# What are appropriate techniques to ensure homogeneous solidification of an agar-based aqueous solution?

I am making a T1 weighted phantom for an MRI project. I'm trying to see if I can calculate how much a persons head heats up during an MRI scan. It consists of distilled boiling water, $\pu{7 g/L}$ agar, $\pu{10 g/L}$ $\ce{NaCl}$ and $\pu{1 g/L}$ $\ce{CuSO4}$. As the phantom cools it will get a jelly like texture. What I would like is for the phantom to be as homogeneous as reasonably possible when it solidifies. What do you think is the best way to do so?

I was thinking of having the mixture on a vibrating plate, or maybe a centrifuge or even just inverting the mixture at regular intervals. Any ideas would be greatly appreciated.

• Neat. I think this question might not be well-received since you aren't asking for something specific for this problem, while simultaneously not asking the real question, which is how to make a uniformly-disperse solid colloid. – pentavalentcarbon May 22 '17 at 15:47
• Thank you for that! Would you have any advice on making a uniformly dispersed solid colloid by any chance? :D Or should I make a new topic? – James Blackwell May 22 '17 at 15:50
• If it doesn't matter how quickly the phantom solidifies, you could use any aggressive mixing technique and then do a rapid cooling to lock the suspension in place. – J. Ari May 22 '17 at 17:29

Let me suggest two avenues :

Vertical set up à la potter's wheel: At school we once did something related to your aim: using an old $\pu{75 rpm}$ record player, obviously no longer needed, with a beaker containing previously molten bees wax in the centre. Under constant rotation (rather than the revolution by a centrifuge), the surface of the wax formed a nice meniscus which frooze by the slow cooling. While I am not aware if the alread-gel particles would keep nicely floating in your suspension, until all is solidified, it may be worth to giv it a try. Centering of the whole ensemble is important (unbalance and spill), especially if the sample to be generated should really become as large as a human head. Scaling up appears possible (larger beaker / bucket, and potter's wheel).

Horizontal set up: If you were interested in a smalller sample, a roller mixer would be an alternative:

A wide mouth bottle (like the still cylindrical $\pu{1 L}$ polymer centrifuge bottles used in biochem) would be filled by immersion with liquid and -- still immersed in solution -- closed with the lid to ensure absence of air bubbles in the bottle. After external cleaning of the bottle, they would be layed along the rolls. Subsequently, you would turn on and crank up the rolling and hopefully the gel would form sooner than the liquid dropping out from the bottles. (One may orient the bottles with the lids facing you, and place a bucket underneath them to collect potential spill.) You may find such a roller mixer in biochem groups cultivating their own cell cultures.

• As the gel solidifies, couldn't the horizontal setup have issues with radial mixing (meaning from the long axis of the cylinder out to the circumference)? – J. Ari May 22 '17 at 17:26
• Thanks a million for the detailed answer! They look like some really interesting methods to try! – James Blackwell May 22 '17 at 20:28
• @J.Ari Depending on the model of the mixers, some keep the bottles like on a conveyor belt (like in a market), inducing axial rotation only. Other models include optional or even adjustable precession motion which offers mixing in more than one direction -- as in the example above video, where the bottles contain air bubbles. (Except air bubbles could be trapped into the gel, i) rendering the phantom likely mechanically less similar to human tissue. And more importantly, ii) changing the $t_1$ / relaxation times (since in this regard (f)MRT often is closer to 1H than 13C NMR spectroscopy). – Buttonwood May 22 '17 at 21:18
• @JamesBlackwell Maybe i) a brief degassing (e.g. some pump thaw cycles while using an ultrasound bath) of the freshly prepared solution prior to the addition of agar-agar and ii) admixing agar-agar without trapping much air would be beneficial to keep the solution, and eventually the forming gel, "air free". I'm curious about the outcome, too. – Buttonwood May 22 '17 at 21:23
• Hi Buttonwood, I think a small amount of ethanol can be used as a degassing agent and it shouldn't affect the properties of it too much. I was also wondering about an autostirrer? I've seen some papers where they use one at 120RPM and allow it too cool very slowly. Mix while heated for 30min and then 30min as it cools. What do you think? – James Blackwell May 23 '17 at 9:19