X-Message-Number: 729 Date: 13 Apr 92 03:09:17 EDT From: Brian Wowk <> Subject: fixation and hi-res scanning To: >INTERNET: I'd like to expand a bit on comments made by Thomas Donaldson in response to Richard Fleischer's questions about chemical fixation and high resolution brain scanning. When tissue is frozen by slow cooling there is a tendency for cells to dehydrate by osmosis (ice forming outside cells creates a hypertonic solution). This dehydration is generally beneficial; it's less damaging for water to freeze outside a cell than in it. Regarding this phenomenon, Dr. Gregory Fahy reported at Alcor's recent anniversary dinner that chemical fixation renders normally semi- permeable cell membranes impermeable, preventing cellular dehydration during cooling. This results in the formation of highly-damaging intracellular ice. Chemical fixation procedures such as embalming are therefore not appropriate for a cryonic suspension protocol. I'm not sure whether this information was available when Drexler wrote Engines of Creation. There may have been good arguments around at that time in support of fixation before vitrification (or freezing). The important points to realize are that any technique involving storage below -130'C (the temperature at which even unfrozen water becomes rock solid) will be sufficient for "long-term biostasis", and that no such techniques are currently reversible at an organ level. Because we don't yet have nanotechnology to recover preserved organs, it's very difficult optimize preservation protocols. (What criteria does one use to judge preservation quality?) As Drexler points out in his book, cryonicists have traditionally taken the conservative approach of preserving tissue function as best as possible. (We know, for example, that at least the individual cells of our suspension patients are still viable.) Hindsight now reveals the wisdom of this approach. It will likely continue in the future as we push toward demonstrably reversible brain preservation protocols. On the subject of high resolution brain scanning, I don't think CT scanning will ever be practical for this application. The radiation dose deposited in tissue scales as the sixth (yes, I said 6th) power of CT scanner resolution. For scanning at the cellular scale, the radiation damage would be prohibitively large. Interestingly, no such restrictions exist for MRI. If the formidable engineering obstacles can be overcome, MRI in conjunction with Thomas Donaldson's nanorobotic imaging probes may well be the brain and memory recording tools of the future. --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=729