How to make your own ultrasound gel (which is also sterile and edible and environmentally friendly) **UPDATED--NEW RECIPE**
Ultrasound requires an aqueous interface between the transducer and the skin or else all you see is black. Ultrasound gel is a clear goo, looks like hair gel or aloe vera, and is made by several companies out of various combinations of propylene glycol, glycerine, perfume, dyes, phenoxyethanol or carbapol R 940 polymer along with lots of water. It is hard to find this information, but it is available in the material data safety sheets for the various companies that make it. The recipes are proprietary. Ultrasound gel is not super expensive, but it is not that easy to find in a store or in a developing country. It costs about $25 for 5 liters on Amazon, or $5 for a nice 8.7 oz squeeze bottle. It smells ever so slightly medicinal and leaves a sticky, then dry white residue as it dries.
There should really be some sort of powder that you mix up with water that makes ultrasound gel so we don't have to be shipping the water part of it, which is undoubtedly about 99% of the contents, long distances. But there isn't a powder. I have been looking. No instant ultrasound gel.
With a mixture of optimism and singularity of purpose I went to the kitchen and tried out 6 different recipes for an aqueous goo that would transmit sound waves. I thought that I could make ultrasound jam out of water and pectin, but that doesn't really work. Obviously there is something magic about fruit that makes pectin gel, maybe the acid or the sugar. Without fruit, even no-sugar pectin becomes about the consistency of spit. (I also tried spit, which does work, but has various obvious drawbacks.) I tried plain gelatin and water and got beautiful clear jello, which falls off the transducer, but kind of works, but is also messy. I tried corn starch and water, as if making extremely boring gravy. That was lovely and white, but the water wants to come out of it so it just slides off the transducer. I tried tapioca flour which I boiled with water, producing a nice clear, very mucoid gel which dries like glue on the skin and is very uncomfortable. I tried xanthan gum, a bacterial polysaccharide used to bind and thicken, boiled and cooled, and although it thickens the water it is slimy and falls off the transducer and makes a mess.
The recipe that worked (and worked great) is guar gum, salt and water. Guar gum has been used for a very long time in countries like India and Pakistan to thicken food and is now used often by people who can't eat gluten, to thicken gravies and make breads. Guar gum is the ground endosperm of the guar bean, which is very rich in a carbohydrate that avidly absorbs water. Guar beans are also eaten green and the pods are used as a vegetable ingredient after shelling out the beans. Guar gum is available in the flour section of many grocery stores and costs about $10 for a 220 gram bag. It is purported to be good for diarrhea, constipation, diabetes and lowering cholesterol. It has been added to infant tube feed formula in intensive care units to decrease stool frequency.
I messed with the recipe awhile and came up with a very nice slightly caramel tinged ultrasound gel this way:
1. Mix 2 teaspoons of guar gum with 1-2 teaspoons of salt. (The amount of salt isn't vitally important since it is just added to keep the guar gum from clumping. Using slightly less than a teaspoon of salt per 2 cups makes a gel with which is isotonic, which would be ideal for use near eyes or other mucus membranes or on open wounds).
2. Boil two cups of water.
3. Slowly sprinkle the guar gum/salt mixture into the boiling water while stirring vigorously with a fork or whisk.
4. Boil for about 1-2 minutes until thick and well mixed.
5. Cool before using. Save lives.
This is wonderful ultrasound gel (see photo above). I tried it and it works at least as well as the proprietary stuff, and probably doesn't dry out quite as fast. It wipes off easily and doesn't leave a sticky film. Even though it is not entirely transparent, there is no reduction in the quality of the ultrasound image compared with the standard clear ultrasound gel. It costs about 25 cents for a half pint, is sterile when you have finished making it and is completely non-toxic. The ingredients are available in many developing countries, not to mention the US. It is edible. It is not particularly bacteriostatic, though it could be made bacteriostatic with a little EDTA (but then it wouldn't be edible). It is probably best made and used for a couple or 3 days, then discarded if unused, though I kept some in a clean bottle at room temperature and it was stable and smelled fresh for over a week.
It is quite thick, like regular ultrasound gel, so it is a bit of a trick to get it into a squeeze bottle. A large bore funnel works, or the cooled gel can be squeezed into the bottle out of the cut end of a plastic bag. It can also be kept in a jar and used with a spoon.
This is kind of exciting. Now I will no longer be dependent on ultrasound gel manufacturers. If I was in Haiti, either I or someone at the house where I was staying could make up a batch of this the night before clinic and I would have fresh clean ultrasound gel with which I could be generous in my scans. The water wouldn't even have to be sterile since the stuff is boiled when it is made. Let there now be ultrasound in places that Amazon.com does not reach!
Here's a YouTube video of how to do it.
**This article has been very popular and readers have left all sorts of new good ideas as comments. In order to make good ultrasound gel it is really only necessary to have some kind of a powder that, when mixed with water, creates a mostly transparent gel which clings to the ultrasound transducer. Polysaccharides are good for this, and guar gum is one of the least expensive that is available worldwide. A reader, however, just told me that he used glucomannan powder in a proportion of 1/2 teaspoon to a cup of water. I just tried it and it is EVEN BETTER THAN GUAR GUM. It, like guar gum, is a thickener and emulsifier, it is used by dieters to decrease appetite and is safe both topically and internally. It is available online and probably in health food sections of grocery stores as a dietary supplement. Glucommanan is a cell wall component of many plants, including the roots of the Konjac plant. Unlike guar gum it does not clump and can be mixed in cold water then allowed to thicken over a few minutes. If it is mixed into boiling water its texture is smoother than when it is made with cold water, and of course it is also sterile, which is very useful. It is almost completely clear, has no flavor or smell and leaves very little residue. Thank you commenter who goes by the name "addedupon"!
***Recently an article has come out in PLOS 1 looking at making gel with different kinds of flour-type substances readily available in markets in resource poor settings. They recommend using cassava flour which is actually the same thing as tapioca flour. The problem with this gel is that it is very glue-like and when I tried to wipe it and even wash it off of my skin, the remaining residue was very uncomfortable. When I have done ultrasound in Africa there is usually nothing available except maybe some flimsy toilet paper to wipe ultrasound gel off of the patient. It is certainly worth experimenting with different combinations (they also mention sorghum flour which sounds promising) but it is important to use them on yourself to see what they feel like when they are incompletely wiped off, as is the usual case.