An investigation into the role of mucus thickness on mucoadhesion in the gastrointestinal tract of pig

Mucoadhesion in the gastrointestinal tract is a complex phenomenon and both formulation and physiological features need to be well understood and considered. Mucus thickness has been inferred to play a role in this process; however no definitive influence has been established. This study aimed to investigate the influence of mucus thickness on the mucoadhesion process, using a large animal (pig) as a model to closely resemble the human physiological features. The mucus thickness of different regions of the gastrointestinal tract of pig was fully measured by means of a histochemical method (hematoxilin/eosin) employing cryostat sections. Mucoadhesion was evaluated ex vivo on porcine mucosa by tensiometry using a polyacrylic acid polymer (Carbopol 974P NF) as a mucoadhesive model material, both in a dry and swollen state. Mucus was thickest in the stomach (body 67.9 ± 54.7 μm) and mucus thickness increased from proximal to distal segments in both the small intestine (duodenum 25.9 ± 11.8 μm, ileum 31.0 ± 15.7 μm) and large intestine (caecum 19.4 ± 8.7 μm, ascending colon 31.9 ± 17.2 μm, descending colon 35.1 ± 16.0 μm and rectum 40.8 ± 12.5 μm). Swollen polymer exhibited lower mucoadhesion than the dry form in all sections analysed. Mucus thickness plays a role on the mucoadhesion, as thicker mucus provides deeper polymer chain diffusion and entanglements; however, other factors are also involved in this complex process.