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String-Pull (Paperfuge)

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Hand-Cranked Centrifuge
Simple designs achievable with basic tools
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String-Pull (Paperfuge)
Ultra-simple centrifuge for field medical testing

A string-pull centrifuge is a light disc on a loop of string that, when the string is pulled and released repeatedly, spins fast enough to separate a drop of blood in a capillary tube with no power and no gearing. The carrier article gives 20,000-100,000 RPM, but the force on the sample depends on how far the tube sits from the axis, and a disc that is spun faster than its material can bear will burst.

Part of Centrifuge Building

A paperfuge is a hand-powered disc centrifuge that uses a twisted string as a flexible drive, built for field medical testing where no electricity or gearing is available.

The String Is A Drive That Needs No Gears

The carrier article describes it: a disc of cardboard, wood or plastic with two holes near the center and a loop of string threaded through them. Pull both ends outward and the disc spins; relax, and the string rewinds in the opposite sense for the next pull. The string twist and untwist store energy and let the disc build up speed with each stroke. The carrier says it achieves 20,000-100,000 RPM with a lightweight disc.

Worked Example: Speed Is Not The Same As Force

The force on a sample depends on RPM and radius together. For a capillary tube lying along the disc at a distance r from the axis, RCF = 1.118 × 10⁻⁵ × r × N². Take the sample 1 cm from the axis:

  • At 20,000 RPM: 1.118 × 10⁻⁵ × 1 × 20,000² = 4,472 g.
  • At 100,000 RPM: 1.118 × 10⁻⁵ × 1 × 100,000² = 111,800 g.

The large gap is because N is squared. Now check the disc itself. Rim speed is v = 2π × r × N ÷ 60. For a disc of radius 4 cm:

  • At 20,000 RPM: v = 2π × 0.04 × 20,000 ÷ 60 = 83.8 m/s.
  • At 100,000 RPM: v = 419 m/s, more than twice the 179 m/s at which mild steel yields (derived in Speed Limits and Material Stress). A cardboard, wood or plastic disc has far lower strength than steel, so a disc anywhere near 4 cm in radius cannot reach the top of the carrier’s range.

Whichever speed a device reaches, the sample must be at a radius small enough that the disc rim is not moving faster than the material can bear. The stress in a spinning ring is its density multiplied by the rim speed squared. This wiki does not give a strength for cardboard, wood or plastic, so no maximum can be stated here: test a new disc at low speed behind a shield and never run a disc that is cracked or warped.

What Is Reported For The Medical Use

The carrier article reports the device developed for malaria diagnosis in field conditions, with a drop of blood in a capillary tube spun for 1.5 minutes. Published versions are reported to separate plasma in a minute or two and to concentrate malaria parasites in roughly a quarter of an hour, but those are reported figures and the carrier’s single time is not a full protocol. Use them as a guide to what to try, not as a diagnostic standard. Reading the result needs a microscope; see Compound Microscope and Microscopic Observation.

What You Need

  • A flat, light, rigid disc, centered and drilled with two holes close to the middle
  • Strong cord, threaded as a loop through the two holes and with handles on the ends
  • A capillary tube sealed at one end, a way to hold it flat on the disc, and clean gloves
  • Eye protection and a shield; see Containment Shield

Method

Step 1: Make the disc round and centered, with the two holes equal distances from the center. A lopsided disc cannot be balanced by hand.

Step 2: Fix the tube along a radius so it is balanced by a second tube or by weight on the opposite side.

Step 3: Pull steadily and in a rhythm; let the string unwind and rewind with the disc, rather than yanking.

Step 4: Stop by slowing the pulls, not by touching the disc.

Step 5: Read the layers with the tube still sealed.

Common Mistakes

  • Quoting RPM as if it were g-force. The radius matters as much.
  • Over-speeding a weak disc. More speed means more stress, and the stress grows as the square.
  • Frayed string. A snapped cord whips toward the face.
  • Opening the tube without disinfection. The blood is infectious.

Safety

A cracked disc at speed is a projectile and blood is infectious

A disc that fails at a rim speed of tens of meters per second sends fragments in all directions: wear eye protection and spin behind a screen. A capillary tube can crack and cut; handle blood with gloves, seal the tube, and disinfect afterwards (see Sterilization Methods). Do not touch a spinning disc.

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