Thesis: Studies on the Electrodeposition of Calcium Phosphate Coatings for Orthopaedic
Applications and the Potential Incorporation of Apatite-Coated Liposomes
Supervisors: Jan T. Czernuszka
About
I was awarded funding by the Engineering and Physical Sciences Research Council for this independent
research project to develop novel, improved coatings for orthopaedic implants with drug-delivery capability.
Analysis techniques that I used included scanning and transmission electron microscopy, electron diffraction
and X-ray powder diffraction crystallography.
When not in the lab, I spent time as the Chair of the departmental Joint Consultative Committee for Graduates
and was the Social Secretary of St. Catherine's College Middle Common where I helped to organise parties and
events for hundreds of graduate students. In addition I sat on the college IT committee and worked as a college
porter and in a student IT support role.
Abstract
A low temperature electrodeposition technique has been used to
coat metal substrates with calcium phosphate (CaP) phases at
37℃ in aqueous solution buffered at pH 7.4.
Experiments have also been carried out on liposome vesicles, some
populations of which were coated in CaP, the final aim being to
combine the liposome vesicles with the coating methodology in
order to produce composite coatings with a drug delivery
capability.
Factors relating to the coating procedure that were altered
included: the initial coating solution concentration, the drying
method and the time for which the samples were allowed to remain
in solution (maturation). The effects upon the coatings were
studied using X-ray diffraction (XRD), low voltage-scanning
electron microscopy (LV-SEM), transmission electron microscopy
(TEM), electron diffraction and measurement of coating shear
strengths.
LV-SEM revealed that one hour or more of maturation time in
solution, after one electrodeposition coating cycle, led to a
dramatic change in coating morphology from a thin particulate
coating, to one with a coherent structure of interconnected
crystallites. The initial coating cycle allowed the formation of a
thin amorphous layer of CaP, which then acts as a seeding site for
subsequent coating growth. TEM and electron diffraction of
coatings revealed that the coating crystallinity increased with
maturation time. The coating was determined to be hydroxyapatite
(HAp) with a preferred orientation of crystallites with their
c-axis perpendicular to the substrate that became more
pronounced with increased maturation time.
Critical point drying (CPD) appeared to protect the structure of
the coatings when compared to air drying. Shear strengths were
modulated by alterations in maturation time. Penetration of the
coating structure by epoxy resin during the testing procedure may
have been a problem.
The results of these studies show that the treatment of the
initial calcium phosphate layer deposited onto a metal substrate
during coating has profound implications for coating structure and
strength.
Lipid vesicles, manufactured from egg phosphatidylcholine (EPC)
containing 30 mol % cholesterol and 4 mol % α-tocopherol,
were characterised in terms of entrapment efficiency and release
of both the marker molecule calcein and the antibiotic gentamicin.
A higher percentage release of calcein was measured from
CaP-coated liposomes when compared to uncoated vesicles when
incubated for periods in excess of 70 h at 37℃,
indicating that CaP ultimately increased the permeability of the
liposomal membranes.
Addition of CaP-coated liposomes to coating solutions, during the
electrodeposition of calcium phosphate, was found to moderate the
decrease in HAp and octacalcium phosphate supersaturation that was
normally seen in their absence. Composite coatings, incorporating
liposomes that contained calcein were also manufactured.