Scientists Have Found a Way to 'Tattoo' Living Cells With Gold

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Tattoos are designs imprinted on the body by injecting ink with needles just under the skin. But recently, scientists at Johns Hopkins University (Baltimore, Maryland) are working on a different sort of "tattoo". They have created technology to label a cell and not with pictures but with gold particles. They are doing this not for pretty designs, but to "monitor and control the state of individual cells and the environment surrounding those cells in real time". How is this labelling done?

A cell with gold nanodots imprinted on it (sciencealert.com)

How would you lay out a grid of sensors in an organized pattern on the human body? Think of placing a net on your skin, and at every place where the fibers cross, there is a sensor. Place that on the arm, back, or wherever, and the sensors can measure temperature, hormones (like insulin), blood flow, etc. Or do it with very thin circuit boards. But that's on skin. Below is a 10-year-old prototype of a skin-mounted sensor to measure body temperature, blood pressure, and electronic signals from muscles or the heart.

(University of Tokyo design, from International Business Times, 10/2014)

There are many technologies that implant such circuitry on the skin, or just under the skin like this one from Dermal Abyss that changes color when pH or glucose or albumin levels in the body change.

This type of technology has even been designed to fit deeper in the body. Here is a picture of sensor circuitry attached to a rat brain.

From the Johns Hopkins University researchers

What about going much smaller to the level of marking an individual cell? David Gracias of the Johns Hopkins University team said that if cells could be labeled with sensors, they could be used to not only monitor cells but control them in some way, maybe even control their environment, too. No further details were given.

Laying a network of electronic sensors on something as small as cells requires more precision than on or under the skin. Just how do you make them stick and not kill the cell in the process?

The technology is called nanoimprint lithography (NIL). Regular lithography is done with drawings on stone (litho-) using oil or greasy utensils, and when paper is placed on top and pressed firmly, the recording (-graphy) copy is transferred to the paper. See the short video demonstration below.

Stone lithography process (YouTube)

So, it's essentially one way of making a copy of a picture or text.

"Nano" refers to something very small, so nanoimprinting involves copying something at a microscopic level. Rather than jumping to imprinting circuitry first, the initial step that researchers have taken is to use metallic (gold) dots instead of sensors just to see if the overall process is feasible. Here's what they did at John Hopkins University. Gold, by the way, is often used for body sensors because it prevents signal loss or distortion when used in electronics.

1. Start with a silicone wafer. Coat that with a thin layer of a polymer called PMGI. On top of that, put a thin layer of another material called "NIL resist". Then use a silicone stamp to imprint a pattern on the NIL resist. Coat the exposed surfaces of that template with gold.

2. Using sound waves, remove the NIL resist and unnecessary bits of gold. Cover it all with another polymer called PMMA.

3. In order to lift the gold and PMMA off the silicone wafer, the PMGI is first dissolved chemically. Then, a glass cover slip is put under the PMMA/gold dot array. 

4. An oxygen plasma etching process removes the PMMA to leave behind gold dots in their original imprinted pattern, now sitting on the cover slip. To lift this like lithograph paper from a stone drawing, an organic compound called cysteamine is put on the gold, so it will allow the gold to be held by a layer of gel coating.

5. In a second lithographic-like step, the gold and gel are flipped over with gel side down to expose the bare gold nanodots' surface. Gold won't stick to cells by itself, so a layer of gelatin is put on it. Then, a suspension of living cells is put on top. Cells stick to the gelatin-gold nanodots.

6. In a final lithographic-like step, all of this is flipped over again and placed in a culture dish, and the gelatin is removed chemically. Now the cells can grow in the culture dish, and the gold nanodots bonded to them can be seen on top.

If you're trying to imagine how all this works, here's a condensed diagram of the process from the point where cells are put on the gold, then flipped. The nanodots in the real photo were colored in artificially with computer imagery.

The Johns Hopkins University researchers tried the process on a non-living bead-like microparticle first. It was very close in overall size to a cell, and the nanodot pattern seemed to hold. But would this work on a not-so-smooth, not-so-round living cell? They tried it on living fibroblast cells, and as you can see from the inset picture on the right, it worked fairly well.

Microparticle with nanodots (left); fibroblast cells with nanodots (right) (From Kwok, et al., bioRxiv)

The fibroblast cells flexed and moved about gently as they grew, but the nanodots stayed attached for up to 16 hours.

All of this merely shows the feasibility of affixing metallic material in an organized array directly onto living cells. None of the nanodots performed any function, nor were they connected by wires like circuits in the picture of the rat brain. But this lays the groundwork for such a thing at the microscopic level. As Gracias said, "It's the first step toward attaching sensors and electronics on live cells." This could be used by doctors to hypothetically track the health of isolated cells, and potentially identify, diagnose, and treat diseases sooner than is done now.

Gracias and his team have also labeled fibroblasts with small sheets of gold wires. You can see two cells moving about with them in this short video (time lapse of 16 hours). They don't seem to be moving any differently than unlabeled cells.

Video footage from Johns Hopkins

Fibroblasts are the most common type of cell in the connective tissue of our bodies. They are not only useful in the body's structure but also for healing of wounds. They are routinely used in many types of research experiments. It's not known why the Johns Hopkins researchers chose them for their first cell experiments, but they expressed interest in labeling other types of cells for future work. 

BONUS INFORMATION

Here is a 4:36 video from MIT that shows how some gold tattoos can be used decoratively or functionally (to control smartphones or computers, or to show body temperature).