Anyone who has made Jello knows how difficult it can be to spring the wobbly treat from its mold intact. Now, imagine trying to dislodge something 10 times softer than gelatin, while keeping every detail unscathed down to a microscopic level. That was the problem faced by U-M biomedical engineering professor Shu Takayama.
His team is working with a type of silicone called Sylgard 527. It’s so soft that just a few cells can squeeze it out of shape.
“Soft silicone structures are useful for studying human cells outside the body,” Takayama said. “We can use them to measure the very small squeezing effect that cells generate during wound healing. This enables us to test the effects of drugs using very small samples of human cells, instead of testing on actual patients.”
The solution came when a PhD student and avid cook was trying a new recipe for homemade cotton candy. The cotton candy was a total failure but when he took the hardened mass out of the pan, he noticed that the sugar retained every detail of the pan it came out of. Why not use hardened sugar as a mold for super-soft silicone? They could pour in the silicone, wait for it to cure, then dissolve the mold in water, leaving perfectly cast pillars of soft silicone.
The sugar molds turn out perfect soft silicone pillars every time.
The pillar-making process begins with a hard epoxy “negative mold” — a mirror image of the sugar mold used to cast the final pillars. They pour in hard silicone to create an initial plastic mold. Next, the molten sugar mixture is poured into this initial plastic mold and left to cool, hardening into what looks a lot like a piece of hard candy. The hardened sugar is popped out of the initial plastic mold and the sugar is then used as a mold for the silicone. The researchers pour the silicone into the sugar mold and cure the concoction in an oven. Finally, the silicone and sugar mold are put into a water bath. The sugar dissolves, while the water-repellent silicone stays intact.
The candy molding process is detailed in a paper published in the journal Lab on a Chip.