SCOPE Field note · Diagnostics

A simpler schisto filter, cut with Occam's razor

June 24, 2025 · 5 min read

Foldscope · S. mansoni

Will McCarthy Health in Your Hands · SCOPE program

We find ourselves again asking the same question: what is the simplest, most user-friendly way to view schistosome eggs with the Foldscope, a low-cost, paper microscope?

My friend at the Prakash lab, Anesta, recently asked a deceptively simple question: what about a reusable mesh filter? It made me want to re-approach the idea of a flat filter from a frugal-science perspective. Caring deeply about sustainability and accessibility, the question becomes one steeped in three things at once: materials, durability, and optics.

The constraints

Materials

Low-cost and accessible: nothing exotic, nothing hard to source.

Durability

Hard to break, easy to clean, good for many uses in a row.

Optics

You have to be able to see the eggs. Not in shadow, not covered up, not too far from the viewing platform.

Microscopy of S. mansoni eggs on nylon mesh — S. mansoni eggs, filtered with nylon, viewed through the Foldscope.

The material

Mesh from Dr. Lee's lab

Sabona, in Dr. Lee’s lab, showed us a trick they use to filter cercariae out of water: nylon mesh. It turns out to be fairly affordable. We remain interested in PET and stainless-steel approaches, but for now, let’s give nylon some thought.

If it’s held taut (say, with a plastic tube melted onto it), it’s easy to clean. And when I asked about durability, a member of Dr. Lee’s lab just said, “I haven’t broken one yet.” Sold.

A reusable Foldscope slide card with nylon filter, paperclips, acrylic coverslips and a syringe laid out on a desk — The makings of a reusable nylon filter, laid out on the bench.

The build

First, filtering

Nylon mesh held in a Sterlitech filter casing — Squishing the nylon between Sterlitech's casing worked for me.

To review: we already know we can use a reusable acrylic coverslip with paperclips to reliably visualize the eggs — a thin, transparent stand-in for a quartz or glass coverslip.

A freshly laser-cut acrylic coverslip held up — Jon with a fresh coverslip from the laser cutter.

Checking the nylon mesh under a benchtop microscope — Simon checks the nylon under the big gun.

So all that was left was to look at some eggs.

Microscopy showing a wrinkled, uneven mesh surface — One problem: a wrinkly, uneven surface, even under the paperclipped coverslip.

A clean view of eggs caught in the mesh — Easy breezy.

We thought of a few improvements, and we’ll test them soon — thank you, Phil, for helping us laser-cut some acrylic and PET!

A Foldscope-card-size backing prototype held in hand — Mahwish with the Foldscope-card-size backing, v1.

While the eggs do catch, the viewing area becomes the next issue. We’re working to circumnavigate both the viewing area and the filtering process with a low-tech solution, currently in the works. Thinking caps are on.

Counting out eggs for a filtering test — Caroline, up to no good — counting out eggs for filtering tests.

A vivid evening sky outside the lab — Sometimes you step outside for fresh air and the world is painted vibrantly.

Always follow through on dinner-napkin ideas — ideally next-day.

A whiteboard of requirements for the next mesh-filter casing — Brainstorming requirements for the next iteration and the mesh-filter casing.

Phil sketching a new approach — Phil pondering new approaches.

A sketch on a paper napkin — The dinner-napkin sketch that started the next iteration.

Notes outlining the next plan — The next plan.

Until next time.

Filed under SCOPE

Focus Diagnostics R&D · Place Houston · Stanford

From the bench

Frugal science runs on simple ideas

Thank you for following the build. These field notes are the messy middle of making diagnostics accessible. Please share them with anyone who’d want a look behind the scenes of what this work takes.

Donate to this work About the SCOPE project More field notes

Materials

Durability

Optics

Mesh from Dr. Lee's lab

First, filtering

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