Surface discontinuity detection using bacterial suspensions

The main problem when detecting microsurface discontinuities in the microfabrication scale is the poor sensitivity of existing nondestructive testing (NDT). Although advanced techniques to detect microdiscontinuities based on ultrasonic testing (UT) or eddy current testing (ET) methods exist, these are not suitable for microcomponents, since the probes cannot be coupled to the parts under inspection. Penetrant testing is the NDT method that allows the detection of the smallest discontinuities, though the excess penetrant removal is critical, mainly if the depth of a discontinuity is small comparatively to its open area at the surface. So, this technique is also inadequate to detect small-sized shallow discontinuities but is appropriate to detect tight and tiny discontinuities where the depth/width ratio of discontinuities is high (for example 1/20, linear crack size down to 5 μm deep and 0.5 μm wide on standardized Ni-Cr test panels). Recently, a new NDT technique was proposed to overcome this drawback. The technique exploits a wide range of bacterial cell properties, including cell penetration and adherence to discontinuities, motility by using an electrical or magnetic field and visualization by adding fluorochromes. This paper describes the technique and its application to the detection of discontinuities to different materials.