Optimized miciroviscosimeter using optical probing; application to biological membranes

In this article, we report on studies aimed at sensing of the stiffness of membranes, particularly in the case of vesicles. A local approach could be done with AFM techniques but the local information is not pertinent for non homogeneous membranes. To solve this problem, we have developed and checked a specific sensor based on a vibrating sphere. The near-field acoustic wave enables to characterize biological particles which change the apparent viscosity and density of the surrounding fluid. The microsphere is well suited for very small volumes of liquid (typically about one microliter). Globally, the microsensor is based on a silicon cantilever which is glued on a piezoelectric transducer at its clamped end. The sphere is connected to the cantilever with a small glass rod (the core of an optical fiber). When operating, the sphere is immersed inside the investigated liquid and the piezoelectric actuator is excited with a low frequency generator. The vibration of the cantilever is sensed with a heterodyne laser probe. The plot of the response of the sensor (Bode plot) allows computing the properties of the liquid. When biological cells or vesicles are in the fluid, the effect of the biological elements is detectable and can be discriminated. We will present the microsensor and the optical probe which have allowed doing the described measurements. The theoretical study will show the influence of the different parameters and the crucial role of the optical probe to detect the low amplitude vibrations of the cantilever. The experimental results demonstrate the high sensitivity of the sensor to small variation of the composition of the fluid (water), particularly in the case of small vesicles of different kinds. Practically, this is a simulation of cells sensing.