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A self-healing nanofiber sensor for temperature-sensitive logistics
(Nanowerk Spotlight) Perishable goods, especially for food and pharmaceuticals, must be kept within certain, low temperature ranges during manufacture, storage and transport in order to prevent degradation and spoilage. Small devices called time-temperature indicator (TTI) are used to detect and record temperature changes throughout the entire cold-supply chain.
"Unfortunately, conventional TTIs are very limited to be broadly used for many practical applications," Dr. Dongyeop Oh, a Senior Researcher at the Research Center for Industrial Chemical Biotechnology at Korea Research Institute of Chemical Technology (KRICT), tells Nanowerk. "For example, diffusion-based TTI is susceptible to forced impact, punctures, and cuts. Chemical reaction-based TTI is expensive and poses a risk of potential chemical leakage."
To address these issues, KRICT researchers fabricated an innovative type of time-temperature sensor that uses a self-healing material. This sensor only requires a single material with one phase, and functions as a result of its own intrinsic characteristics.
"What we developed contains superior characteristics in all aspects compared to a conventional TTI," Oh points out. "Our TTI functions even after it is subject to forced impact, punctures, high load, and even cuts. Moreover, it is highly flexible, not at risk of chemical leakage, and cost-effective."
As the team, including first author Sejin Choi and senior co-authors Sung Yeon Hwang and Jeyoung Park, reports in Advanced Materials ("A Self-Healing Nanofiber-Based Self-Responsive Time-Temperature Indicator for Securing a Cold-Supply Chain"), their TTI is based on an electrospun, self-healing, nanofiber nonwoven material.
An innovative type of time-temperature indicator fabricated using self-healing nanofibers
An innovative type of TTI fabricated using self-healing nanofibers. The time-temperature dependent change in surface area and corresponding light transmittance is driven by the flow of thermodynamic free energy, and operates on the same timescale as the deterioration of perishable foods. (Image: KRICT)
The functionality of this novel TTI is based on the unique properties of a self-healing material, namely, that its surface area decreases with temperature and time on the same timescale as perishable foods spoil, because the surface area is proportional to the thermodynamic free energy.
"To implement this idea, we used our recently reported (Advanced Materials, "Superior Toughness and Fast Self-Healing at Room Temperature Engineered by Transparent Elastomers") self-healing material, which is an aromatic disulfide-based thermoplastic polyurethane (TPU) that works efficiently at room temperature and has superior mechanical properties," explains Oh. "To maximize its surface area and obtain a flexible and thin form, we electrospun the self-healing material into a nonwoven nanofiber mat."
Self-healing polymers are advanced materials currently very popular with researchers. One of their characteristics is quintessentially minimizing the surface area; whereas nanofibers are a surface-area-maximized material.
By combining these two contrasting characteristics in one device, the self-healing nanofiber-based TTI goes through irreversible and dramatic phase change behavior over time to stabilize the thermodynamic free energy.
Oh points out that self-healing polymers are widely used in research, but the fabrication of nanofibers with a pure self-healing material itself has so far never been reported. To that end, the team successfully manufactured, for the first time, a single-component nanofiber from self-healing elastomer through electrospinning.
Electrospun nonwoven materials are generally opaque due to the reflection and scattering of light. However, the opaque nanofiber mat developed by the KRICT team gradually becomes transparent above a specific temperature with the evolution of time.
SEM images of the time-temperature dependent changes of the display and nanofiber mats
SEM images of the time-temperature dependent changes of the display and nanofiber mats at 20 °C over a 24 hour period. (Image: KRICT)
Having demonstrated the basic functionality of their TTI device at two different temperatures – at 2 °C for chilled food and -20 °C for frozen food – the researchers are now working on improvements needed to detect various ranges of temperature in order to make the TTI suitable for practical logistic applications.
A limitation of the current fabrication method is that electrospinning is not very suitable for mass production. To address this issue, the team is pondering various solutions including melt-blown spinning.
"Compared to electronic devices, the type of stimuli-responsive materials used in our device is cost-effective and simply to manufacture," Oh concludes. "However, they tend to possess a lack of sensitivity and only provide limited information. Our ongoing research efforts are focused on addressing these limitations.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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