Quantitative 3-dimensional profiling of channel networks within transparent ‘lab-on-a-chip’ microreactors using a digital imaging method

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Abstract
We have developed a method for the quantitative 3-dimensional profiling of micron sized channel networks within optically transparent “lab-on-a-chip” microreactor devices. The method involves capturing digitised microscope images of the channel network filled with an optically absorbing dye. The microscope is operated in transmission mode using light filtered through a narrow bandpass filter with a maximum transmission wavelength matching the wavelength of the absorbance maximum of the dye solution. Digitised images of a chip filled with solvent and dye solution are analysed pixel by pixel to yield a spatially resolved array of absorbance values. This array is then converted to optical path length values using the Beer–Lambert law, thereby providing the 3D profile of the channel network. The method is capable of measuring channel depths from 10 to 500 µm (and probably even smaller depths) with an accuracy of a few percent. Lateral spatial resolution of less than 1 µm is achievable. It has been established that distortion of the measured profiles resulting from a mismatch in refractive index between the dye solution and the glass of the microreactors is insignificant. The method has been successfully used here to investigate the effects of thermal bonding and etch time on channel profiles. The technique provides a convenient, accurate and non-destructive method required to determine channel profiles; information which is essential to enable optimisation of the operating characteristics of microreactor devices for particular applications.