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Programmable cake

April 2011

In my research group there is something of a tradition to make novelty cakes for the farewell gatherings when someone leaves. I'm not going to lie to you: this is a tradition invented and driven by two of us and tolerated by the others. It began with a multimeter cake for a departing lab technician:

Over time things started to get out of control, with subsequent efforts over the years encompassing entirely edible renditions of:

• A gated silicon quantum dot
• Don Eigler's quantum corral
• A silicon nanowire, along with a laptop measuring transport through it
• The Kane quantum computer

At some point along the way I got it into my head that the pinnacle of cake achievement would be one of either:

• A cake with (edible) moving parts
• A cake with (edible) electronic functionality

As someone who thinks all mechanical solutions begin and end with a hot glue gun, I recognize that my best hope of success lies with edible electronics.

You might not be surprised to learn that very few electrical components can be rendered in an edible form. Even the most basic element, low resistance wiring, is difficult. The first (edible) thing that comes to mind is something based on salt, but there are two problems with this:

1. Nobody wants to eat a cake decorated with some kind of gross salt paste, with the possible exception of northern Europeans and their salty liquorice. I am all about putting gross salt paste on my toast, but not my cake.
2. Ionic conductivity is not very good. Seawater has a typical resistivity of 20 Ohmcm, which means a 10cm long, 5mm diameter tube of it would be about 1kOhm. This is not outrageous, but suggests that you would need to use something very salty to get reasonable wires.

The breakthrough came when I discovered that edible silver foil is a real thing, that you can buy. It's popular in Indian sweets, and provided it's relatively pure it is generally regarded as safe to eat - the foil is only a few microns thick so the quantities involved are low.

The book The Hungry Scientist Handbook put me onto this, and furthermore they have even gone ahead and already done exactly what I had in mind! Their approach (see here) uses silver foil wrapped around liquorice strips to illuminate a chain of LEDs on a cake. The LEDs are not edible, but hey, you have to start somewhere.

Clearly it would be necessary to raise the bar. Instead of a chain of static, battery driven LEDs on a cake, I want ... a scrolling LED sign cake.

And that's exactly what we did. Writing something in icing is so last century. By coating only one side of a liquorice strip with the foil, you can cross wires without them shorting to each other. Then you just have to lay out a matrix and stick your carefully cleaned LEDs into the straps. A microcontroller is driving the LEDs to scroll out a message.

It all sounds very easy, and I have so far been describing it in the manner of somebody who has realized absolute success in their venture. Sadly that is not quite the case. The truth is that the cake was something of a failure, saved only by the temerity of its vision (and also its deliciousness). It turns out that silver foil is extremely difficult to work with. In the final product we ended up with a lot of hairline cracks in the foil which resulted in a lot of broken connections. A few LEDs still flashed in time, but not enough to carry the effect.

So the cake ended up being salty anyway. Salty from my tears of shame.

I've put this up here for two reasons:

1. Despite failing in the end, it was still awesome.
2. Hopefully some kindred spirit out there takes what I've learned and has another, more competent attempt at it. Please do, and please let me know how it goes!

P.S. I've since seen edible silver paint being used which looks much easier to work with.

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