contact detection (force resolution: 38.5nN) and the other for gripping force measurements (force resolution: 19.9nN). Through nanoNewton force measurements, closed-loop force control, and visual tracking, the system quantified the Young’s modulus values and viscoelastic parameters of alginate microcapsules, demonstrating an easy-to-operate, accurate compression testing technique for characterizing soft, micrometer-sized biomaterials.

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