When silk and gold come together

The clever use of silk and gold in high-tech sensors can help us track the health of stored foods, according to recent research, writes Kollegala Sharma.

Well, what could happen if silk and gold – two of Indian women’s favourites put together? Not just the heritage Zartaari, but very high-tech sensors could also be the spin-offs, says a report published in a recent issue of Advanced Materials?

A combined American team of aerospace engineers, biomedical engineers and material engineers have put together silk and gold to design a easy-to-wrap and use biosensors. In a ‘proof of the concept’ experiment, the team led by the biomedical engineer at Tuft’s University, Fiorenze Omenetto, has demonstrated that food sensors thinner than an onion peel can keep track the health of an ageing banana or souring milk.

Sensors that keep track of the health of a stored food item are much sought after. Smart sensors that give out the quality as well as the age of the packed food are needed in these days in these days of processed and stocked foods. Omenetto’s team feels that silk fits the bill neatly for the purpose. For silk is not just a yarn, but a wonderful material which can be dissolved into near transparent solution, formed into gels, foams and sponges besides the beautiful fabric. It is also biodegradable and tiny amounts of silk can also be eaten and digested. 

Silk with electronics

Combining these qualities of silk with electronics, Omenetto’s team has ventured to fabricate tight-fitting silky sensors – a sort of antenna, which radio out messages about the health of the food. The sensor contains a very thin film of silk over which electronic circuits in gold are laid. To prepare the film the raw silk is first dissolved into a solution, and then cast into thin films.

The gold circuit works as an antenna and can be fabricated to detect very wide range of electromagnetic waves. “We have developed a variety of techniques for chemical-free fabrication of silk sensors,” reports Omenetto’s team. The team has used four different techniques successfully to fabricate different-sized silk sensors having a wide range of electronic circuits. It has also confirmed, through experiments, that antennas which can detect from microwaves to terrahertz waves can be easily fabricated using these techniques. 

Just as the radio antennas respond to a specific ‘tuned’ frequency, the Zari silk antenna gives out a signal when it detects a specific frequency of wave that ‘tunes in’ with the sensor’s ‘resonating frequency’. The resonating frequency of the antenna can change depending upon its shape or its immediate chemical environment. Omenetto’s team plans to use this change in the resonating frequency of the antenna in tracking food items just like the RFID (radiofrequency identification) tags keep track of goods.

While the RFID chips hidden inside the superstore goods can only detect the whereabouts of the goods, the Zari silk sensor can do more.  Another advantage, as Omenetto’s team points out, is that the Zari silk sensors are not as stiff or as hard as the silicon RFID chips.

So they can fit on an object of any shape. This is particularly useful in detecting the changes happening in the fresh foods. The silk sensor is first wet by hot steam which softens the silk.

The softened silk is stick and can be tightly wrapped around the food under study.  Once stuck, it stays there till sliced off. The Tufts University team has stuck these sensors on such varied objects as apples, eggs and banana.

Tracking food quality

To prove their point, Omenetto and colleagues stuck a silk sensor on raw bananas and tracked  the fruit’s ripening over a period of nine days. As it ripens, the raw banana softens and changes in shape too. Though very slight, the altered shape modifies the ‘resonating frequency’ of the antenna.

As the shape keeps continuously changing, the resonating frequency also keeps shifting which can be detected using lab equipments.  Omenetto's team observed the resonating frequency increase from 36.1 MHz to 42.6 MHz as the banana ripened.

In a similar proof of principle experiments, gold-silk sensors were applied onto the surface of cheese and to the inside of a milk bottle’s cap.

In both cases the sensors reported the changes happening in cheese and milk through a change in their resonating frequencies. While in case of cheese, it corresponded with the increasing bacterial contamination, in case of milk it related to the increasing sourness indicating their usefulness in tracking spoilage of such perishable foods.
 
Are the sensors edible?

Could we eat the sensors along with the food? Although the sensor is edible with a gold film that is several times thinner than the gold and silver films that are used in bakery and confectionery industry, and the silk is biocompatible, meaning it does not cause harm to the body when consumed, for those coy about consuming anything unnatural, Omenetto’s team suggests that “the portion of the food with the antenna can be easily sliced off and disposed off”.

Alternatively, the silk sensor can be applied on the package or other substrates since it is easy to fabricate and wrap around on any shape.

So what does it mean for our crisis ridden silk industry? May not be much, since a very small quantity of silk is required for sensor fabrication.

But the fresh foods on which the sensors are stuck surely are going to be valued for both their quality which is assured by the sensors, and the precious yellow metal on the sensor.