A calcium phosphate cryogel for alkaline phosphatase encapsulation

There has been significant interest in the development of biomaterials that can localise drugs, enzymes and other therapeutic molecules and be well tolerated in the body. To date, the majority of research in this area has focussed on the formulation and refinement of silica-based gels. Whilst significant progress has been made in optimising silica gel materials, their manufacture typically requires the use of toxic precursors. Here we report the encapsulation of alkaline phosphatase (ALP) in a calcium phosphate-based cryogel. The activity of the ALP following encapsulation, over a period of 14 days, was evaluated and compared with the activity of horseradish peroxidise (HRP), a model enzyme often used in encapsulation studies. Furthermore, the chemical and structural properties exhibited by the gel were determined using X-ray diffraction, helium pycnometry and mercury porosimetry. It was found that when encapsulated in the gel, the activity of ALP was preserved and remained higher than when aged for an equivalent amount of time free in solution. In the case of HRP, however, encapsulation reduced enzyme activity. This was attributed to the different sizes and charges exhibited by the substrates of these two enzymes and the associated diffusional limitations through the mesopores of the gel structure.