Retinal Organoids On-a-Chip: A 3D Printed Micro-Millifluidic Bioreactor for Long-Term Retinal Organoid Maintenance

Retinal degeneration is a leading cause of vision impairment and blindness worldwide and medical care for advanced disease does not exist. Stem cell-derived retinal organoids (RtOgs) became an emerging tool for tissue replacement therapy. However, existing RtOg production methods are highly heterogeneous. Controlled and predictable methodology and tools are needed to standardize RtOg production and maintenance. In this study, we designed a shear stress-free micro-millifluidic bioreactor. We used a stereolithography (SLA) 3D printer to fabricate a mold from which Polydimethylsiloxane (PDMS) was cast. The multi-chamber bioreactor design and fabrications methods easily combined micro and millimeter features with very low cost and short manufacturing time. We optimized the chip design using in silico simulations and in vitro evaluation to optimize mass transfer efficiency and concentration uniformity in each culture chamber. We successfully cultured RtOgs on an optimized bioreactor chip for 37 days. We also characterized the RtOgs produced by static dish culture and chip culture methods using qualitative and quantitative techniques. Phase contrast imaging showed that both conventional and chip-cultured RtOgs developed a transparent outermost surface structure. Fluorescence lifetime imaging (FLIM) showed that RtOgs on the chip had significantly lower long lifetime species (LLS) ratio than static cultured ones, which demonstrated that bioreactor cultured RtOgs exhibited less oxidative stress. RtOgs in bioreactor culture demonstrated higher NADH signal overall, but both bioreactor and conventional cultures showed similar free/bound NADH ratio over time, which indicated normal differentiation time course. RtOg gene expression was examined by fluorescence imaging and quantitative polymerase chain reaction (qPCR) analyses. RtOgs in both groups showed thick nuclear outer layers expressing CRX on day 120 of differentiation. The gene profiling showed both groups expressed retinal progenitor genes and most of the tested photoreceptor markers. We, therefore, validated an autonomous micro-millifluidic device with significantly reduced shear stress and lower oxidative stress to produce RtOgs of equal or greater quality than those maintained in conventional static culture.