Introduction: EcoPatches: Maker-Friendly Chemical-Based UV Sensing
This Instructable walks through the process of creating wearable UV-sensitive patches called EcoPatches. EcoPatches change their color when they are exposed to UV radiation from the sun. The more they change their color, the more they have been exposed to UV during the day. EcoPatches are not reversible (i.e., they do not return to their original color).
Unlike existing products (e.g., My UV Patch, LogicInk), EcoPatches can be made using an inkjet printer and off-the-shelf materials. This means that you can create whatever designs or formulations you want to fit your needs. For more information about how EcoPatches work, please refer to our academic publication: (link pending)
Warning: Just like many other inks, the ink that will be created in this Instructable should not be ingested or placed directly on skin. Please wear personal protective equipment (gloves and eyewear) and read the accompanying datasheets while working with chemicals.
- Inkjet printer with refillable ink cartridges and ink (link)
- Syringe for filling ink cartridges
- An opaque container with a lid
- UV radiometer or dosimeter (link, optional)
- Photography color chart (link, optional)
- Photoacid generator (e.g., 0.1 M diphenyliodonium chloride)
- pH indicator dye (e.g., phenol red)
- Basic buffer (e.g., 1 M sodium hydroxide)
- 95% ethanol
Step 1: Deciding What Exposure Levels Are Important
UV intensity is measured in UV Index (UVI) and cumulative UV exposure is measured in UVI-hours (UVI-hrs). The amount of cumulative UV exposure that leads to sunburn for an individual depends on the amount of melanin in their skin. Darker skin tones contain more melanin, making them more resistant to UV irradiation. The Fitzpatrick scale is a widely recognized classification scheme for human skin color. The picture above shows the Fitzpatrick scale and the cumulative UV exposure levels at which people with different skin tones are likely to become sunburnt. Note that as a person's skin tone becomes darker, the minimum amount of UV irradiation that leads to sunburn risk grows exponentially.
EcoPatches are very similar to pH strips. They change their color to convey information about how they have been exposed. Once you have seen enough of them, you can memorize the meaning of different colors and estimate their measurements on your own. However, it is easiest to put them next to a reference color scale so you can make a side-by-side comparison.
With that in mind, you will want to think about which values are important to include in your reference color scale. Do you just care about the moment when you are likely to get sunburnt? Do you want to know when you've reached the halfway mark? Keep in mind the number of values in your desired scale and their associated values for later on.
Step 2: Creating an Image File
You will want to plan out your EcoPatch design so that it includes a region that will change its color when exposed to UV radiation and a collection of regions that will act as a reference color scale. Other than that, you have complete control over how your EcoPatches will look. For example, the picture you see above uses the sky as the UV-sensitive region and the mountain as the scale itself.
You can create your design using your favorite image editing program (e.g., PhotoShop, GIMP). The image file should be split into two layers:
(1) Dynamic layer
This layer will include all of the elements in your design that will change color when they are exposed to UV radiation. Therefore, the colors that you use to draw this layer will not be the final colors in your design. Instead, this layer should be a single color corresponding to one of the cartridges in your printer (e.g., magenta, yellow).
(2) Static layer
This layer will include all of the reference color scale and all of the aesthetic elements in your design that will stay the same regardless of UV exposure. Unfortunately, you won't know what the color scale should look like until you have tested out your UV-sensitive ink; for now, just pick whatever colors you like!
Step 3: Making UV-sensitive Ink
Before we describe how to mix the ingredients for the UV-sensitive ink, we should first explain the science behind it so you can come up with the right components and ratios for your needs.
Under UV irradiation, photoacid generators (PAGs) produce hydrogen ions from photon absorption. The hydrogen lowers the pH value of the solution, causing the pH indicator to change color. Basic buffer neutralizes hydrogen ions to slow this process down. The addition of ethanol optimizes the ink's viscosity for inkjet printing.
The choice of pH indicator affects both the range and discernability of colors the ink will exhibit as it accumulates UV irradiation. For example, phenol red changes from yellow to red between pH 6 and pH 8, and bromothymol blue changes from yellow to blue between pH 2 and pH 7. pH indicators that exhibit more drastic changes across smaller pH differences are generally preferred since those indicators will be more sensitive to smaller doses of irradiation.
The ink's rate of change is dependent on its buffer concentration. Inks with more buffer will undergo slower color transitions than inks with less buffer.
The UV-sensitive ink should be mixed in an opaque container so that it does not change its colors while you are mixing it. To create a one-size-fits-all EcoPatch that is useful for most skin tones and goes from purple-to-pink, mix the following components:
- 1 part 0.1M diphenyliodonium chloride (PAG)
- 1 part 1M sodium hydroxide (basic buffer)
- 10 parts phenol red (indicator dye)
- 10 parts 95% ethanol
In other words, 140 mL of ink would be made using 10 mL PAG, 10 mL NaOH, 100 mL phenol red, and 100 mL ethanol. Remember to always wear personal protection and read the materials datasheet when you’re handling chemicals.
