The disclosure concerns oral formulations that enable delivery of therapeutic agents that are sensitive to stomach pH and/or have reduced ability to be absorbed. In one aspect, the oral formulation comprises: (a) phenyl boric acid (PBA)-functionalized chitosan grafted with branched polyethyleneimine (PEI) and (b) a therapeutic agent, wherein the therapeutic agent is encapsulated by the PBA-functionalized chitosan grafted with a branched PEI. In other aspects, the oral formulation comprises: (a) chitosan grafted with branched polyethyleneimine (PEI), (b) a therapeutic agent, wherein the therapeutic agent is encapsulated by the chitosan grafted with a branched PEI, and optionally (c) a eukaryotic cell membrane fragment. The disclosure also concerns methods of making and administering such oral formulations.