Understanding of the mechanism of colonic electrolyte transport has markedly increased over the past three decades. This article provides a brief summary of the critical features of Na, Cl, and K transport in the large intestine and how these processes may be altered in diarrhea. Less understood is the mechanism of colonic HCO3 secretion. Recent progress in the regulation of HCO3 secretion in the distal colon is summarized with emphasis on the interrelationship between Cl-dependent, short-chain fatty acid (SCFA)-dependent, and cAMP-induced HCO3 secretion. cAMP down-regulates Cl-dependent HCO3 secretion, while SCFA stimulates HCO3 secretion but also inhibits both Cl-dependent and cAMP-induced HCO3 secretion. As SCFAs are the primary anions in stool, it is likely that SCFA-dependent HCO3 secretion is the primary mechanism of HCO3 secretion in the mammalian colon. Future studies will undoubtedly provide increased understanding of the mechanism of HCO3 secretion in health and disease.

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Micro embedded ring resonator electro-optic switching in InGaAsP based quantum well structures is briefly reviewed and all optic switching is presented. A 20Gbps electro-optic modulator with >12dB extinction ratio has been proposed using quantum confined stark effect tuning, for which a novel high speed coplanar-microstrip inverted ground plane transition has also been designed. All optical tuning is used to theoretically demonstrate a 20G packets/s switching with a 5dB switching ratio. The fabricated micro embedded ring resonator ring using e-beam lithography and Al lift off mask has been demonstrated and the measured device spectrum is found to match well with the simulated results.