MEMS-Based Mechanical Characterization of Micrometer-Sized Biomaterials

Links

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.

Cited Works (11)

Year	Entry
1988	Theret DP, Levesque MJ, Sato M, Nerem RM, Wheeler LT. The application of a homogeneous half-space model in the analysis of endothelial cell micropipette measurements. J Biomech Eng. August 1988;110(3):190-199.
1999	Shin D, Athanasiou K. Cytoindentation for obtaining cell biomechanical properties. J Orthop Res. November 1999;17(6):880-890.
2003	Koay EJ, Shieh AC, Athanasiou KA. Creep indentation of single cells. J Biomech Eng. June 2003;125(3):334-341.
2005	Leipzig ND, Athanasiou KA. Unconfined creep compression of chondrocytes. J Biomech. January 2005;38(1):77-85.
2004	Trickey WR, Vail TP, Guilak F. The role of the cytoskeleton in the viscoelastic properties of human articular chondrocytes. J Orthop Res. January 2004;22(1):131-139.
1990	Sato M, Theret DP, Wheeler LT, Ohshima N, Nerem RM. Application of the micropipette technique to the measurement of cultured porcine aortic endothelial cell viscoelastic properties. J Biomech Eng. August 1990;112(3):263-268.
2006	Eppell SJ, Smith BN, Kahn H, Ballarini R. Nano measurements with micro-devices: mechanical properties of hydrated collagen fibrils. J R Soc Interface. February 22, 2006;3(6):117-121.
2000	Hochmuth RM. Micropipette aspiration of living cells. J Biomech. January 2000;33(1):15-22.
1970	Timoshenko SP, Goodier JN. Theory of Elasticity. 3rd ed. New York, NY: McGraw-Hill Book Co; 1970.
1965	Sneddon IN. The relation between load and penetration in the axisymmetric Boussinesq problem for a punch of arbitrary profile. Int J Eng Sci. May 1965;3(1):47-57.
2006	Darling EM, Zauscher S, Guilak F. Viscoelastic properties of zonal articular chondrocytes measured by atomic force microscopy. Osteoarthritis Cartilage. June 2006;14(6):571-579.