Abstract Introduction and Objective: Recently, various microfluidics-based circulating tumor cells (CTC) isolation technologies have been developed to isolate, enumerate, and characterize CTCs, but there are few studies reporting CTC release after selective capture. Existing devices face impediments to the easy release of captured cells due to the conjugation of antibodies to permanent structures within the devices. Here we present a tunable microfluidic polymer graphene oxide (GO) Chip based on a temperature-sensitive graphene oxide-polymer composite for selective capture and efficient subsequent release of CTCs. Methods: A blend film of GO sheets and thermo-responsive polymers serves as a platform enabling cell capture and release in our microfluidic devices. The polymers with a tunable lower critical solution temperature (LCST) allow for stimulated release of the captured cells when the device temperature is below the LCST. The GO sheets serve as a carrier for phosophlipid-PEG-amine, which is used for the immobilization of an antibody against the epithelial-cell-adhesion-molecule (EpCAM). To investigate the capture efficiency, MCF-7 cells labeled with cell tracker dye were spiked into 1 mL of whole blood and flowed through the microfluidic device. After thoroughly washing the device with PBS at 25 °C to remove any non-specifically bound cells, we dissolved the thermo-responsive composite with 1 mL of PBS at 5 °C in a cold room and collected released cells from the device outlet. The released cells were cytospun and stained to identify CTCs as cytokeratin positive, DAPI positive, and CD45 negative cells. We also tested peripheral blood samples obtained from patients with metastatic breast cancer in a similar manner. Results: The graphene oxide-polymer based CTC-chip showed a capture efficiency of 95.21% at 95.21% and a release efficiency of 92.45% for MCF-7 breast cancer cells. This new GO Chip was also able to isolate viable CTCs from blood samples of metastatic breast cancer patients for further molecular characterizations. Conclusion: We present a sensitive and tunable microfluidic chip based on a GO-polymer composite to capture, release, identify, and characterize extremely rare CTCs. The CTCs released from the chip are viable, suggesting that this polymer based GO Chip offers great potential for clinical applications due to its high-affinity cell capture and the ability to enable various downstream analyses of captured CTCs. Citation Format: Hyeun Joong Yoon, Apoorv Shanker, Yang Wang, Molly Kozminsky, Shamileh Fouladdel, Monika L. Burness, Ebrahim Azizi, Max S. Wicha, Jinsang Kim, Sunitha Nagrath. Capture and release of circulating tumor cells by temperature-sensitive graphene oxide-polymer composite. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 376. doi:10.1158/1538-7445.AM2015-376