applications of hydroxyapatite thermochemistry to ... in...applications of hydroxyapatite...

Applications of Hydroxyapatite Thermochemistry to Biomaterials Synthesis

R. E. Riman, C. Mossaad, M. Starr and D. T. Denhardt and L. Shimp

2010 OLI Simulation Conference

Materials Science and EngineeringMaterials Science and Engineering


2

Acknowledgements

• National Science Foundation/Rutgers IGERT on Biointerfaces, DGE 0333196

• Rutgers University Roger G. Ackerman Fellowship

• NASA GSRP Grant NNG04GO44H• Rutgers University Technology

Commercialization Fund• Osteotech, Inc.



3

Outline

• Biomaterials• DBM-Hydroxyapatite nanocomposites• Thermodynamic simulations• Hydroxyapatite nanomaterials• Hydroxyapatite-collagen (DBM) nanocomposites



4

Issues in Hard Tissue Replacement• Injury and Osteoporosis

- Cancer treatment- Celiac- Diabetes- Parathyroid issues

• Limited joint replacement lifetimes- Stress shielding causes loosening- Fibrous capsule causes loosening- Wear debris cause inflammation

• Current materials- Polymer/HA blends- Titanium alloy/HA coating- Stainless Steel- Cobalt alloy/HA coating- Pure polymers- Demineralized bone for non-load

bearing applicationsGoodman et. al. “Effects of orthopaedic wear particles on osteoprogenitor cells” Biomaterials 27 (2006) 6096-6101

Drees et. al. “Mechanisms of Disease: molecular insights into aseptic loosening of orthopedic implants” Nature Clinical Practice: Rheumatology 3(3) (2007) 165



5

DBM-HA Nanocomposites

• Bone is made of Hydroxyapatite and collagen - Materials recognized and processed by native tissues

• DBM (Demineralized Bone Matrix) collagen forms- Matrix of collagen with residual proteins and mineral- Used in non-load bearing applications- High osteoinductivity (forms new bone)- Easily disintegrates in aqueous media

• Hydroxyapatite (Ca10(PO4)6(OH)2)- Implants, drugs, chromatography, supplements & toothpaste- Osteoconductive (forms bone on implant surface)- Possibly osteoinductive- Particle size very small in bone- Formed in aqueous environment

• Marry advantageous properties of both materials• Start with non-load bearing material 1emedicine.medscape.com

2http://www.graftondbm.com

Demineralized bone matrix1,2



6

Paradigm hydroxyapatite synthesis system

• Precipitates at or below 37oC• Neutral pH (7-7.8)• Non-toxic precursors• Chemically robust• High yield• Scalable (>1 g/100 ml solution)• Short reaction time(< 4 h)• Can integrate process with collagen mats, fibers, and other

biomaterials• Current methods and chemistries do not meet all criteria

- Harsh chemical environment (ss rxn, hydrothermal, mechanochemical)- Low process yield and slow (simulated body fluid)- pH too high (aqueous precipitation, phase transition)



Rational Approach to Crystallization

• Compute thermodynamic equilibria as a function of the processing variables for phase of interest

• Generate equilibrium diagrams to map processing variable space for phase of interest

• Design hydrothermal experiments to test and validate computed diagrams

• Utilize processing variable space maps to explore opportunities for control of reaction and crystallization kinetics



Thermodynamic Modeling• List all relevant equilibrium species in the solid, aqueous and

vapor/gaseous state• Write k independent reactions for i species• Write the equation for the Gibbs free energy change for each

independent reaction, equations for respective equilibrium constants and collect the associated standard state thermochemical data tocompute these properties as a function of temperature

• Choose activity coefficient models and collect data for activitycoefficient computation

• Generate additional equations that invoke mass balance and charge neutrality to enable solution of all equations

• Solve all equations simultaneously using property minimization methods (e.g., Newton’s method) that minimize the free energy for the system

• Repeat this calculation for a wide range of processing variable space and plot the phase boundaries for the equilibrium diagram.



