CHAPTER

22Chemical StructureOf Biomaterials

2.1 Introduction: Bonding and Structure of Biomaterials

Crystalline vs. Amorphous

2.2 Structure of Metals

2.2.1 Crystal Structures Unit cellCoordinate number Atomic packing factor

(1) Face-centered cubic structure(FCC)

a, cell edge length; r, atomic radius(2) Body-centered cubic structure (BCC)

(3) Hexagonal close-packed (HCP)

2.2.2 Crystal Systems

lattice structures / lattice points / lattice parameters

Miller indices:

location of points

orientation of planes

Example Problem 2.2

2.2.3. Defects in Crystal Structure (1) Point Defects

vacancy & self-interstitialentropy increase lattice strain

(2) Impurities

solid solution a) interstitial solution b) substitution solution (solute/solvent ratio)

Hume-Rothery rules ex) alloys: strength, corrosion resistance, electrical properties

2.2.4. Solid State Diffusion

self-diffusion vs. inter-diffusion (impurity diffusion)

(1) Diffusion Mechanisms

atomic jumps

a) vacancy diffusionb) interstitial diffusion

(2) Modeling of Diffusion

Diffusion flux and Conc. gradientFick’s 1st Law

2.3 Structure of Ceramics

ionic rather than atomic

2.3.1. Crystal Structures

Ceramic crystal structure 1) magnitude of the electrical charge 2) physical size of ions rc < ra

Stability ---- Maximum ion’s coordinate number

--- rc/ra ratio

(1) AX crystal structures the same charge, an equal number of A and X

(2) AmXp crystal structures no equal charges between A and X charge neutrality

ZSCAP ceramicsFECAP ceramics

(3) Carbon based materials graphite (?) pyrolytic carbon (heart valve)

single-walled nanotubes (SWNT) multi-walled nanotubes (MWNT)

Degree of chirality or twist in SWNTCh = na1 + ma2 armchair tube / zigzag tube / chiral tubemechanical reinforcement of biomaterials

2.3.2. Defects in Crystal Structures

(1) Point defects Groups of defects (electroneutrality)

a) Schottky defectvacancies in + and – in the correct ratio

b) Frenkel defectvacancy/interstitial pair

(2) Impurities

solid solutions substitutional solutioninterstitial solution

electoneutrality similar ions for substitution

otherwise, lattice compensation

diffusion of point defects and impurities

2.4. Structure of Polymers

hydrogen-carbon covalent bonds

2.4.1. General Structure

(1) Repeat Units

mer

Polymers saturated vs. unsaturated

functionality [bifunctional and trifunctional]

(2) Molecular weight determination

a) number-average mol. wt.

b) weight-average mol. wt.

c) polydispersity index (PI)

(3) Mer configuration

single macromolecule ---- multiple shapes

conformation and configuration

polymer configuration [isotactic / syndiotactic / atactic]

(4) Polymer structure

linear / branched / crosslinked / network

2.4.2. Polymer Synthesis

(1) Additional polymerization

identical between mer and polymerinitiator requirement

1) initiation 2) propagation 3) termination

Free radical polymerization Ionic polymerization

(2) condensation polymerization

step reaction

(3) polymer production via genetic engineering

2.4.3. Copolymers

1) random copolymers 2) alternating copolymers 3) block copolymers 4) graft copolymers

2.4.4. Methods of Polymerization

(1) bulk polymerization heat dissipation (highly exothemic)

(2) solution polymerization small polymer yield / reaction volume solvent removal

(3) suspension polymerization small droplet reactor

(4) emulsion polymerization polymer beads or rods

(5) gaseous polymerization (6) solid-state polymerization

monomers in crystalline state (7) plasma polymerization uniform thin films of polymer

2.4.5. Crystal Structures and Defects

(1) Crystal structures [tacticity, degree of branching, bulk chain] (2) Point defects and impurities

2.5. Techniques: Introduction to Material Characterization

spectroscopy and chromatography

2.5.1. X-ray Diffraction X-ray: high energy electromagnetic radiation ---- core electrons

1) energy dispersive X-ray spectroscopy (EDS) 2) wavelength dispersive X-ray spectroscopy (WDS) 3) electron spectroscopy for chemical analysis (ESCA) 4) microcomputated tomography (CT)

(1) Diffraction

constructive same wavelength

2 x amplitude(in-phase)

destructive no diffraction (out-of-phase)(path-length difference)

partial reinforcement

most cases: no diffraction certain atomic arrangement

--- diffraction

Miller indices (h, k, l) interplanar distance dhkl

(2) instrumentation

aluminum powder

intensity vs. 2

(3) Information provided

size and geometry of a unit cellatomic arrangement

2.5.2 Ultraviolet and Visible Light Spectroscopy

(1) UV-VISa) absorptionb) relaxation

(2) Instrumentation

poly(lactic-co-glycolic acid) PLGA

Transmittance Absorbance

(3) Information provided

a) identification of chemicalgroups

b) quantitation of the light absorbing species

Beer-Lambert’s Law

2.5.3. Intrared spectroscopy

ball and spring model Freq of IR radiation

= Freq of bond vibration

bond with permanent dipole

Mode of vibrations a) stretching b) bending c) rocking

Different freq for different chemical groups

(2) Instrumentation

% transmittance vs. wavelength

FT-IR mirrors of interferometer short scan time --- # of scans --- high signal-to-noise ratio

(3) Information provides a) quantification of substances b) molecular fingerprint c) relative change over time of certain peaks

2.5.4. Nuclear Magnetic Resonance Spectroscopy

changes in spin-state of nucleus under strong magnetic field resonance frequency

shielding effect vs. deshielding effect

molecular structure

(2) Instrumentation

maintaining a constant magnetic field varying the frequency of radiowaves

(3) Information provided

inorganic and organic molecules types of bonds

chemical shifts polymer after synthesis

product quality and purity

2.5.5. Mass Spectrometry (1) Basic principles

atomic and molecular masses deflection of charged species under magnetic field

(2) Instrumetation

(3) Information provided isotope ratios strength of various bonds

SIMS

2.5.6. High Performance Liquid Chromatography

(1) Size exclusion chromatography

mobile and stationary phase elution

a) diffusionb) phase transitionc) Eddie diffusion

(2) Instrumentation

Gel filtration chromatographyGel permeation chromatography

(3) Information provided

molecular wt.

Ion-exchange chromatography affinity chromatography reversed-phase chromatographyhydrophobic interaction

chromatography