Jon Moulton

A comparison of the delivery of Atto488-bovine serum albumin into cultured human sarcoma cells by nine different commercial delivery products, with an exploration of a photochemical technique enhancing endosomal escape:

Mellert K, Lamla M, Scheffzek K, Wittig R, Kaufmann D. Enhancing endosomal escape of transduced proteins by photochemical internalisation. PLoS One. 2012;7(12):e52473. doi: 10.1371/journal.pone.0052473. Epub 2012 Dec 21.

http://www.plosone.o...al.pone.0052473

You can design the oligo slightly longer than the guide strand, extending into the loop or past the Drosha cleavage site into the stem.  This improves the oligo affinity for the pre-miRNA/pri-miRNA and helps in inhibiting maturation of the microRNA.  This is a standard strategy for Morpholino design targeting miRNAs and was originally explored in:
Kloosterman WP, Lagendijk AK, Ketting RF, Moulton JD, Plasterk RH.  Targeted Inhibition of miRNA Maturation with Morpholinos Reveals a Role for miR-375 in Pancreatic Islet Development.  PLoS Biol. 2007 Jul 24;5(8):e203 [Epub ahead of print]
http://biology.plosj...al.pbio.0050203

This strategy is also useful if there is sequence within the guide strand that would require a stable self-complementarity in the oligo sequence.  The oligo target can be shifted farther into the flanking sequence to break the self-complementarity.  An oligo targeted in this fashion will not bind well to pre-processed miRNA on RISC but can be a very effective inhibitor of miRNA maturation.