Cell micropatterning, a method to place cells at arbitrary regions, is becoming an essential tool to conduct cell biology and tissue engineering. Conventional cell patterning techniques usually allow only single patterning with single cell type on the same culture surface. However, biomedical research today requires even sophisticated fabrication methods that require spatiotemporal control of multiple cell arrangements. Here we introduce in situ cell micropatterning system which enables stepwise cell patterning using a photoresponsive cell culture surface (PRCS) whose cell adhesiveness could be altered by the UV irradiation. To demonstrate an application to tissue engineering, a liver-mimic tissue array was fabricated and liver-specific gene expressions were quantified with real time PCR. Patterned co-culture systems composed of HepG2 spheroids with Balb/3T3 were fabricated, and the optimum spheroid diameter, which yielded the highest cellular functions, was determined to be 150 mu m. After 20 days of patterned co-culture of HepG2 spheroids and Balb/3T3, CYP3A4 expression increased 50-fold higher than conventionally cultured HepG2; CYP3A4 expression was 20% higher than randomly co-cultured HepG2 and Balb/3T3. Thus the combination of PRCS and the photomask-free irradiation apparatus showed the versatility of experimental setups and proved to be a powerful tool for biomedical studies. Biotechnol. Bioeng. 2009;103: 552-561. (C) 2009 Wiley Periodicals, Inc.