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<p>In this study, a novel smart indicator pad was fabricated using coaxial 3D printing, featuring a core–shell structure with a curcumin-loaded Ca<sup>2</sup>⁺/CMC core and an alkaline sodium alginate–curcumin shell. During printing, in situ Ca<sup>2</sup>⁺–SA ionic crosslinking formed a stable three-dimensional network, while curcumin enabled dynamic colorimetric response under alkaline conditions. Systematic investigations showed that crosslinking pathway, infill rate, and structural thickness significantly affected gas-responsive behavior, with the inward-to-outward crosslinking mechanism showing superior performance. The curcumin emulsion system improved stability, reducing curcumin loss from 27.56% to 10.86%, and the printed pads exhibited rapid CO₂ responsiveness, with ΔE (60%-0.6) values reaching 20.90 within 2&#xa0;min. During storage tests, the indicator pads displayed clear color changes corresponding to fruit and vegetable spoilage. For instance, mushroom pads showed ΔE increasing from 8.20 on day 1 to 24.99 on day 5, strawberries reached 24.81, and average daily ΔE change rates were approximately 5.00 ΔE/day, reflecting high sensitivity to respiration-generated CO₂. The overall colorimetric response ranking was mushroom &gt; strawberry &gt; Malanto (<i>Kalimeris indica</i>) &gt; bok choy &gt; grape, consistent with produce-specific respiration and moisture characteristics. These results demonstrate that the fabricated 3D-printed indicator pads provide an intuitive, quantitative, and real-time approach for monitoring freshness, offering promising potential for industrial intelligent packaging applications.</p>

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Coaxially 3D Printed Core–Shell Hydrogel Colorimetric Indicator for Real Time Freshness Monitoring of Fruits and Vegetables

  • 期刊论文

  • 发表日期：

    2026年4月28日

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  • Tiantian Tang,
  • Min Zhang,
  • Benu Adhikari,
  • Dongcui Fan,
  • Zhimei Guo

摘要

In this study, a novel smart indicator pad was fabricated using coaxial 3D printing, featuring a core–shell structure with a curcumin-loaded Ca2⁺/CMC core and an alkaline sodium alginate–curcumin shell. During printing, in situ Ca2⁺–SA ionic crosslinking formed a stable three-dimensional network, while curcumin enabled dynamic colorimetric response under alkaline conditions. Systematic investigations showed that crosslinking pathway, infill rate, and structural thickness significantly affected gas-responsive behavior, with the inward-to-outward crosslinking mechanism showing superior performance. The curcumin emulsion system improved stability, reducing curcumin loss from 27.56% to 10.86%, and the printed pads exhibited rapid CO₂ responsiveness, with ΔE (60%-0.6) values reaching 20.90 within 2 min. During storage tests, the indicator pads displayed clear color changes corresponding to fruit and vegetable spoilage. For instance, mushroom pads showed ΔE increasing from 8.20 on day 1 to 24.99 on day 5, strawberries reached 24.81, and average daily ΔE change rates were approximately 5.00 ΔE/day, reflecting high sensitivity to respiration-generated CO₂. The overall colorimetric response ranking was mushroom > strawberry > Malanto (Kalimeris indica) > bok choy > grape, consistent with produce-specific respiration and moisture characteristics. These results demonstrate that the fabricated 3D-printed indicator pads provide an intuitive, quantitative, and real-time approach for monitoring freshness, offering promising potential for industrial intelligent packaging applications.