Liver spheroids on chips as emerging platforms for drug screening

Liver plays a critical role in drug metabolism and nowadays multi-cellular culture systems aiming at imitating liver-specific morphology and functionality are advancing robustly. Numerous immortalized or stem cell-induced hepatic cell lines have been investigated to enhance the hepatic phenotype in establishing co-culture systems. Owing to the robust progresses of microtechnology and bioengineering, two-dimensional (2D) co-cultures such as sandwich culture and micropatterned co-culture systems have emerged. Controllably arranging the hepatocytes and fibroblasts allows bio-mimic homotypic and heterotypic interactions, and the miniaturized co-culture platform realizes high-throughput and sensitive drug screening. Yet, to address the rapid dedifferentiation and the decreased maintenance of hepatic functions existing in 2D cellular systems, various three-dimensional (3D) and dynamic hepatocyte co-culturing formats have been established to obtain more physiologically relevant liver microsystems and prolonged hepatic functionalities, such as spheroid barcodes, liver organoids, bioengineered hepatic spheroids, and microfluidic perfused liver-on-a-chip. In this review, we first introduce the typical structural color spheroid barcodes which facilitate multiple screening and testing. Then we sketch various advances in liver-imitating cellular co-culture protocols through representative examples. Meanwhile, we summarize current difficulties and propose the prospects of bioengineered liver-on-barcode systems for reliable and high-throughput drug screening.