effect in numerous applications, including single-cell sequencing, [4,5] digital diagnostics, [6,7] nanoparticle synthesis, [8,9] drug screening, [10] directed evolution [11] and enzyme kinetic analysis. [12] Droplets can be generated in either a passive or active manner. Passive methods, that leverage geometric variations in microchannel structures have proved to be exceptionally effective in allowing the robust generation of pL-volume droplets at high speeds. The most common passive methods, include the use of T-junctions, [13,14] flow focusing geometries, [15] and coflow structures. [16] Under certain circumstances, these geometries can be used to generate femtoliter (fL)-volume droplets via tip-streaming. [17] Although, fL-volume droplets can be formed in a robust manner, it is exceptionally difficult to control both droplet volumes and generation frequencies at the same time. [18] In this regard, step-emulsification strategies provide some advantage, since droplet size is primarily controlled by the geometry and size of the microchannel itself, with only a weak dependence on the flow conditions. [19] Although, step-emulsification has been shown to be effective in generating fL-volume droplets at high speed, [20] it should be noted that the strategy is poorly suited for controlling droplet payloads.Active droplet generation methods are based on the principle of introducing additional energy into the fluidic system via mechanical, thermal, electrical, or optical means. [21][22][23][24] When compared to passive methods, active methods offer improved flexibility in regard to controlling droplet size and payloads, and in some cases enable on-demand droplet generation. [18] Amongst active strategies, pneumatic valving has been shown to be effective in generating, [25] splitting, [26] sorting, [27] and merging [25] droplets. Large numbers of pneumatic valves can be integrated within microfluidic devices, and typically comprise a control channel layer and flow channel layer, separated by a thin membrane. [28] The flow and control channels are aligned vertically, with the shared membrane between them acting as the valve. When pressure is applied to the control channel, the membrane is deformed and the flow is interrupted. Multiple valves can be integrated into a single device at high densities owing to the small size of each unit. [29] Microscale valves have proved to be exceptionally useful when performing complex experimental workflows. They can be used to isolate functional components and perform a range of droplet manipulations, including sorting, isolation, immobilization, and coalescence. [30] Through careful design, the operation of each individual functional unit can occur in isolation Contemporary droplet-based microfluidic platforms generate large numbers of sub-nanoliter (<10 −9 L) volume droplets on short timescales. The controllable generation of femtoliter (10 −15 L) volume droplets is however a far more challenging task. Herein, the design, fabrication, and testing of a valve-based droplet-on-demand ge...

Glioma is the most common malignant intracranial tumor with low 5-year survival rate. In this study, we constructed a plasmid expressing anti-HAAH single-chain antibody and sTRAIL fusion protein (scFv-sTRAIL), and explored the effects of the double gene modified human umbilical cord mesenchyreal stem cells (hucMSCs) on the growth of glioma in vitro and in vivo. The isolated hucMSCs were identified by detecting the adipogenic differentiation ability and the osteogenic differentiation ability. The phenotypes of hucMSCs were determined by the flow cytometry. The hucMSCs were infected with lentivirus expression scFv-sTRAIL fusion protein. The expression of sTRAIL in hucMSCs were detected by immunofluorescence staining, western blot and ELISA. The tropism of hucMSCs toward U87G cells was assessed by transwell assay. The inhibitory effect of hucMSCs on U87G cells were explored by CCK8 and apoptosis assay. The xenograft tumor was established by subcutaneously injection of U87G cells into the back of mice. The hucMSCs were injected via tail veins. The inhibitory effect of hucMSCs on glioma in vivo was assessed by TUNEL assay. The hucMSCs migrated into the xenograft tumor were revealed by detecting the green fluorescent. The results showed that the scFv-sTRAIL expression did not affect the phenotypes of hucMSCs. The scFv-sTRAIL expression promoted the tropism of hucMSCs toward U87G cells, enhanced the inhibitory effect and tumor killing effect of hucMSCs on U87G cells. The in vivo study showed that hucMSCs expressing scFv-sTRAIL demonstrated significantly higher inhibitory effect and tumor killing effect than hucMSCs expressing sTRAIL. The green fluorescence intensity in the mice injected with hucMSCs expressing scFv-sTRAIL was significantly higher than that injected with hucMSCs expressing sTRAIL. These data suggested that the scFv conferred the targeting effect of hucMSCs tropism towards the xenograft tumor. In conclusion, the hucMSCs expressing scFv-sTRAIL fusion protein gained the capability to target and kill gliomas cells in vitro and in vivo. These findings shed light on a potential therapy for glioma treatment.