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We present the design, fabrication, and characterization of a sensor capable of detecting minute changes in pressure within a polydimethylsiloxane (PDMS) microchannel. The device consists of a guided-mode resonance (GMR) grating with a resonant wavelength of 731 nm embedded in an 85 μm thick PDMS membrane which acts as one wall of the microchannel. As pressure within the channel is increased, the membrane expands, straining the PDMS as well as the embedded grating. This strain results in an increase in the pitch of the grating, which in turn produces a shift in the resonance of the device. By measuring the resulting change in the reflectivity spectrum of the device, changes in relative pressure as small as 50 pascal may be detected over a range of over 5 kilopascal.