Flexible Microfluidic Device for Mechanical Property Characterization of Soft Viscoelastic Solids Such as Bacterial Biofilms

Abstract

We introduce a flexible microfluidic device to characterize the mechanical properties of soft viscoelastic solids such as bacterial biofilms. In the device, stress is imposed on a test specimen by application of a fixed pressure to a thin, flexible poly(dimethyl siloxane) (PDMS) membrane that is in contact with the specimen. The stress is applied by pressurizing a microfabricated air channel located above the test area. The strain resulting from the applied stress is quantified by measuring the membrane deflection with a confocal laser-scanning microscope. The deflection is governed by the viscoelastic properties of the PDMS membrane and of the test specimen. The relative contributions of the membrane and test material to the measured deformation are quantified by comparing a finite element analysis and an independent (control) measurement of the PDMS membrane mechanical properties. The flexible microfluidic rheometer was used to characterize both the steady-state elastic modulus and transient strain recoil of two soft materials: gellan gums and bacterial biofilms. The measured linear elastic moduli and viscoelastic relaxation times of gellan gum solutions were in good agreement with the results of conventional mechanical rheometry. The linear Young’s moduli of biofilms of Staphylococcus epidermidis and Klebsiella pneumoniae, which could not be measured using conventional methods, were found to be 3.2 kPa and 1.1 kPa, respectively, and the relaxation time of the S. epidermidis biofilm was 13.8 s. Additionally, strain hardening was observed in all the biofilms studied. Finally, design parameters and detection limits of the method show that the device is capable of characterizing soft viscoelastic solids with elastic moduli in the range of 102 – 105 Pa. The flexible microfluidic rheometer addresses a need for mechanical property characterization of soft viscoelastic solids common in fields such as biomaterials, food and consumer products. It requires only ~ 200 pL of test specimen.

Blood viscoelasticity measurement using steady and transient flow controls of blood in a microfluidic analogue of Wheastone-bridge channel

“…17 Most of these methods require fully integrated sensors or additional tedious calibration procedure using a standard reference. On the other hand, viscoelasticity measurement of fluids has been conducted using ferromagnetic micro-beads, 18 optically-induced force, 19 squeeze flows, 20,21 flexible polymer-based deflection, 22,23 micro-PIV, 24 and extensional flow. 25,26 However, most of these previous studies focused on measuring the relaxation time rather than elasticity itself because these methods may not have the ability to concurrently measure fluid viscosity.…”

Contribution of the Klebsiella pneumoniae Capsule to Bacterial Aggregate and Biofilm Microstructures

“…In addition to passive observations of community architecture, modifications of the currently described image analysis strategy might be used for more sophisticated structural analysis. We have recently described a microfluidic channel-based rheometer for evaluating biofilm viscoelasticity on structures of comparable sizes to those measured here (12). Our cell-locating algorithm, if applied repeatedly to a community of bacteria undergoing mechanical deformation over time, might be used to extract local viscoelasticity determinations (e.g., elastic modulus and Poisson ratio) within aggregates or biofilms.…”

Recent advances in microfluidic techniques for single-cell biophysical characterization

“…145 This design can also be extended to the mechanical characterization of other specimens, such as biofilms. 146 Alternatively, hydrostatic pressure can apply a uniform compressive force without physical contact. 147 Micropost arrays have been used to measure the traction force generated by a cell at the adhesion sites.…”