# Technologies and Fabrication of Intelligent Packaging for Perishable Products
## 2 Intelligent Packaging Techniques
### Sensing Techniques  
Indicators and sensors are two main sensing technologies used in intelligent packaging to measure the product status or the surrounding environmental factors. Indicators are simple devices that have information about the product. This allows for the product status or environmental factors to be known in real-time from the packaging itself by observing it. There are three typical indicators: time-temperature indicators, gas indicators, and freshness indicators. A time-temperature indicator (TTI) is a simple and inexpensive device. It is usually fixed on the packaging of products or shipping containers. By observing TTI on the packaging, consumers can directly know the measurable current at that instant or the historical environmental temperature based on the color changes, text display, etc. on the packaging of the products [17]. In the market, there are three types of TTIs: (1) critical temperature indicators that show warnings when the temperature is above or below a reference value, (2) partial history indicators that can display an indication when a product has been exposed to an environmental temperature that may result in a change in product quality, and (3) full history indicators that can provide all the historical environmental temperaturesthat a product has experienced [18]. [PERSON] et al. developed and patented a time and temperature dependent tamper-proof barcode indicator. Based on time and temperature dependent migration of the color particles, it allows the machine-readable barcode to change by printing the barcode using a specialized coloring agent [19]. Gas indicators are used to detect the gas condition in the packaging. Gasses can significantly affect the quality, safety, and integrity of perishables--for example, ethylene can promote fruit ripening, while shortening its shelf life. Gas indicators can either be small devices in the form of a package label, or they can be directly printed on packaging films for the purpose of monitoring [11]. [PERSON] and [PERSON] developed a novel water-resistant UV-activated oxygen indicator for intelligent food packaging using alginate, a polymer that issued to prevent dyes from leaching out of colorimetric oxygen indicator films. However, the dye-based oxygen indicator film suffers from dye leaching upon contact with water [20]. Freshness indicators can show whether packaged goods are fresh or spoilt through changes in the color of a label or text on the package [1]. [PERSON] et al. developed a colorimetric indicator label for monitoring the freshness of freshly-cut green bell peppers based on pH-sensitive indicators [21]. [PERSON] et al. developed a simple colorimetric sensor to serve as an inexpensive visual indicator to monitor the freshness of a fish using PANI films. The PANI films are reusable and are regenerated using an acid solution [22]. [PERSON] et al. developed a combined pH and dye-based colorimetric freshness indicator as a \"chemical barcode\" for the real-time monitoring of the spoilage of skinless chicken breasts [23]. Sensors are more complex than indicators and are commonly used in cases where the extensive historical status of products or surrounding environmental factors of perishable products must be known. Sensors are used to detect, locate, or quantify energy or matter by generating a signal following the detection or measurement of a physical or chemical property to which the device responds [24]. A typical sensor is composed of four parts: a receptor, a transducer, signal processing electronics, and a display unit. However, sensors used in intelligent packaging must also have a micro-controller, an external memory and a power source. The structure of a typical sensor is shown in Figure 3. In recent years, flexible sensors have received great attention for their role facilitating intelligent packaging of perishable products. [PERSON] et al.