Using simple tools, a fine glass tube (capillary) can be drawn to a point fine enough to dissect a single red blood cell—an object about 6-8 μm (6-8 thousandths of a millimeter). The capillary’s interior channel will diminish in proportion to the walls. By removing the very tip, a syringe is made that is fine enough to inject material into, or remove material from, individual living cells. While other materials can produce tool tips of microscopic fineness, none combine the workability and chemical neutrality of glass.Of course, other instruments are needed to mount these glass tools, to manoeuver them in space, to contain the sample, and observe the process. This apparatus centers on the microscope and micromanipulator. In the late 19th and early 20th centuries when the technology of micromanipulation (referred to more succinctly as ‘micrurgy’) was being developed, all of this equipment could be purchased commercially or manufactured in local workshops except for the glass microtools which are too delicate and disposable to be made outside the lab. Making useable tools in sufficient quantity took a great deal of practice.
Here I want to trace the various early methods for making glass microtools that were described by the people who first developed them. Because the process was inherently difficult, investigators created new instruments to make this process more reliable while also making possible more specialized and elaborate microtools.
How to make a micropipette
Begin with a glass tube of about 3-5mm in diameter. (Solid rods can be used as well, but are only useful for dissection.) This can be made either of “soft” (soda-lime) or “hard” (borosilicate) glass. Hard glass is more durable and alkali free but is also harder to work because it softens at a higher temperature. Soft glass can be drawn into a slightly finer point. Heat the tube in a Bunsen flame and when the heated section becomes soft, pull on both ends to draw it into a narrow pipette between 0.3 – 0.8 mm in diameter. (These measurements vary somewhat depending on the source.) Then use a much smaller heat source to pull a finished tip on this pipette.This tip is especially difficult to make because of the relatively small amount of heat necessary to soften the fine pipette and the challenge of pulling it in such a way as to produce a tip that is sturdy and straight. It is very easy to apply too much heat resulting in a fine, flexible “hair” (think of a fibre optic cable), or to end up with a bent, broken, or poorly centered tip. Certain tip profiles are best suited to certain kinds of work. The variability involved in making these tools by hand typically meany that one would produce a number of them, then sort them according to their profiles.
There are various ways to make these tips, each of which comes with its own complications. The simplest and earliest was described by the French biologist Laurent Chabry (1855-1893) in a paper published in 1887. Chabry’s method involved placing the fine glass filament against an incandescent object (for instance a heated bead of glass) and then withdrawing it quickly. This method was very imprecise, however. In a morning’s work one could only expect to make four or five serviceable points. [Chabry 1887, 176]
A second method was introduced by Marshall A. Barber, a professor of bacteriology at the University of Kansas. This involved a microburner—an improvised device that attached to the laboratory gas supply which could produce a useful flame of about 2mm. This tiny heat source can be used to soften the glass of the filament just enough that it can be pulled apart by hand with both separated halves tapering to a fine tip.
The microburner was far from a perfect tool. In 1932, J. Arthur Reyniers, a bacteriologist at the University of Notre Dame, stated bluntly that “Gas micro-flames are useless in this work because they cannot be accurately regulated.” [Reyniers 1933, 271] Reyniers was among several researchers who developed new instruments to produce these essential glass tools more easily and reliably.
Efforts to make it easier to create glass microtools began very early in the development of micrurgy. Chabry developed a system that involved sliding the glass pipette along a grooved surface. By sliding the pipette into contact with the incandescent platinum blade of an instrument used for cautery, then quickly withdrawing it, he was able to produce straight tool tips with reasonable consistency. [Chabry 1887, 177]
A new series of pipette-making instruments emerged from a 1908 invention by Keith Lucas, a Fellow at Trinity College, Cambridge, who worked in the physiological laboratory. Lucas’s instrument used a loop of electrified platinum wire that encircled the central part of the pipette to be softened. The pipette was clamped on both ends and under tension. Because both heat and tension could be regulated precisely, this technology made it possible to repeatedly produce good tips and even to precisely create tips of a particular kind. [Lucas 1908, xxviii-xxx]
A number of instruments were developed based on Lucas’s method. A simple, compact device, developed in 1931 by Delafield Du Bois of New York University, was distributed by the Leitz company over a long period. Today, sophisticated programmable machines such as the MicroData Instrument PMP series will produce a variety of tips to very precise specifications.New tools, new possibilities
There is more to this story than the familiar mechanization of a skill-intensive laboratory process. Glass micropipettes were simply the most basic and adaptable of an ever-growing family of purpose-built microtools ranging from microelectrodes, to tiny crucibles used in chemical operations, to elaborate apparatus meant, for instance, to measure pressure in the smallest capillaries. (A number of these are described in this digitized source.) Fabricating these instruments would typically have required the skills of a laboratory machinist and/or glass blower.At least one tool did emerge that expanded the researcher’s ability to make glass tools. The “microforge” was introduced in 1937 by Pierre de Fonbrune (1901-1962), Chef de Laboratoire at the Pasteur Institute in Paris. It provided a means to make elaborate tool tips that would have been impossible using earlier techniques.
Like the common forge, the microforge was a craftsperson’s tool, permitting the user to apply heat and pressure to shape glass at a microscopic level. Heat was again provided by an incandescent wire. Pressure came from jets of air. Both could be positioned relative to the glass workpiece. Work was done under magnification. De Fonbrune, provided detailed instructions for making various tools. Modern versions of this instrument are still being made.In this very brief survey, I don’t want to suggest that there are no similarities between laboratory work and craft work and that the making of glass microtools represents some kind of unlikely exception. On the contrary, many historians and STS scholars have pointed to the importance of manual skills in the production of scientific facts.
Reading the instructions left by early experts in the field, though, I’m struck by the extent to which the process of making glass instruments depends on mastering complex and intuitive manual skills through practice. This certainly isn’t the case with all experimental labour. In my next post, I will explore this process of bodily (or ‘gestural’) knowledge by discussing the experience of making my own glass micropipettes.