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From Sour Milk to Advanced Sensing Materials

Posted on March 15th, 2017 by in New Materials & Applications


I’ve attended the Consumer Electronics Show (CES) three out of the last four years.  One of the trends that has persisted through the nearly schizophrenic shows has been the Internet of Things (IoT).  The idea is that a network of sensors communicating through the internet will open up an automated system that regulates anything that can be measured.  The goal is to optimize the interaction among things.  However, the assumption is that the sensors must be microelectronic devices.  What if they were sophisticated materials instead?  How could this change the seedling market and affect materials producers?

IoT was first depicted as your refrigerator communicating with your HVAC system so that both would run at different times and avoid peak electric usages to minimize your electric bill.  This was intriguing because it promised a large reduction in wasted energy and products.  I figured if my air conditioner could communicate with my refrigerator, it would be an easy step for my milk to talk with my phone and tell me, “I’m going to go bad in two days so either drink me or replace me!”  Sour milk, and its older, more disgusting brother, chunky milk, is simply milk that has generated too much lactic acid, and the casein protein molecules have lumped together forming curds. The pH decreases as lactic acid increases until it becomes unpalatable, somewhere around a pH of 5.0.

Its this milk idea that got me thinking.  Would each carton of milk need its own sensor, or could a material be a substitute?  Maybe a square that turned from white to green with an increase in lactic acid content would be enough compared to a whole system.  Another option could be a material that swells with an increase in acid and then sends a signal through a RFID to the nearest router in a home.  Though the latter is more in-line with an IoT sensor system, the former would add value to the consumer while keeping infrastructure costs down.  Instead of a complex sensor system, all that is needed is information at a quick glance.  At this point it was time for some research, because as one of my mentors, Doug, used to say, “An hour in the library is worth ten in the lab.”

One of the first pieces I found was from National Geographic, by Rebecca Rupp, PhD (1).  Dr. Rupp outlined the milk problem very well and came to a similar conclusion that a way to measure lactic acid in milk could be the basis of a sensor.  She pointed to recent work by a group of researchers from the University of California at Berkeley, and the National Chiao Tung University in Taiwan (2).  Interestingly, the team used 3D printing to make the sensor cap, taking advantage of another technology trend that is popular at CES.  The sensor itself was printed, including several injections of silver ink for the wiring.  Though this is a good demonstration of an advance material and fabrication of simple electronics, it would be difficult to scale such a process for mass consumer use. Still, it is a cool way to measure milk freshness!

The 3D printed solution proved that a sensor system could be made and implemented. However, this solution used complex manufacturing and microelectronics.  It also required milk to be splashed against the cap, taking just as much time as sniffing the milk. Something faster, simpler, and cheaper is needed, and a layered material system starts to make sense at this point.  My initial thought was something that swelled.  An acid-base reaction is simple and litmus paper is basic chemistry. A more sophisticated litmus test has to exist, and still be safe for food contact.

Thinking about materials that are safe for human use I started contemplating chemicals used in baby products, which society holds to a high degree of quality.  Diapers and other such products use hydrogels as an active component.  A decent amount of research has gone into hydrogels because they can be tuned fairly easily, especially in drug delivery applications.  Part of the research includes adjustments to respond to different acid levels.  There has been some research into this specific area indicating that swelling of a hydrogel could work (3).  Thinking about this, the potential exists to tune a hydrogel system to swell at a certain pH.  If a layer of hydrogel can be tuned to respond to increasing lactic acid in milk, it could close a circuit, open a valve or activate a symbol.  A variety of mechanisms exist to provide a quick visual indication that your milk is in the funky zone.

After chatting with Marc, a friend who is a technology leader in the medical industry and holds a PhD. in polymer chemistry, he thought of a different solution and suggested using an indicator.  He recalled that a pH indicator can be formed from red cabbage juice.  Since it is food-based, it would have a much easier route to be considered safe than starting outside of the food chain and then working in.  He imagined including an indicator mixed with the high-density polyethylene (HDPE) used to form common milk jugs.  If the jug is white, the milk is ok.  If the jug is red, don’t drink it!

Solutions that adapt to existing infrastructure and offer clear value to customers are well positioned for success.  As IoT struggles to establish itself in the market, a gap exists between standard dumb products of today and the promise of the future.  Smart materials can fill the gap.  Simple color changes, switches, and other signals can help consumers improve existing products and overall quality of life.  Wouldn’t it be great if your milk container turned green before you did!


  1. Rupp, R., (September 8th, 2016). When Good Milk Goes Bad. National Geographic. Retrieved from
  2. Wu, S., Yang, C., Hsu, W., Lin, L., (July 20th, 2015). 3D-printed microelectronics for integrated circuitry and passive wireless sensors. Microsystems & Nanoengineering, 1. Retrieved from
  3. Seddiki, N., Aliouche, D., (November 23rd, 2013). Synthesis, characterization and rheological behavior of pH sensitive poly(acrylamide-co-acrylic acid) hydrogels. Arabian Journal of Chemistry. Retrieved from



All opinions shared in this post are the author’s own.

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