Reconstruction of Anorganic Mammalian Bone by Surface-Initiated Polymerization ofL-Lactide

Abstract

Polymerization of L-lactide within the pores of anorganic mammalian bone is described. No additional solvent or catalyst is used. The resulting composites exhibit macroscopic morphologies and mechanical properties similar to that of the original bone. We observe an average compressive strength of 194 MPa and an elastic modulus of 8.8 GPa for composites comprised of poly-L-lactide and anorganic bone derived from bovine femurs. Modeling of the reaction kinetics with synthetic sources of crystalline hydroxyapatite powder suggests that polymerization proceeds via a surface-initiated mechanism that is first order in surface area of hydroxyapatite and first order in mole fraction of L-lactide.

Biomimetic composites by surface‐initiated polymerization of cyclic lactones at anorganic bone: Preparation and in vitro evaluation of osteoblast and osteoclast competence

“…As we observed previously for the polymerization of l ‐lactide using synthetic HA, the polymerization of cyclic lactones with thermally treated AB is generally fit well by the integrated rate equation for pseudo first‐order kinetics: where X t is the mole fraction of monomer at time t > 0, and X 0 ( X 0 = 1) is the mole fraction of monomer at time t = 0. The pseudo first‐order rate constant, k app , is constant for individual reactions but is proportional to the surface area ( A ), i.e., where k is the intrinsic heterogeneous rate constant. In short, the overall reactions are first order in mole fraction of monomer and first order in surface area because the surface area determines the (constant) number of nucleophiles available to initiate and propagate the growth of the polymer chains.…”

“…It is important to note that the materials we produce are not mere mixtures of HA and a polymeric cyclic lactone but are, by virtue of the interfacial bonding produced by the surface‐initiated polymerization, true composite materials in which the properties of the product are different from those of either the polymer phase or the native HA phase alone. The elastic modulus and compressive strength of HA/PLLA composites produced from anorganic bone (AB) are found to be in close agreement with those of the bone tissues from which they were derived . Figure illustrates a PLLA/HA composite rat skull that demonstrates the potential complexity of the devices using this material.…”

“…Additionally, the free hydroxide remaining on the surface of the HA can cleave the ester bonds in the polymer, reducing its molecular weight. Using HA as the initiator results in a carboxylate end group that is electrostatically bound to a calcium ions in the HA matrix . This also reduces the amount of free hydroxide on the surface that can cleave the polymer and reduce its molecular weight.…”

“…HA has been shown to induce ring opening polymerization of cyclic lactones including ε−caprolactone, δ‐valerolactone, and l ‐lactide to produce biologically benign polymers . We recently reported the use of HA derived from mammalian bone to initiate the polymerization of l ‐lactide to poly‐ l ‐lactide (PLLA) . Our goal was to remove organic components that might lead to undesired immune responses while producing a composite that more closely mimics the mechanical properties of living bone.…”

“…[4][5][6][7] We recently reported the use of HA derived from mammalian bone to initiate the polymerization of L-lactide to poly-L-lactide (PLLA). 8 Our goal was to remove organic components that might lead to undesired immune responses while producing a composite that more closely mimics the mechanical properties of living bone. Composites produced using this general methodology were shown to conserve both the macroscopic and microscopic morphologies of the original bone tissue.…”