Contrary to popular opinion, in cryonics the blood of the patient is replaced with a cryoprotective agent to reduce freezing, or more recently, to eliminate ice formation altogether through vitrification. This procedure requires surgical access to the circulatory system of the patient to wash out the blood and replace it with a cryoprotective agent. But how do we know what the concentration of the cryoprotective agent is in the brain of the patient?
There are a number of methods to estimate the concentration of the cryoprotective agent including specific gravity (SG) and freezing point depression osmometry. Such methods can be impractical and time consuming. In the following article from Long Life Magazine (1978), Mike Darwin introduced refractometry as a method to determine cryoprotective agent concentration. Measuring the refractive index of the perfusate to estimate the concentration of the cryoprotectant in the patient is still practiced in cryonics today by taking samples of the arterial, venous, and burr hole effluent.
The sort of handheld refractometer that is discussed in Darwin’s article was used by Alcor for manual refractometry measurements until the organization obtained a benchtop refractometer in the mid-2000’s. Alcor also collects continuous refractive index measurements using LabView and in-line industrial refractometers. In 2007, Aschwin de Wolf recommended the Reichert AR2000, a digital handheld refractometer, which is convenient to use, offers a wide reading range and a choice of different scales, and can be used with software to automatically record data. The Reichert AR2000 is currently used by the Cryonics Institute.