In the previous Alcor News we reported that Alcor has acquired the prototype of an Intermediate Temperature Storage (ITS) device developed by Brian Wowk of 21st Century Medicine. (For an explanation of why ITS is desirable, please check the explanatory section in Alcor News #13 dated July 1st, 2003.)
Our new lab assistant, Todd Huffman, has been studying Brian Wowk’s design and has visited 21st Century Medicine to discuss reliability issues and possible modifications. Since the ITS pod will be cooled by positioning it above a pool of liquid nitrogen inside a Dewar, we have been debating which type of Dewar to use. An off-the-shelf industrial design is available, large enough to contain seven ITS pods (one in the center and six around it, in a hexagonal pattern). However, our proprietary “bigfoot” Dewar design is much taller, only slightly more expensive, and we have had extensive experience operating them over the past decade. A “bigfoot” seems our best option at this point.
In theory, it would be tall enough to contain a stack of three layers of ITS pods, with seven pods in each layer. The problem is that a tall Dewar will allow a more severe temperature gradient. In other words, the pods at the top will tend to be warmer than the pods at the bottom, and ITS requires that the temperature should be controlled with some precision.
One way around the problem is to use an internal framework of metal such as aluminum, which is a good conductor of heat and could minimize the temperature gradient. Another possibility is to fill the lower part of the Dewar with conventional neuropatients fully immersed in liquid nitrogen, with a single layer of ITS pods above them at the top. The disadvantage of this configuration is that we would have to modify our standard neuropatient containment shell. Also the ITS pods would get in the way during insertion or removal of neuropatients.
Another issue which Todd is investigating is the optimal refill system to maintain the reservoir of liquid nitrogen. A gravity feed would provide the security of constant refill without pumps, but would be grossly inefficient since the pipe connecting it with the Dewar cannot be optimally insulated and will promote nitrogen boiloff.
Using a pump for automatic refill sounds intuitively risky, but low-temperature pump design has been perfected in industrial applications, and a Dewar refill pump probably would run only for a few minutes per week. Two pumps could be installed in parallel for redundancy.
Todd has been tabulating every conceivable failure mode, including liquid-nitrogen level sensor malfunction (Dewar will boil dry), failure of temperature sensor inside ITS pod (patient may become too cold or too warm), wire-break failure, pod heater failure, control system failure, and many others.
Probably we will need four months to explore all these failure modes and develop satisfactory solutions, after which the construction and testing of actual patient storage units may take another two months. This is longer than we would like, but obviously the system must be absolutely reliable before we can offer it as an option to our members.
We can’t predict how much ITS will cost relative to conventional Dewar storage until we have established all the components in the system and have measured the liquid nitrogen boiloff rate.