# Three-Dimensional-Bioprinted Liver Chips and Challenges
## 4 Future Perspective  
Future liver-on-chip studies may potentially use hepatocytes derived from induced pluripotent stem cells (iPSCs) to cover greater population variability, which would ultimately improve the thoroughness of drug screening tests (e.g., idiosyncratic drug-induced liver injury is difficult to predict with current clinical trials) [3]. Although the immune system plays a critical role in liver infections, drug-induced hepatotoxicity, and disease progression by altering the secretion of cytokines and histamine from immune cells in the liver, there is almost no liver chip that takes into account the immune system [6; 73]. A liver immuno-chip could contribute significantly to a more accurate examination of the liver. Furthermore, because the liver is closely associated with lungs, skin, and kidneys, combining the liver with other organs on a chip can be used to monitor drug effects on multiple organs [73]. A variety of multi-organ-on-a-chip technologies can be used for in vitro drug toxicity assessment; examples include a lung/liver-on-a-chip platform to study the toxicity of inhaled aerosols [88] and a kidney/liver-on-a-chip model to study the toxicity of drug metabolites [89]. An ultimate goal of OOCs is the implementation of body-on-a-chip platforms that facilitates patient-specific drug screening and disease investigation considering all organs. However, the perfusion media used on OOC platforms are mostly optimized for a specific organ. Thus, finding a single culture medium (i.e., an alternative for blood) that is compatible with multiple organs would be an intriguing area of research for the future. Perfusion is imperative to improve cell proliferation and survival. However, the use of micropumps for simultaneous perfusion of multiple cell cultures increases the complexity and overall cost of the chip. Passive, gravity-driven perfusion methods based on differential fluid pressure in the reservoirs can be a simple, low-cost alternative without the need for expert users to perform the cell culture [6]. Paper provides a biocompatible, perfusable, and porous natural support medium that physically emulates the cellular microenvironment for cell culturing [90; 91; 92]. Future studies can develop paper-based 3D-bioprinted platforms for cell cultures, benefiting from the advantages of both 3D bioprinting and paper substrates (e.g., biocompatibility, low cost, disposability, modifiable pore size, and passive fluid transportation). Moreover, external fields such as magnetic or acoustic fields can play a role in convenient cell separation/isolation/characterization in microfluidic channels at the outlet of chips to enable an analysis of the subtle changes in the concentration of elements in the perfusion medium [93; 94; 95]. Because OOC platforms consist of a small number of cells, detection and sampling of small secreted cellular products are challenging [96]. Increasing the length of the culture channel, decreasing the perfusion flow rate, and using multiple culture chambers in parallel can be a number of possible solutions to obtain a sufficient amount of secreted products. Furthermore, the lack of a stable readout and uniform testing standards are two of the reasons why almost none of the liver chips have received approval from the Food and Drug Administration (FDA) [73]. Integration of molecular reporters and nanoscale biosensors with liver-on-chip platforms enables dynamic real-time screening/imaging of multiple physiological processes on a chip to enable stable, high-throughput readout [33; 73]. Since contemporary microscopes can provide a large amount of readout data (images/videos), interpretation of acquired data (e.g., detection and quantification of target cells such as cancer cells, as well as classification of cells based on their morphology) outpaces the ability of human experts.