The invention provides proteinoid microsphere made up of a mixture of thermally condensed amino acids that are crosslinked with a bis dithiol crosslinker reagent. The proteinoid microspheres of the invention may be used to encapsulate a material or a compound and to provide slow, sustained or timed release of the material or compound. The proteinoid microspheres are stable in solution until exposed to a reducing agent. However, the in vivo environment provides a sufficient reduction potential to provide slow, sustained release of materials contained therein from the proteinoid microspheres of the present invention.