'Paperfuge' Could be Used for Blood Separation, Malaria Diagnostics

NEW YORK (360Dx) – Inspired by a children's toy, researchers at Stanford University have developed what may be the world's fastest non-motorized centrifuge.

Called a "paperfuge," the device is capable of separating plasma from whole blood and purifying malaria parasites from blood samples, and could potentially be used for rapid, inexpensive diagnostics.

As described in a paper published Tuesday in Nature Biomedical Engineering, the paper-based device costs $.20 to make. The paperfuge can achieve maximum speeds of 125,000 RPM for an equivalent centrifugal force of 30,000 g, approximately the maximum force of a standard benchtop microcentrifuge used for small tubes of sample.

Researcher Manu Prakash and co-authors took inspiration from a children's toy called a whirligig. This toy, examples of which date to 3,000 B.C., consists of a small wheel in the middle of a length of coiled string attached to handles. Stretching the string causes the string to unwind and the toy rotates. Releasing the string causes it to coil up again and the toy to spin in the opposite direction.

The researchers first modeled the engineering properties of a whirligig toy, which they refer to as "a nonlinear, non-conservative oscillator," deriving the equation that governs its angular acceleration and torque. From this, the group developed theories about design that they used to guide and optimize development of the paperfuge.

The Stanford team's ultimate device was made of paper disks mounted with tiny plastic capillary tubes.

The group was able to demonstrate complete separation of plasma from red blood cells in 1.5 minutes using a finger-prick sample, or about 20 microliters, of whole blood. Experiments with human blood samples showed the device yielded 100 percent pure plasma.

The team also experimented with a design that included a plastic float in the capillary tube for quantitative buffy coat analysis.

Buffy coat typically contains white blood cells and platelets. However, this technique can be a critical component of certain kinds of blood parasite testing, since buffy coat mixed with the dye acridine orange will cause parasites to fluoresce under UV light.

Buffy coat separation using the hand-operated paperfuge took 15 minutes of spinning. In proof-of-concept experiments using 30 microliters of blood sample spiked with 7.5 percent Plasmodium falcipuram parasitemia, the group showed it could identify the parasites with fluorescent microscopy after buffy coat separation and that quantitative results were similar to those obtained using a commercial electric centrifuge.

Other devices have been proposed for centrifuging in low-resource settings. Researchers at Texas A&M University, for example, developed a method using the blades of a quadcopter, or drone, while others have pioneered methods using salad spinners.

The Stanford group also developed 3-D printed versions of the paperfuge, as well as a microfluidic disc made of polydimethylsiloxane, which they called a "PDMS-fuge" and suggested could be easily mass produced.

The latter, "[o]pens up possibilities to design integrated lab-on-a-chip devices that do not require external pumps or electricity," the researchers wrote. However, they noted, "the choice of paper as a substrate further opens opportunities for incorporating origami-based geometries, embedding optics, paper-based microfluidics, and ultimately integrated lateral-flow rapid diagnostic assays."