In the late 1930s Barry Green, a research chemist at the National Cash Register Company in Dayton, began investigating how the concept of microencapsulation might have potential application in copying documents. If specks of dye could be covered with a special fusible coating, forming a microcapsule, the use of ink could prove much less messy and more efficient. Scientists had long been intrigued by the possibilities of controlling the release of an active ingredient by encapsulating it. Theoretically, microencapsulation was fairly straightforward; practically, getting the conditions right had proved exceedingly difficult. Green’s breakthrough would become the heart of the technology by which copiers and printers produce documents. Today microencapsulation is central to many other technologies, including time-released pesticides and pharmaceuticals.
Before xerography, a clerk often made copies of a document by using multipage forms interleaved with carbon paper. These packets were inherently messy because users had to pull and then discard the carbon-paper pages. Reading the last copy in a stack was often a challenge. By 1942 Green, working with Lowell Schleicher, had developed a working method of microencapsulating ink and a prototype of carbonless carbon paper. Over the next dozen years he painstakingly refined his methods, scaled them up to production levels, and worked with Thomas Busch of Appleton Coated Paper in Appleton, Wisconsin, on the tricky process of applying the microcapsules to paper in a thin, flexible layer.
The product had three layers: the paper; a film of acid-sensitive dye packaged in microcapsules; and a layer of acidic clay to develop the dye from transparent to dark blue or black. Pressure from a writing implement broke the microcapsules of dye on the underside of each sheet (except the last one); when the dye was released, it reacted with the acidic layer on the surface of the next sheet. Considerable effort went into designing capsule walls that were sturdy enough to withstand processing but would rupture under the pressure of a pencil.
To harden the cell walls, Green used gelatin, a protein that consists of long chains of chemically linked amino acids. When gelatin is treated with a reactive chemical such as formaldehyde, glutaraldehyde, or tannic acid, new chemical links form between the chains. The result is a three-dimensional network called a cross-linked gelatin, which is harder and less soluble than regular gelatin, yielding a tougher and more durable microcapsule.
To make the microcapsules, dye was dissolved in a high-boiling organic solvent, and the resulting solution was stirred at high speed in the presence of gelatin and gum arabic in water. As a result of the vigorous agitation, the oily dye solution formed a dispersion of fine droplets in the water layer. Changing the acidity of the water solution made the gelatin and gum become less soluble and caused them to precipitate in the form of a coating on the droplets. Formaldehyde hardened the coatings, and the resulting microcapsules were separated from the mixture with a sieve and dried.
Meanwhile, within a few years of the introduction of carbonless carbon paper, microencapsulation made possible another technology that forever altered office procedures. In the late 1940s an inventor named Chester Carlson enlisted the aid of the Haloid Company of Rochester, New York, to help commercialize his new copying process, known as xerography, a dry photocopying process that used toner consisting of microencapsulated dyes. The development work culminated in 1959 with the introduction of the revolutionary Xerox 914. Although it was bulky and needed frequent attention, this machine made it possible for the first time to produce faithful copies of virtually any document without resorting to messy wet processes.
Microcapsule technology found applications in agriculture as well. A number of pesticides are now available in encapsulated form to control the release rate. A further benefit is that some pesticides, such as the corn borer insecticide fonofos, are highly toxic to humans. Because of this, application of the normally formulated compound requires stringent protective measures. Microencapsulation provides a barrier between the operator and the pesticide and decreases the applicator’s exposure.
In the field of medicine, pharmaceutical companies developed microencapsulated drugs that could mask unpleasant tastes and withstand the highly acidic conditions of the stomach but later dissolve in the nearly neutral environment of the small intestine, where most drugs are absorbed into the bloodstream. The choice of capsule wall material also controlled the rate at which a drug leaches out, making it possible to provide a steady outflow of a drug for as long as 24 hours.
Microencapsulation also found application in liquid crystals. In the early 1970s researchers succeeded in microencapsulating thermochromic (temperature-sensitive) liquid crystals. When the microcapsules are dissolved in a solvent and sprayed onto a surface, the liquid crystals they contain will indicate the temperature of the surface by changing color. This is useful for many diagnostic procedures in engineering and medicine, as well as in thermometers. Without microencapsulation, the crystals would deteriorate from contact with the surface and the surrounding air.