Open source optomechanics, following the Openflexure project. This is the second of a series of technical posts on the design of Cytkit, the open source cytometer.

As was pointed out by Richard Bowman, the founder of Openflexure, traditional optomechanics do not lend themselves well to open source 3D printed instruments. Each degree of freedom requires sliding joints (great in machined aluminium or brass, but poor in 3D printed plastic), plus toothed gears, rack and pinion. Moreover these parts need to be assembled, which adds to the cost and complexity. Instead, Openflexure pioneered flexure mechanics: by putting parallelograms into the structure, one could design several degrees of freedom in a single 3D-printed part [1].

Remarkably they reported less than 20 microns drift in their structure over one week, without temperature stabilisation [1]. Sounds ideal for many other applications of optomechanics…. actually I would venture to say that their ideas may be more powerful in cytometry than microscopy, because in a cytometer, one needs a few degrees of freedom and micron-scale precision of adjustment. However, one needs range of only a millimetre or so in each.

I set out to design optomechanics for a 3D printed cytometer based on their ideas. The first problem to solve was, what is the minimal number of degrees of freedom for this cytometer? Here we are talking about a single laser, forward/side scatter, and a single spectral array for a set of fluorescent markers. The answer is 5 – far fewer than in a commercial instrument. I can explain very briefly why. In a single 3D-printed body, we rely on the accuracy of the printer (~0.1 mm) for coarse placement of optical parts. Considering that the detector chips are each approximately 3 mm square, this accuracy is sufficient where the laser is not focused, However the laser focus itself is much smaller and has to be aligned accurately with the stream of cells, say within 10 microns. Since the stream is translationally symmetric in one direction, that means 2 axes are required for laser positioning, i.e. the laser focus lens is on an XY platform. Then the fluorescent light coming off the cells has to be collected and directed to an array of the detectors. One axis is required to focus the condenser lens, and two to point the collimated light in the right direction towards the grating and detector array. Considering that it is more compact to have 2 flexures in the same place rather than 3, the condenser lens is on an XY platform, while the laser is on a tilting flexure.

That totals 5 degrees of freedom: two XY stages and one tilt. The current design can be downloaded on the Cytkit repo on Github – follow links from the website in comments below.

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