Due to the emerging study of cell biology,micro-scale cell patterning has attracted increasing attention.The micro-scale cell patterning can be applied not only in the field of tissue engineering but also in cell-cell interaction [1],drug screening [2],and biosensors [3].This study proposes a fabrication method for magnetic microrings for magnetic cell patterning.We used dried ferrofluid as the magnetic material of the magnetic rings,and used these rings to perform the magnetic cell patterning.We generated ferrofluid (EMG 705) droplets by an immiscible fluid (TPGDA) in a T-junction microchannel,and collected the droplets in a reservoir downstream the T-junction (see Fig.1(a)).During the above process,an out-of-plane magnetic field H was applied to control the droplet size and the distance between droplets in the reservoir.Afterward we solidified the TPGDA by applying UV light to create a substrate for cell patterning followed by partial air-dry of the ferrofluid droplets to form the ring shaped magnetic layer.We then took the substrate out from the microchannel and washed the remaining ferrofluid out to complete the magnetic ring fabrication (see Figs.1(b) and 1(c)).To perform the cell patterning,a PDMS film was coated on the substrate to prevent the cells from direct contact with the dried ferrofluid rings (see Fig.1(d)); RAW cells contained magnetic nanoparticles by endocytosis were then added and were attracted to the magnetic rings under an out-of-plane external magnetic field (see the red arrow in Fig.1(d)).Figure 1(e) shows OM images of the cell patterning results; the inner and outer diameter of the magnetic rings are 43.5±5.1 μm 81.4±2.7 μm,respectively,and the distance between each two adjacent rings is 152±7.7 μm,where the small errors of the ring size and the center distance reveal the good uniformity of the ring array.In addition,we can see the most of the cells were located on the rings but not on the other regions; this result indicates the feasibility of our approach for magnetic cell patterning.In summary,by using our method,we can massively produce magnetic microrings and uniform ring arrays,and the outer diameter and the center distance between rings can be easily controlled by the magnetic field strength during the fabrication.Furthermore,it is also easy to control the inner diameter of the rings by selecting different air-dry time.The experimental result of the cell patterning also shows the future potential of our method.