User:Danny6Lai/sandbox/gustatory technology

Gustatory technology is an engineering discipline focusing on digital simulation of tastes .Virtual tastes and textures of real food can be generated by stimulating tongue with different temperatures and electric currents.

In the last decade, gustatory technology developed out of the increasing demand on virtual reality. The desire of vivid virtual reality effectively promotes the researches on taste simulation. Gustatory technology gains most of its publicity from "digital lollipop", a research done in 2012 at the National University of Singapore.

Nowadays, more research teams have participated in simulating real food through multiple methods, including sound enhancement, thermal stimulation and electrical stimulation on tongue or masseter mussel on cheek. As more researches when on, gustatory technology demonstrates its potential in multi-sensory communication, entertainment, food industry and human health. However, commercial products of gustatory technology are yet to be promoted.

Progress


In 2012, a research team leading by Nimesha Ranasinghe at the National University of Singapore started a research on digital taste interference. Nimesha Ranasinghe and his team designed a tongue interface and a control system to study the effect of thermal and electrical stimulus on tongue. The tongue interface contains a silver electrodes to pass electric currents and a heat sink to control the temperature. The research team then created a digital lollipop to specialize on studying the relationship between electric currents and sourness.

In 2016, the same team did a research on virtual sweet. They created a box-shape device with a heat sink inside and a metal plate on it to simulate sweetness through thermal stimulation on the tip of the tongue.

At the same year, a team from the University of Tokyo researched about creating real food texture trough electrical muscle stimulation (EMS). They put electrodes on the masseter muscle and passed currents with frequency varys from 100 Hz to 250 Hz and duration from 100 ms to 250 ms. At the end, they concluded that the higher the frequency, the harder the texture and the longer the duration the more elastic people feel.

In 2017, Nimesha Ranasinghe and his team made virtual lemonade and digital cocktail. Both of them has electrodes on the edge of the cup to simulate the taste and lights on the bottom to mimic the color of real drink. These to devices were built base on their early research on taste simulation.

Thermal Taste
Thermal taste refers to the phenomenon that people experience a sensation of taste when the tongue is warmed or cooled. It is one of the most common method. Multiple variables can effect the resulting taste, including the original tongue temperature, the change in temperature, and the part of tongue being stimulated.

Generally, warming the tongue can result in a sensation of sweetness while cooling it will result in sourness. Researches also indicate that there exist sections on tongue that can maximize the experience of thermal taste. Peak sensitivity on thermal sweetness, the taste of sweetness due to warming, locates on the tip of the tongue; and for thermal sourness, it exists on the lateral or side of the tongue tip. Warming the "sweet-best site" of the tongue from 20°C to 30°C results in highest intensity on sweetness. Cooling the "sour-best site" from 35°C to 15°C  results in highest intensity on sourness. If the "sour-best site" is cooled 10°C more, to 5°C, it will result a diminishing on sour taste and increasing on salty taste. For "sweet-best site", if it is cooled from 35°C to 5°C, salty will be the dominant taste.

The reason behind thermal taste is due to the thermal sensitivity of chorda tympani and glossopharyngeal nerves, which are two kind of nerves that convey taste information from tongue to the brain. When the food start contacting with the tongue, taste recepto r cell on the taste buds will detect ligands of different taste and send electrical signal through nerves. Different receptors has specialized sensing taste. G-protein-coupled receptors (GPCRs) is relative to the sucrose (sweet) or bitter taste while HCN1 and HCN4 (HCN channels) contributes to sour and salty taste. These taste receptors are innervated by chorda tympani and glossopharyngeal nerves, which means the nerves can effect the sensation of receptors. Changing in temperature would influence the nerves, and further change the activeness of receptors. In the case of thermal taste, GPCRs are triggered by warming and that HCN1 and HCN4 are triggered by cooling.

Electric Currents
Electric stimulation is also commonly used to produce taste. It is most effective in producing sour and salty sensation, which are generated when ions depolarize taste receptors. Sourness is formed by free hydrogen ions (H+) reacting with HCN1 and HCN4 receptors and salty taste is formed by sodium (Na+ ) ions. Electric current has a large influence on ions as well as the ion channels, which gate the passing ion in nerves. For example, a current of 140μA can induce a strong taste of sour. The sensation of sweet and bitter is harder to be simulated by electricity since both of them rely on sensing more complex moleculars. TAS1R2+TAS1R3 heterodimer receptor function as the sweet receptor by binding with sugar. The TAS2R proteins function as bitter taste receptors and react with bitter ligands, such as cycloheximide, denatonium, PROP (6-n-propyl-2-thiouracil), PTC (phenylthiocarbamide), and β-glucopyranosides.

Application
Gustatory technology have the potential application in multiple aspects, inceluding as multi-sensory communication, entertainment, food industry and human health. Virtual reality will be one of its major application since taste and smell are two big sensation of current virtual reality. Gustatory technology could also be used in future's social media or communication that virtue taste can be transfered through Internet and shared with friends. Food and beverage with less salt or surge could be produced and benefit people with diabetes or obesity. It is even possible that people with ageusia, the inability to taste, can taste again since gustatory technology evoke taste through physical stimulation instead of chemical stimulation.