Machine Design

Multiphysics software helps build a better blood monitor

Slava Djakov
Sensor Development Director

Microvisk Technologies
Denbighshire, North Wales, U. K.
Edited by Leslie Gordon
[email protected]

Comsol Inc.

Microvisk Technologies

Handheld devices for monitoring blood viscosity typically use reactant-coated electrodes to induce and detect chemical reactions in blood samples. This technology has remained unchanged for many years. Recently, though, Comsol Multiphysics software from Comsol Inc., Burlington, Mass., helped Microvisk develop a handheld unit for home use that employs microelectromechanical sensors (MEMS) on a disposable strip which contains a small cantilever to measure viscosity. The discreetly sized monitor checks blood viscosity in a onestage process based on physics rather than chemistry.

Patients need only slip the test strip into the handheld device, prick their finger to produce a tiny drop of blood, about 3 microliters of volume, and touch it with the free tip of the test strip. The capillary action takes the blood straight to the MEMS sensor and the test commences. The device passes signals through the microcantilever layers, each of which deflects in a different way. The cantilever movements indicate the sample’s rheometric properties and dynamic changes in real time. In blood-coagulation tests, such changes are fed back to the handheld unit for detecting the onset of clotting.

Other cantilever designs found in atomic-force microscopy or in biological research for probing and assessing DNA, protein, and aptamer bindings with drugs or antibodies, use crystalline-silicon (cSi) rigid cantilevers. Because of their rigidity, cSi and similar structures are delicate, brittle, and provide restricted movement. Although cSi cantilevers can be sensitive, through actuation in resonant mode, their restricted mobility impedes performance once the microcantilevers are immersed in liquids.

In contrast, the MEMS-based device’s microcantilevers comprise layers of different polymers. The cantilevers’ free ends can deflect significantly from their resting positions, resulting in efficient and accurate responses.

The big challenge in developing the monitor lay in its sophisticated design, which required a holistic approach to integration, packaging, and signal processing. Once the concept was proven, the big questions in MEMS-based microchip design were: How likely are the chips to perform, and what are the set points? It was also necessary to account for mechanical response and reproducibility as well as reliability aspects such as cycling times and performance deterioration.

When the research began on the early units several years ago, there was no suitable software-modeling option. Thus, it was not possible to conduct multiple analyses of the MEMS. We were forced to rely on past experience, basic know-how, and gut feeling. Needless to say, coming up with the initial design was a long and tedious process involving countless laboratory experiments and real-life tests.

The multiphysics software helped speed this process because it complemented the design flow and real-life testing with simulation. The software let us analyze individual materials — each with unique thermal and electric properties — tangled together. The program helped answer such questions as: Which materials are the most critical and how will they behave in the presence of fluid?

The software can simulate complex designs because it lets users analyze combined mechanics and statics of beams systems, thermal and electric properties of structural materials, and currents being applied to MEMS structures immersed in and interacting with fluids. A current not only changes electrostatic fields, it also alters mechanical structures and creates thermal effects.

Comsol Multiphysics let us simulate the microchips mechanically, thermally, and electrostatically. It also permitted analysis of the microfluids and their properties and how these interface with the chips and moving cantilevers. By linking all the physical properties of the design, the software speeded iterations, reduced prototyping, and cut development time.

The software also eliminated the tedious method of solving one problem, then another, and plotting a graph after each step. Previously, we collected data from a number of test strips, which had to be analyzed, understood, and verified. One iteration typically assessed 20 different design options and the manufacturing and assembly implications of each.

The multiphysics software eliminated this problem, letting us quickly start selecting the most promising simulation options, which we then confirmed with laboratory testing. We estimate the software took four or five months off monitor-development time.

Medical-diagnostic equipment has stringent requirements such as the time allowed for blood-sample testing. Because blood clotting begins as soon as the finger is pricked, the process had to be quick. Here, the software came in handy in simulating and optimizing flow through the microcapillary channels that feed fluid samples onto the microchips. The microcantilever is immersed immediately, so tests can begin with just a quarter of the volume needed by existing monitors. This results in less pain for patients. In addition, it is unnecessary to drip blood onto a certain area of the strip. The monitor shows test results in just 30 sec.

Point-of-care testing and medical devices for home use are emerging markets. The potential for home blood testing is similar to that of glucose testing by patients diagnosed with diabetes, a market in which 160 companies are now established. Microvisk plans to launch the new device in the last quarter of 2011.

© 2011 Penton Media, Inc.

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