Given the wide adoption of polydimethylsiloxane (PDMS) for the rapid fabrication of microfluidic networks and the utility of polyacrylamide gel electrophoresis (PAGE), we develop a technique for fabrication of PAGE molecular sieving gels in PDMS microchannel networks. In developing the fabrication protocol, we trade-off constraints on materials properties of these two polymer materials: PDMS is permeable to O2 and the presence of O2 inhibits the polymerization of polyacrylamide. We present a fabrication method compatible with performing PAGE protein separations in a composite PDMS-glass microdevice, that toggles from an "enclosed" microchannel for PAGE and blotting to an "open" PA gel lane for immunoprobing and readout. To overcome the inhibitory effects of O2, we coat the PDMS channel with a 10% benzophenone solution, which quenches the inhibiting effect of O2 when exposed to UV, resulting in a PAGE-in-PDMS device. We then characterize the PAGE separation performance. Using a ladder of small-to-mid mass proteins (Trypsin Inhibitor (TI); Ovalbumin (OVA); Bovine Serum Albumin (BSA)), we observe resolution of the markers in <60 s, with separation resolution exceeding 1.0 and CVs of 8.4% for BSA-OVA and 2.4% for OVA-TI, with comparable reproducibility to glass microdevice PAGE. We show that benzophenone groups incorporated into the gel through methacrylamide can be UV-activated multiple times to photocapture protein. PDMS microchannel network is reversibly bonded to a glass slide allowing direct access to separated proteins and subsequent in situ diffusion-driven immunoprobing and total protein Sypro red staining. We see this PAGE-in-PDMS fabrication technique as expanding the application and use of microfluidic PAGE without the need for a glass microfabrication infrastructure.