A Life-saving Whirligig

Modern science is big: the gravitational wave detector (LIGO) cost over a billion dollars, and the large hadron collider (LHC) in Geneva took decades to build and cost almost five billion euros. It may seem that scientific advances require enormous financial investment. So, it is refreshing to read in Nature Biomedical Engineering (Vol 1, Article 9) about the development of an ultra-cheap centrifuge that costs only a few cents to manufacture [TM111 or search for “thatsmaths” at irishtimes.com].

SAMSUNG

Whirligig, made from a plastic disk and handles and some string

The design of the centrifuge was inspired by a simple spinning toy, often called a whirligig. This toy consists of a circular disc such as a button with a loop of twine threaded through it. Some readers may recall making these toys from tin-lids and wool. As the string loop is pulled and relaxed, it induces successive rapid winding and unwinding phases with alternate clockwise and anticlockwise spinning of the disc. What may surprise readers is that, if the toy is correctly operated, spin-rates of many thousands of revolutions per minute (rpm) can be achieved.

whirligig-schematic

Schematic diagram of the whirligig centrifuge (from Bhamla et al, 2017).

The designers of the centrifuge, from the Department of Bioengineering, Stanford University (Bhamla, M. S. et al., 2017) call their invention a “paperfuge”, as the disc is made from stiff paper card. Two paper discs, a loop of string or nylon and two wooden or plastic handles are essentially all that is required to make it. The simplicity of the device means that mass production at very low cost is feasible.

Commercial centrifuges are heavy and expensive and need electric power. Egg-beaters and salad-spinners have been tried, but without much success as spin-rates are far below the levels required for practical use. The paperfuge device is capable of spinning much more rapidly. In trials, the smallest disc, of radius 5mm, reached peak angular speeds of 125,000 rpm using only human power. The disc most suitable for applications, with a 50mm radius, reached over 20,000 rpm, comparable to commercial centrifuges.

Mathematical Model

The designers developed a comprehensive mathematical model of the device to analyse its performance. Newton’s law of motion states that the rate of change of angular momentum is equal to the total torque or turning effect. The equation of motion can be written

Whirligig-Equation
where
I is the moment of inertia of the disc, φ is the angle of the disc and the τ-terms denote components of the torque.

whirligig-theoryexpt

There are three turning effects, one due to the manual input force applied by the operator, one due to the twisted state of the string and one due to the air drag on the disc. These torques were parameterised, that is, approximated by bulk formulae using measurements from several experiments. Then the theoretical motion of the disc was calculated from the equation of motion. Comparison of the theoretical results and experimental data showed that the mathematical model simulated the motion faithfully.

whirligig-bloodplasmaIn practical use, small capillaries of whole blood are mounted radially on the paper disk, and the device is spun for a short time. The figure here shows the separation of a blood sample into plasma and red cells as the paperfuge is spun over a period of two minutes. Critical separation is achieved well within this period.

Blood analysis using conventional centrifuges is impractical in many circumstances, due to high equipment costs or to lack of electric power. The paperfuge is suitable for point-of-care medical diagnostics in under-developed regions as it does not require and electric power. It is hand-powered, and can separate whole blood samples into plasma and red blood cells in less than two minutes.

This research is a good example of frugal science. It shows how the complex dynamics of a simple toy can be exploited for valuable global health applications. The simplicity of design and manufacture of the hand-operated centrifuge will enable mass production and distribution, providing a low-cost solution suitable for medical applications in resource-poor regions.

Sources

Bhamla, M. S. et al. (2017): Hand-powered ultra-low cost paper centrifuge. Nat. Biomed. Eng. 1, 0009.

Supplementary Information:

http://www.nature.com/article-assets/npg/natbiomedeng/2017/s41551-016-0009/extref/s41551-016-0009-s1.pdf

http://www.nature.com/article-assets/npg/natbiomedeng/2017/s41551-016-0009/extref/s41551-016-0009-s1.pfd


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