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Threshold pressure P c th (kPa), a parameter controlling rock's sealing performance in cases of CO 2 geological sequestration, was studied in a supercritical CO 2 -water system under conditions of 1000 m depth (10 MPa and 40°C). With respect to correlation between P c th and another important parameter of permeability, k (mdarcy), the closest-packing structure of spherical particles is defined theoretically as a line having slope of À0.5 on a double logarithmic plot. This study found this line by measuring P c th of a capillary plate with known throat diameter as P c th = 188.8 k -0.5 . This function is directly applicable to different depth and salinity conditions, although it requires minor correction at some temperature conditions. As a first step in determining the range of variation of rocks' P c th and k from internal particle structures, we used sintered compacts of uniform spherical silica particles with diameters of 0.1-10 μm. Results show that measured values of sintered compacts were scattered around the closest-packing line, with the difference of packing state depending on different sintered additives and sintering temperatures. The change of packing state, occurring independently of resultant changes of porosity, varied sensitively according to P c th . Therefore, P c th is inferred to depend strongly on the local structure within rocks. The Knudsen number estimated for supercritical CO 2 at 1000 m depth indicated that CO 2 transmuted into a noncontinuum at k < 0.1 μdarcy in the closest-packing structure. Future studies should consider whether the concept of P c th is applicable in this situation.

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Induced pluripotent stem (iPS) cells have received remarkable attention as the cell sources for clinical applications of regenerative medicine including stem cell therapy. Additionally, labeling technology is in high demand for tracing transplanted cells used in stem cell therapy. In this study, we used quantum dots (QDs), which have distinct fluorescence abilities in comparison with traditional probes, as the labeling materials and investigated whether iPS cells could be labeled with QDs with no cytotoxicity. iPS cells could not be labeled with QDs alone but required the use of cell-penetrating peptides such as octaarginine (R8). No significant cytotoxicity to iPS cells was confirmed by up to 8 nM QDs, and the iPS cells labeled with QDs maintained their undifferentiated state and pluripotency. These data suggest that QDs can be used for fluorescence labeling of iPS cells.