Via Attila Csordas, a Nature report on a project in which Harvard chemist Derek Bruzewicz and colleagued converted an old desktop plotter into "an impressively simple microfluidics device that can be produced without a clean room or photolithographic equipment."

The system works like this. By replica moulding, the pens of the plotter are replaced with PDMS [organic polymer poly(dimethylsiloxane)] versions that can deliver various types of 'ink'. The purpose of the ink, when cured, is to create channels in a filter-paper substrate, and after experimenting with the possibilities Bruzewicz et al. found that a syrupy mixture of 3:1 PDMS:hexane did just fine. Having chosen the appropriate paper, the trick then is to use the plotter to draw channel shapes, with the PDMS syrup penetrating the full depth of the paper to create water-tight chambers in various patterns....

The authors have tested different types of the device with well-tried colorimetric assays for identifying excess protein and glucose in urine, and found they performed well, with no cross-contamination between channels.

According to the HP Computer Museum (not actually part of HP), the particular plotter used in this project, a 7550

was the most advanced small plotter ever built. It had an incredible acceleration of 6g, making it one of the fastest plotters ever (and the most fun to watch). The 7550 had 8 pens and could plot on many types of media including paper, transparency film, vellum and polyester film.

It was introduced in 1984, and cost $3900 at the time; you can get them on eBay for $50.