The Sensor

MEMS Fabricated Sensor

Utilizing advanced micro-electro-mechanical systems fabrication methods (MEMS), a fluidic micro-channel is embedded inside the silicon resonator. By suspending the silicon lever in vacuum and embedding the micro-fluidic channel within it, LifeScale is able to exploit the high Q of a vacuum system whilst at the same time allowing microbes suspended in fluid to pass through the resonator. In this way biological samples, such as living microbes, can be measured under physiological conditions.


To measure a microbe, the change in resonant frequency of the sensor is monitored as the microbe passes along the micro-channel resonator from position 1 to 3. 


As the microbe enters the channel, label 1, the effect of the added mass can be seen in the frequency data. The maximum frequency shift occurs when the microbe is at the tip of the resonator, label 2, and is proportional to the microbe's buoyant mass, i.e. the mass of the microbe over that of the fluid it displaces. As the microbe leaves the channel the measured frequency returns to that of the baseline resonant frequency, label 3. By measuring the duration of the excursion from the resonance frequency, label 1 to 3, it is possible to calculate the microbe’s transit time through the sensor. As the geometry of the sensor is precisely known the transit time of individual microbes can be used to determine the total volume of inoculate passing through the sensor in a given time. By simply counting individual microbes as they pass through the sensor and calculating their velocity an accurate measure of the microbe concentration in counts per ml is obtained.


Frequency shifts in the resonant frequency of the sensor serve as both a measure of the microbe's mass and the transit time. 


The MEMS fabricated sensor is supplied pre-mounted for ease of handling.

N.B. The actual sensor is too small to be visible at the image below.


Sensor length:   200 microns
Sensor aperture:   8 microns
Sensor mass:   70 nano grams
Sensor volume:   25 pico litres
Resonance:   400 kHz
Frequency shift:   <0.01 Hz (20 ppb)


A micrograph taken at a higher magnification shows the sensor with the inlet to the micro-channel highlighted, yellow box. Microbes enter on the left, flow along the micro-channel and out of the sensor on the right, red line.



Higher sample throughput improves the statistics and hence, frequency shifts for a large number of microbes are individually measured. Each shift reflects the mass measurement of an individual microbe and from this information, Archimedes' Principle is used to generate the distribution of mass and size for the microbe population. Sampling a culture at regular time intervals allows LifeScale to measure growth in both microbe mass and number as a function of time. This data may be plotted to yield traditional growth curves, concentration, as well as curves showing how the mean mass varies during the same time period.

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