9

Thermodynamic Modeling Search Criterion

• HA synthesis goals- Room Temperature synthesis (T=25oC)- Ambient Pressure (p=1 atm, no high pressure reactor needed)- Neutral pH- Low cost precursors- Non toxic ions produced in solution for ease of washing

• Tasks- Survey and model various calcium sources with a chosen phosphate- Survey and model various phosphates with a chosen calcium- Survey other reaction variables

Temperature, pH, composition- Analyze model outputs: Chemical/phase diagrams

Yield, Stability- Verify selected conditions with wet-chemical experiments

• Software used: OLI Stream Analyzer 2.0 by OLI Systems, Inc.



10

Relevant Hydroxyapatite Chemical Equilibria



11

Phase Equilibrium Systems for the Paradigm

Precursor (pH) K2HPO4 (9.00) H3PO4 (1.32) Na3PO4/K3PO4 (12.13/12.24)

CaCl2 (6.73) N N HCa(OH) 2 (12.39) H H H

Ca(C2H3O2)2 (8.92) N N H

• Ca/P=1.67, 0.5 m Ca-species, 0.3 m P-species, T= 25oC• H – Hydroxyapatite is the dominant phase somewhere• N – Hydroxyapatite not the dominant phase anywhere



Modeling of the CaCl2-Na3PO4 System

12Mossaad, C.; Starr, M.; Patil, S.; Riman, R. E., Chem. Mater., 22(1), 2010, 36-46.



Modeling of the CaCl2-K2HPO4 System




Modeling of the Ca(OH)2-H3PO4 System




Ca(CH3COO)2-K3PO4-H2O System




16

Universal pH dependent Stability Diagram

• Ca/P controls width of yield region

• Maintaining the pH above 6.2 avoids secondary products, but below 7-7.8 allow the co-processing with tissue

Mossaad, C.; Starr, M.; Patil, S.; Riman, R. E., Chem. Mater., 22(1), 2010, 36-46.



17

Temperature Effects on Yield Diagram




18

Validation of the Yield Diagram




19

XRD of Verification Points on the Calcium Acetate/Tribasic Phosphate Model (Unwashed)

20 30 40 50 60

AP 6

AP 5

AP 4

AP 3

AP 1Inte

nsity

(Arb

itrar

y)

2 Theta (Degrees)

HA

Monetite

AP 2

Unwashed Powders as Prepared

20 30 40 50 60

APHT 6

APHT 5

APHT 4

APHT 3

APHT 2

Inte

nsity

(Arb

itrar

y)

2 Theta (Degrees)

HA

KCP

APHT 1

Unwashed Powders Heat Treated




20

TEM – Very Small Nanoparticles! Very Difficult to Isolate.

Mossaad, C.; Tan, M.C.; Starr, M.; Payzant, E.A.; Howe, J.Y..; Riman, R. E., Cryst. Growth Des., in press, 2010.



21

Matrix Matrix with RU HA Surface Layer

Biomineralization of Hydroxyapatite onto DBM



22

Micro-CT of Surface Mineralized DBM (10/6 mM)



23

TEM of Mineralized DBM Fibers



24

FTIR and XRD Confirms HA Presence

4000 3500 3000 2500 2000 1500 1000 500 0

1255

12581355

1560

1655

1656

1563 1407

1023

% T

rans

mitt

ance

(arb

itrar

y)

Wavenumber (cm-1)

DBM

HA+DBM

1019

10 20 30 40 50 60 70 80

Inte

nsity

(Arb

itrar

y)

2 Theta (Degrees)

Synthesized HA HA Standard



25

TGA Studies show mineral content is similar to bone

7 wt%



26

12 Bony-Site Sheep Study

Putty

Matrix



27

Conclusions

• Thermodynamic computations were used to engineer a new mild process for synthesizing nanosized hydroxyapatite

• The process could be integrated with collagen fibers to make DBM-HA nanocomposites

• An aseptic adaptation of the synthesis method was accomplished

• A bioactive implant was prepared that stimulated bone growth- More stable interface afforded by preventing rapid aqueous dissociation

of the fibers- High surface area nanocomposite

applications of hydroxyapatite thermochemistry to ... in...applications of hydroxyapatite...

Documents