In this critical review, we describe the recent developments in the field of organs-on-chips: in vitro tissue models that feature physiologically realistic, mic

In the fields of MicroElectroMechanical Systems (MEMS) and Lab On a Chip (LOC), a device is often fabricated using diverse substrates which are processed separately and finally assembled together using a bonding process to yield the final device. Here we describe and demonstrate a novel straightforward, rapid and low-temperature bonding technique for the assembly of complete microfluidic devices, at the chip level, by employing an intermediate layer of gluing material. This technique is applicable to a great variety of materials (e.g., glass, SU-8, parylene, UV-curable adhesive) as demonstrated here when using NOA 81 as gluing material. Bonding is firstly characterized in terms of homogeneity and thickness of the gluing layer. Following this, we verified the resistance of the adhesive layer to various organic solvents, acids, bases and conventional buffers. Finally, the assembled devices are successfully utilized for fluidic experiments.

This paper describes the integration of opto-chemosensors in microfluidics networks. Our technique exploits the internal surface of the network as a platform to build a sensing system by coating the surface with a self-assembled monolayer and subsequently binding a fluorescent sensing molecule to the monolayer. Fluorescent molecules were used that can switch between a fluorescent and a non-fluorescent state, depending on the acidity of the surrounding solution. Two systems were investigated. The first employs surface confinement of a Rhodamine B dye in a glass micro channel that serves as a molecular switch in organic solutions. Upon rinsing the micro channels with acidic or basic solutions it was possible to switch between the fluorescent and non-fluorescent forms reversibly. Moreover, this system could be used to monitor the mixing of two solutions of different acidity along the micro channel. To widen the scope of optical sensing in micro channels an Oregon Green dye derivative was immobilized, which functions as a sensing molecule for pH differences in aqueous solutions. In this case, a hybrid system was used consisting of a glass slide and PDMS channels. The fluorescence intensity was found to be directly correlated to the pH of the solution in contact, indicating the possibility of using such a system as a pH sensor. These systems allow real-time measurements and can be easily implemented in micro- and nanofluidics systems thus enabling analysis of extremely small sample volumes in a fast and reproducible manner.

A ruthenium oxide (RuOx) sensor for potentiometric pH sensing is currently being developed for organs-on-chip purposes. The sensor was fabricated from a Ru(OH)₃ precursor, resulting in RuOx nanorods after heating. An open-circuit potential of the RuOx electrode showed a near-Nernstian response of −58.05 mV/pH, with good selectivity against potentially interfering ions (lithium, sulfate, chloride, and calcium ions). The preconditioned electrode (stored in liquid) had a long-term drift of −0.8 mV/h, and its response rate was less than 2 s. Sensitivity to oxygen was observed at an order of magnitude lower than other reported metal-oxide pH sensors. Together with miniaturizability, the RuOx pH sensor proves to be a suitable pH sensor for organs-on-chip studies.