Quartz 4

Hand Centrifuge

3 min read

Hand Centrifuge

Why We Need It

Cell pelleting requires ~300g for 7 minutes. Lab microcentrifuges run at 10,000-16,000g — cell pelleting is a tiny fraction of maximum speed. The physics are accessible with cheap motors.

Pelleting allows:

  • Concentrating cells from dilute nasal wash into small volume
  • Washing away background histamine, environmental noise, and cromolyn from collection spray
  • Resuspending in controlled volume of clean buffer

G-Force Math

G = (RPM² × radius_cm) / 895

Target: 300g, 7 minutes

At 3,000 RPM: need radius = (300 × 895) / (3000²) = 268,500 / 9,000,000 = ~3cm

At 2,000 RPM: need radius = (300 × 895) / (2000²) = ~6.7cm

Design the rotor radius to match your motor’s RPM. Bake the calculation into the print.

Motor Options

Dollar Store Fan Motor

  • ~$3, available everywhere
  • Estimated 3,000-5,000 RPM unloaded
  • Not regulated — RPM varies under load
  • Sufficient for cell pelleting where precision isn’t critical
  • Problem: Designed for fan blade loading, not tube loading. Balance is the engineering challenge.

Small DC Motor (preferred)

  • Hobby motors, RC car motors — $5-15
  • Known RPM ratings
  • Easier to mount and balance
  • USB-powered options available (5V, 500mA from any phone charger)

Drill (immediate home option)

  • Variable speed, handles rotational loads
  • 3D printed rotor attaches to chuck
  • Run at lowest speed setting
  • Good for Splatspace experiments before dedicated motor

3D Printed Rotor Design

Key requirements:

  • Balance — tubes must be positioned symmetrically. Always load in opposing pairs.
  • Fixed radius — tubes held at consistent distance from center for reproducible G-force
  • Retention — tubes cannot fly out at speed. Snap-fit or friction fit with positive retention.
  • Safety shroud — containment in case of tube failure. Not optional when working with biological samples.

Print in PETG or ABS — better chemical resistance than PLA, more appropriate for lab environment.

Tube slots at calculated radius for target G-force at expected motor RPM.

Kit Product Version

For consumer kit: whirligig/paracord style centrifuge.

Stanford bioengineers demonstrated 125,000 RPM with paper/string whirligig (published PLOS ONE ~2017), sufficient for plasma separation. Cell pelleting is a much lower bar — needs ~300g, trivially achievable.

  • No power required
  • Manufacturable for cents
  • Proven in low-resource diagnostics
  • Consistent with home use format

Design: 3D printed tube holder on paracord. User pumps cord to spin. Timer included in protocol (7 minutes). Low-tech, reliable, no batteries.

Status

  • Design rotor for drill attachment (Splatspace use)
  • Print and test balance
  • Characterize actual RPM with tachometer or phone app
  • Verify cell pelleting at calculated G-force
  • Design whirligig version for kit

Graph View