The invention provides a composition comprising a matrix of chitosan and a tannin wherein the chitosan is electrostatically bonded to the tannin to form a chitosan-tannin composite material. The chitosan can be partially or fully deacetylated, and the tannin can be a monomeric or an oligomeric proanthocyanidin or a hydrolysable tannin. The chitosan-tannin composite material can be a nanoparticle, a hydrogel film, a bio-foam, or a biogel, or the chitosan-tannin composite material can coat a liposome. The composite materials can be used for drug delivery, for antibacterial and/or antifungal applications, for tissue engineering applications, for wound healing applications, or they can be used as adjuvants for vaccination, including oral vaccinations. The invention also provides methods of preparing the composite materials and their various forms.