sure within the PIPAAm-modified capillary at 30 °C and burst release during PIPAAm collapse. This burst is analogous to drug release from collapsing PIPAAm gel disks undergoing analogous thermal treatment. [17] Similarly, just after each temperature return to 30 °C, small fluores-cent spikes are observed. In contrast, oscillatory fluorescent changes have been observed using 0.5 min cyclic intervals (shown in Fig. 3c) indicating subtle PIPAAm swell-shrink transition kinetics affecting flow profiles. Aqueous flow has been blocked completely using air bubbles in PIPAAm-grafted capillaries (j = 200 lm) above the PIPAAm transition temperature, where the microchannel surfaces are hydrophobic, trapping the air bubble. [30] Our microfluidic channels distinguish their flow-control behavior by completely blocking water flow via PIPAAm graft hydration. Considerably higher pressure is required to initiate water flow through PIPAAm-grafted capillaries below PIPAAm's collapse-transition temperature. Thermally regulated flow control in PIPAAm-grafted capillaries is rapid and reversible, facilitating many reliable and repeated open-close cycles. Such reversible, thermally modulated, polymer-grafted on-off switches for controlling microfluidic behavior might be regulated locally on-chip using optical signals (local infrared heating), resistive heating microelements, or external thermostats. Spatial control is possible using patterned grafted capillaries and different polymergrafted areas with chemistries exhibiting distinct phase-transition behavior in designated locations. Opportunities for new microvalve systems exploiting this basic design are substantial.