How to make an open-source sheath? This is the first of a series of technical posts on the design of Cytkit, the open source cytometer.

Almost all the fluidics parts needed for a flow cytometer are available as low cost generic components; most of the mechanical parts are also easy to 3D print. But not so the sheath chamber and cuvette. The development version of Cytkit uses a glass capillary instead of a quartz cuvette, since the former is widely available*. And that leaves the sheath part.

Current leading commercial cytometers mostly use a sheath chamber integrated with the cuvette. This is made of several machined and polished parts of fused silica, diffusion bonded, and available from a couple of specialist companies. These are far from cheap or widely available!

There is very little theory of sheath flow in flow cytometry, at least not in the public domain. What little exists is in the proprietary designs of the flow cytometry companies. Fundamentally the sheath has to make laminar flow: concentric sheath fluid and sample inputs, with the sample input well centred. And it should have nothing to disturb the sheath flow or bias it to one side, e.g. it should not trap bubbles. It should have smooth walls and avoid background fluorescence. A couple of paragraphs about sheath flow can be found in Howard Shapiro’s classic Practical Flow Cytometry. There is not much more theory around than that!

I designed a sheath chamber for 3D-printing. taking advantage of resin printing**. (Earlier versions used FDM, although the resolution wasn’t high enough and it was always tricky to get them to seal.) The latest version is a beauty, even if I do say so myself! The design is *very* minimalist: it has barbs for easy connection of silicone tubing, avoiding junctions (which trap particles) and avoiding microfluidic connectors (unnecessary cost).

The current design can be downloaded on the Cytkit repository on Github – follow links from website in comments below. See the video… it works! Aptly, I have rigged up an Openflexure microscope, custom mounted on its side, to verify the sheath flow by imaging of beads. 

Get in touch if you want to talk about Cytkit, life science tools, or need my help for anything!

* For the potential performance advantages, I’m still considering adding a low-end quartz cuvette, even if it does have to be sourced from specialist companies.

** For the kits, we are considering using a specialist 3D resin-printing microfluidics service. Thanks to Paul Marshall of Rapid Fluidics for advice.

Here’s a teaser video about Cytkit now in development, showing off the minimal optical design and the fully 3D-printed mechanics

Is it possible to make a sub-$5k open-source cytometer that is functionally equivalent to a $100k commercial instrument?

Furthermore, is it possible to build a business selling such an open-source cytometer (as components, kits and fully assembled instruments)? No commercial organisation, no service contracts. The design needs to be minimalist and open-source to be accessible to the users, to install and configure for themselves, maintain and repair. Moreover, the business would rely on the community for the sharing of knowledge and design modifications for user applications.

It would also rely on an untapped resource – the cytometrists. In the past, cytometrists were the experts on building and modifying their hardware. However, this diminished greatly as automated fully-serviced commercial cytometers came to dominate the field. Cytkit would be a platform for the cytometrists to take back the field, to build and customise instruments for their users, enabled by open-source (including the optical model), minimalism of the basic design, community knowledge, and readily available components. So the cytometrists, instead of just charging their users by the hour for access to their instruments, could also provide a service to set-up and maintain open-source, custom instruments for their users.

I just came back from CYTO in Denver, Colorado, where I presented this concept, at David Novo’s Open Cytometry Hardware meeting and at my poster. I spent two days carrying the Cytkit prototype in a little flight case, showing it off to everyone who was curious to see it. People were excited, and now I’m now excited by their responses. Thanks to those who have already signed up on the website or told me privately that they will buy one.

So I hope the answers to the questions above are “yes”. I’m embarking on this business as an interesting experiment. I hope to start a pre-order campaign circa September, once the design is mature. Until then, look out for further posts about the development.

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