Mechanical properties and structure of swollen crosslinked high amylose starch tablets

Crosslinked high amylose starch (CLHAS) has been introduced as an excipient for controlled oral drug delivery. The kinetics of delivery is governed by the diffusion of the medication through a hydrogel matrix, which forms as water penetrates the CLHAS tablet. The formation of the gel from the compressed spray-dried starch powder comes from pseudo-crosslinking of particles by the self-assembly of amylose and amylopectin segments into double helices. Tablets reach equilibrium swelling after more than 24 h and contain about 200% water. They demonstrated nearly reversible non-linear viscoelastic properties. Under compression, water flows out and vice-versa for decompression suggesting a sponge-like behaviour. A series of tablets fabricated under increasing compression force in the dry state showed a decrease of wet equilibrium stiffness with increasing fabrication pressure. Scanning electron microscopy on freeze-dried tablets reveals a fine porous texture at the surface of the tablets, which forms, in the first minutes of water penetration. On the other hand the internal texture which develops over longer periods has much larger pores which account for the high equilibrium water content and the observed sponge-like behaviour.