Step 4: Making a Test Batch of EcoPatches
In order to test out how your ink will react to UV irradiation, you will need to print out a test sheet of EcoPatches. You only need to print out the UV-sensitive regions for this step, but we will describe the entire printing process here anyway.
- Fill an empty ink cartridge with the UV-sensitive ink. Make sure to fill the cartridge that corresponds to the color you’ve assigned to the dynamic region in your design file.
- Load the printer with a sheet of temporary tattoo paper and then print the dynamic (UV-sensitive) layer only. Note that the UV-sensitive ink has a higher viscosity than regular inks, so it comes out of the cartridge at a different rate. To ensure that enough UV ink gets onto the sheet, run the same sheet through the printer 10 times.
- Replace the UV-sensitive ink cartridge with a standard ink cartridge and print the static (non-UV sensitive) layer only. Since this step uses regular printer ink, only one iteration is required in this case.
- While the ink dries for a couple of minutes, mix the archival grade resin and hardener in a 1:1 ratio. Heat the mixture at 65°C for 2 minutes or until the mixture is fluid and can be thoroughly mixed. Immediately after mixing, pour the resin mixture onto printed sheet of EcoPatches. Using a squeegee, press and spread the resin mixture evenly over the sheet.
- Leave the resin-covered sheets in a dark and well-ventilated area at room temperature 48 hours to cure the resin.
- Apply the double-sided adhesive and then the skin-safe adhesive to the sheet.
Note that the resining process is particularly critical to how your EcoPatches will react to UV irradiation. If you do not spread the resin evenly across the sheet, then your EcoPatches will change their color at different rates. The thicker the layer, the more UV exposure the EcoPatch can take before saturating. For our formulation, we found 0.5 mm to be an optimal thickness.
Step 5: Performing Color Calibration
Now that you have printed your first sheet of EcoPatches with your UV-sensitive ink, it is time to see how it works! Unless you have a super-expensive solar simulator, the easiest way to test your EcoPatches is to sit outside and expose them to the sun. You will want to keep track of how much UV exposure they have received so you can determine the colors that correspond to each part of your reference color scale.
There are three ways of tracking cumulative UV exposure, which we have ordered from most to least accurate:
- UV dosimeter: UV dosimeters measure cumulative exposure over time, so just keep the device on and close to the sheet of EcoPatches.
- UV radiometer: UV radiometers measure instantaneous exposure over time. To get a cumulative exposure measurement, you will need to compute a rolling sum or integral over time. For example, if your sheet sits in an environment with a UVI of 2 for one hour and a UVI of 4 for two hours, the cumulative exposure will be (2 UVI * 1 hr) + (4 UVI * 2 hr) = 10 UVI-hrs. You will probably want to calculate the integral at 10-15 minute intervals.
- UV index: Many websites (e.g., National Weather Service, Chrome Dome) report the daily UV Index by zip code. These measurements may not align with the UV Index in your immediate area since factors like angle and shade can affect how much irradiation actually reaches the sheet, but this is the cheapest option of the three. Using the same calculation as with the radiometer, you will want to regularly look at the UVI in your area and calculate the cumulative exposure.
Whenever your EcoPatches have been exposed to a value that you want to show on your reference color scale, take a picture of them with a camera or a smartphone. If you have a photography color chart, include that in the photo so you can get an accurate representation of the colors later on. Repeat this process until your EcoPatches have saturated in color.
Using image-editing software (e.g., Photoshop, GIMP), open up each photograph and extract the color of your UV-sensitive ink. These will be the colors that should go on your reference color scale. If you used a photography color chart, use the corresponding software to color-correct the images before extracting the colors.
Step 6: Iterating on Your Design
There are lots of steps to making EcoPatches and chemistry can be very finicky. You will probably need to go through Steps 3-5 to get the ideal ink formulation and fabrication process. There are many factors that affect how your EcoPatches will respond to UV irradiation. If your EcoPatches are reacting too quickly, try using more basic buffer or a thicker resin layer. If your EcoPatches are reacting too slowly, try using less basic buffer or a thinner resin layer.
Step 7: Making Your Final EcoPatches
Now that you have determined the colors that should go on your reference color scale, you are now ready to create your finished product! Simply add the color references to your image file and then repeat the same process you went through for making your test batch. It is important that you go through the printing process the exact same way you did when you created your test batch. Otherwise, your EcoPatches will behave differently and your reference colors will not match up as they did before.
Assuming you go through the process in the same way as before, then congrats! You've made your first completed sheet of EcoPatches.