Billions of ‘Brood X’ cicadas emerge after 17 years underground to get jiggy with it
Engineers at Columbia University have developed a single-chip medical device so small it can be injected into patients using a hypodermic needle
Billions of periodical cicadas emerged from the ground in early June across 15 states in the US, from Georgia in the south to New York in the north. The insects spend most of their lives as nymphs living less than a metre beneath the surface, feeding on sap from tree roots. Then after either 13 or 17 years, depending on the species, they emerge in vast numbers for a brief adult stage, mate and die.
There are 15 ‘broods’ in total, which are grouped according to the year in which they emerge. Brood X consists of all three 17-year species, and is the largest and most impressive cicada emergence.
So why do they emerge every 17 years? “For a long time the explanation was that the prime-numbered 17year cycle, and the 13-year cycle of some other periodical cicada broods, prevented predators getting into phase with them by having a life cycle that was a divisor – prime numbers having no divisors,” said Prof Adam Hart, an entomologist at the University of Gloucestershire. “But the current thinking is that it prevents hybridisation between broods with different cycles and mathematical models back this up.”
The cicada nymphs emerge after sunset and crawl to a nearby tree, plant or bush. At this stage of their life cycles, they are pale brown and wingless. Shortly after emerging, the nymphs begin the process of moulting their exoskeletons and entering adulthood.
Following the moulting, countless discarded exoskeletons can be found all over the area where the cicadas emerged. When insects moult, they also shed the linings of their trachea, which are used for breathing. These can be seen as white threads in this image.
Cicadas have five eyes – two distinctive red compound eyes and three much smaller ocelli located in the centre of their heads, which are believed to detect light and dark.
Males use a special organ on their abdomen called a ‘tymbal’ to produce their characteristic screeching sound. They do this to attract females to mate with. Cicadas can mate multiple times, and a single female may lay as many as 600 eggs. Once the eggs hatch, the nymphs burrow into the ground to begin the entire cycle all over again.
WHAT’S THE BACKGROUND TO THIS RESEARCH?
Moore’s Law states that you can cram more and more transistors into a certain area on an integrated circuit chip. And that number’s been growing exponentially for the last 30 or 40 years. It’s primarily been used not to make the chips smaller, but to put more transistors on a chip that’s the same size. So, we’ve gone from chips with a thousand transistors to chips with tens of billions. But another thing you could do with that density is use it to make chips that are very, very small.
HOW SMALL ARE WE TALKING?
So this is the smallest autonomous single chip system that we know of that supports both power and bidirectional communication – it’s roughly 300 x 300 microns [one micron = 0.001mm].
WHAT ARE THE MAIN CHALLENGES OF PRODUCING A CHIP THIS TINY?
A chip needs to be powered and you need to be able to communicate with it, otherwise it’s of no use. So what we’ve been doing is an example of a device where the chip is the entire system. There’s nothing else; no external sensor array, no external antenna, no external battery, there’s no external anything. And for a chip to operate as an autonomous system, it needs to meet a few criteria. All of the power and communication to the chip needs to be done wirelessly. So, all the antennae for that wireless powering and communication need to be integrated. And then in the case of these kind of implantables, the chip is also sensing something, so that sensing function has to be integrated.
You’d be very challenged to communicate with a device this small with electromagnetics, such as radio waves, because the wavelength is too large relative to the size of the device. Even at tens of gigahertz, you’re talking about wavelengths in the several millimetre range. This device is much less than a millimetre in size, so that’s why we use ultrasound. This device is powered and communicates with acoustics, not electromagnetics, which is useful because sound waves travel very well in the body.
HOW EXACTLY DO YOU POWER THE CHIP WITH ULTRASOUND?
We’re looking at using these devices to augment ultrasonography, to provide additional information that’s not intrinsically available. The way ultrasound works is that it sends a sound wave into your body. And when there’s an acoustic mismatch, a difference in the acoustic impedance
“So what you see is this tiny chip in your ultrasound image flashing at you. And that flashing is sending information back to you that tells you what it measured locally”
[the amount of resistance an ultrasound beam encounters as it passes through tissue] due to different materials or
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some of that acoustic energy back to the imager. And that’s what you see in an ultrasound image.
But there are many things that aren’t available or known to you. For example, this particular chip [we’ve designed] measures temperature. There’s no way in intrinsic ultrasound imaging that I can know anything about temperature. If I put one of these devices in your body and the ultrasound beam hits it, the energy turns the device on, which then measures the local temperature and
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the ultrasound imager accordingly. So what you see is this tiny chip in your
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back to you that tells you what it measured locally.
The chip also does something called ‘energy harvesting’ – it harvests the energy from the ultrasound beam. And it can do that because we’ve integrated a piezoelectric material into the chip that converts sound to electricity. So, when you apply a pressure wave to this material – which is what sound is, a pressure wave – the material gets squeezed a little bit, which generates a voltage. That voltage is used to power the chip.
HOW DEEPLY CAN YOU IMPLANT THE CHIP INTO THE BODY?
We’re using about 5MHz ultrasound for these devices. Most clinical ultrasound is a little lower frequency, usually about a 1MHz or so. As you go up in frequency, you can penetrate less deeply as the ultrasound is absorbed more in your tissue. But at 1MHz, the wavelengths are too large to communicate with this device. So, at 5MHz, we can go about maybe six or seven centimetres deep, which is pretty substantial.
HOW DO YOU PLACE THEM INSIDE THE BODY?
6he EhiRs are small enough to fit in an 18-gauge hypodermic needle, so that’s how we inject them into the body. They can also be removed in the same way.
HOW DO THEY FUNCTION ONCE THEY’RE IN?
There are two ways the devices could be used. One is where it’s chronically implanted – you simply put it in and leave it alone. But much more testing has to be done to understand the long-term consequences of having something like that in your body. The belief is that being so small will help it to be acceptable and so there’ll be less of a foreign body response. The other way would be that you simply remove it after a period of time. And you can do that using the same kind of hypodermic needle, but guided with ultrasound. You use the ultrasound imager to guide the needle, find the device and then suck it out.
WHAT ARE THE POTENTIAL APPLICATIONS?
Our particular design provides additional information to an ultrasound imager. And that can be used in almost any context in which you’re doing ultrasound imaging. For example, there are many clinical applications in which clinicians apply heat. So, if you wanted to know how much heat you’re applying, you could use the chip that measures temperature. There also might De sReEifiE DiomarMers that you’re looking for, maybe you’re doing continuous ultrasound imaging over time to verify that a tumour hasn’t come back. But it might make sense to implant devices like this that measure biomarkers to indicate even earlier if there’s a concern. We’re also looking at trying to improve healing by monitoring various biomarkers within a wound.
WHAT ARE THE NEXT STEPS?
Well, there are lots of other things you could do with these ‘chip as a system’ implants. There’s a lot of interest right now in interfaces to the central nervous system – brain/computer interfaces and devices that interface with the peripheral nervous system for things like pain management, and interactions with the autonomic nervous system to control things like blood pressure.
What these devices are delivering is
what we Eall nXolumetriE eHfiEienEyo,
a statement of how much function you’re able to get out of the implantable device for a given amount of displaced volume. These devices are the most
XolumetriEally eHfiEient deXiEes you
can imagine, because you can get the maximum amount of function out of these devices for a minimum amount of displaced volume. And that gives them a lot of advantages.