TABLE 1.

Typical materials for OOC applications.

Materials	Major properties	Limitations	Typical applications in OOC
Glass	+ Surface stability + Optically transparent + Electrically insulating	– Not gas permeable – High cost of fabrication	• OOC device substrate • Glass-based chip for transform studies (Kulthong et al., 2018) • Enabling real-time imaging (Li X. et al., 2018)
PDMS	+ High elasticity + High gas permeability + Biocompatibility + Rapid prototyping	– Hydrophobicity – Strong adsorption of biomolecules – Not compatible with organic solvents	• Most common OOC substrate • Biomimetic cell culture scaffold (Kim et al., 2012) • Microvascular model (Zhang W. et al., 2016)
Plastic	+ Optically transparent + Low absorption + Rigid + Suitable for mass production	– Less gas-permeable – Unsuitable for prototyping	• OOC device substrate (Miller and Shuler, 2016) • Porous membrane to model tissue-tissue interfaces (Pocock et al., 2017)
Paper	+ Highly porous + Matrix of cellulose + Potable and low cost	– Limited detection methods – Difficult to integrate microcomponents	• OOC device substrate • TRACER (Young et al., 2018) • Model of respiratory system (Rahimi et al., 2016)
Collagen	+ Biocompatible + Enzymatically degradable + Similar in structural and mechanical properties to native tissues + Good cell adhesion	– Weak mechanical properties	• Microvascular networks (Zheng et al., 2012) • Scaffold mimicking 3D villi structure (Shim et al., 2017) • Neurovascular model (Adriani et al., 2017) • Skin model (Lee S. et al., 2017) • Kidney model (Lee S. J. et al., 2018) • Pumping heart chamber model (Li R. A. et al., 2018) • Liver spheroids, tumor spheroids (Yamada et al., 2015; Jeong et al., 2016)
Gelatin	+ Biocompatible + Biodegradable + Similar in composition to collagen + Good cell adhesion + Tunable properties by the addition of functional group (e.g., GelMA)	– Weak mechanical properties – Rapid degradation	• Heart-on-a-chip (Zhang Y. S. et al., 2016) • Skin model (Zhao et al., 2016) • Microvascular networks (Yang et al., 2016) • Spheroid-based liver model (Bhise et al., 2016)
Alginate	+ Biocompatible + Biodegradable + Easy functionalization + Immediate gelation at mild condition	– Weak mechanical properties – Poor cell adhesion – Uncontrollable degradation	• Scaffolds containing living cells (Ning et al., 2016) • Liver spheroids, tumor spheroids (Chan et al., 2016; Kang et al., 2016) • Hydrogel fibers (Zhu et al., 2017)
PEG and its derivatives (e.g., PEGDA)	+ Biocompatible + Tunable and precise mechanical and degradation properties + Relatively low protein adsorption	– Less cell adhesive – Limited biodegradation	• Self-organizing cardiac microchambers (Ma et al., 2015) • Liver organoids generation (Ng et al., 2018) • Intestinal organoids generation (Cruz-Acuña et al., 2017)