**3.5 Organ-on-a-chip**

Organ-on-a-chip is a biomimetic system that uses fabrication of computerized microchips and microfluids consisting of living cells, mimicking the natural environment of organs from which it is been created. There are several factors that made organ chips be listed in "Top Ten Emerging Technologies" in the World Economic Forum [75] such as shear force, tissue-boundaries, concentration gradients, tissue–organ interactions and cell patterning. Organ chips have intensified in the field of drug therapeutics for their ability of high throughput screening. **Table 2** summarizes the recent researches carried out using various organ chips. These organ chips use microtechnology that provides nutrients to the cells for their better growth and proliferation. Microfluids are one such component that has been used in various studies for efficient treatment in drug sensitivity testing [86]. Talking of this notion, a microfluidic chip was produced in order to monitor and document real-time impedimetric biosensor changes. Other organ-on-a-chip models such as blood–brain barrier chips have been developed to represent the *in vivo* architecture of brain involving micro blood vessels by using type 1 collagen hydrogel. Another model entails the significance of human-on-a-chip that depicts the normal human physiology in combination with single organ chip within a microfluidic system that ultimately forms multi-organ chip [87]. One study dealt with an *in vitro* 3D-tumor-on-a-chip device that illustrated its importance in quorum sensing phenomenon in tumor cells activated by salmonella [88]. Hence, organ-on-a-chip has been diversified in many scientific platforms due to their efficient physiological bio-mimicry.
