There is a serious need for an inexpensive centrifuge that doesn't use electricity in the developing world. People have tried salad spinners, but they're way too slow - about 6oo rpm. Other solutions involve car or motorcycle parts and run on battery power, but performance is low and cost is far too high.
You may have had a children's toy that had a wheel with a coiled string or rubber band coming out from each side. You would pull on the strings and the wheel would spin .. it's a non-linear oscillator. Every time you pull the strings you're putting energy into uncoiling and into spinning the wheel. (this problem is sometimes given in physics mechanics courses). There are some loses, but they can be efficient. Some clever folks at Stanford hit on the idea of designing a version to be very efficient for spinning blood samples - think testing for malaria and other diseases - and made it human powered and cheap. The current model hit 125,000 rpm with 60,000g s at the spinning wheel. 125,000 rpm and it only cost twenty cents. Couple it with a smartphone microscope - people have done that for about ten bucks - and you can do serious good work even sending images to expert people or computers anywhere in the world.
here's a short video to help you understand what's going on..
Nature has posted the paper outside their paywall. It has a bit of undergrad level physics... but go in even if that's not your cup of tea ... there are some great videos at the end of the paper.
Hand-powered ultralow-cost paper centrifuge
M. Saad Bhamla, Brandon Benson, Chew Chai, Georgios Katsikis, Aanchal Johri & Manu Prakash
In a global-health context, commercial centrifuges are expensive, bulky and electricity-powered, and thus constitute a critical bottleneck in the development of decentralized, battery-free point-of-care diagnostic devices. Here, we report an ultralow-cost (20 cents), lightweight (2 g), human-powered paper centrifuge (which we name ‘paperfuge’) designed on the basis of a theoretical model inspired by the fundamental mechanics of an ancient whirligig (or buzzer toy; 3,300 BC). The paperfuge achieves speeds of 125,000 r.p.m. (and equivalent centrifugal forces of 30,000 g), with theoretical limits predicting 1,000,000 r.p.m. We demonstrate that the paperfuge can separate pure plasma from whole blood in less than 1.5 min, and isolate malaria parasites in 15 min. We also show that paperfuge-like centrifugal microfluidic devices can be made of polydimethylsiloxane, plastic and 3D-printed polymeric materials. Ultracheap, power-free centrifuges should open up opportunities for point-of-care diagnostics in resource-poor settings and for applications in science education and field ecology.
A centrifuge is the workhorse of any medical diagnostics facility. From the extraction of plasma from whole blood (for performing immunoassays or determining the haematocrit value), to analysing the concentration of pathogens and parasites in biological fluids, such as blood, urine and stool (for microscopy), centrifugation is the first key-step for most diagnostic assays 1 . In modern diagnostics, separation of unwanted cellular debris is especially critical for the accuracy and reliability of molecular diagnostics tools and lateral-flow-based rapid diagnostic tests 2 that are designed for detecting low levels of infection in diseases such as malaria, human immunodeficiency virus and tuberculosis 3,4,5 . Currently, centrifugation is typically inaccessible under field conditions, because conventional machines are bulky, expensive and electricity-powered 4 . The need for electricity-free centrifugal bio-separation solutions has prompted researchers to use egg-beaters and salad-spinners as proposed devices 6,7 . However, these suffer from bulky designs and extremely low rotational speeds (maximum 1,200 r.p.m.; 300 g), leading to impractical centrifugation times for a simple task of blood plasma separation (>10 min). Thus, a low-cost, portable, human-powered centrifuge that achieves high speeds is an essential, yet unmet need, especially for diagnostics in resource-limited environments 8,9,10 .
We describe the design and implementation of an ultralow-cost (<20 cents, Supplementary Table 5), lightweight (2 g), field-portable centrifuge, henceforth referred to as a ‘paperfuge’ and inspired by historic whirligig (or buzzer) toys (Fig. 1a). We demonstrate that the paperfuge achieves speeds of 125,000 r.p.m. (30,000 g) using only human power. Using a combination of modelling and experimental validation, we uncover the detailed mechanics of the paperfuge and leverage this understanding to construct centrifuges from different materials (in particular, paper and plastic). We demonstrate applications including plasma separation, quantitative buffy coat analysis (QBC) and integrated centrifugal microfluidic devices for point-of-care (POC) diagnostic testing.