CHAPTER3 Physical Properties of Biomaterials 3.1 Introduction: From Atomic Groupings to Bulk Materials Metals and Ceramics: Polycrystalline materials (interactions.

CHAPTER

33Physical Propertiesof Biomaterials

3.1 Introduction: From Atomic Groupings to Bulk Materials

Metals and Ceramics: Polycrystalline materials (interactions of multiple crystals) Amount and type of dislocations

Polymers: Crystalline and amorphous regions (% Crystallinity)

Thermal transition of physical properties

3.2 Crystallinity and Linear Defects

point defects, linear defects, planar defects

3.2.1. Dislocations (1) Edge dislocations

half-planedislocation line

magnitude and direction of atomic displacement

atomic circuit drawing Burger’s vector

(2) Screw and mixed dislocations

Screw dislocation: shear force ---- helical pattern Mixed dislocation: Edge + Screw

(3) Characteristics of dislocations

a) localized lattice strains b) relationship between the Burger’s vector and the dislocation line c) invariant Burger’s vector d) termination of dislocation e) slipping of dislocations (slip planes)

3.2.2. Deformation

plastic (permanent) deformation [dislocation glide] dislocation glide: planes with higher atomic density slip and slip plane

dislocation’s geometry plane = crystallographic slip plane

slip system: crystallographic planes x # of slip directions high --- more deformable (ductile), low --- little deformation (brittle)

Ceramics limited movement

electroneutrality requirement longer Burger’s vector less slip --- brittle ceramics

3.3. Crystallinity and Planar Defects

planar defects: surface and grain boundaries

3.3.1. External surface

atoms at the surface --- no maximum coordination --- higher energy [surface tension]---- thermodynamic instability ---- chemical reaction at the surface

3.3.2. Grain boundaries

metals and ceramics: polycrystalline atoms at grain boundary --- no optimal coordination

---- higher energy ---- higher chemical reactivity

total interfacial energy: low in materials with larger grains

Two types of grain boundaries (1) small-angle grain boundary

tilt boundary (edge dislocations), twist boundary (screw dislocations) (2) high-angle grain boundary

severe misalignment [atomic mismatch ---- energy increase]

cf) twin boundary

3.4 Crystallinity and Volume Defects

volume defects: precipitates and voids voids (pores): 1) accidental formation, 2) creation with porogens and fibers

porogens: 1) solid porogens [salts, gelatin (collagen), waxy materials (lipids or paraffin)]

---- extraction --- pore formation extraction methods amount and shape --- porosity and pore geometry 2) gaseous porogens

N2, CO2 / liberation and bubbling amount, rate, timing of gas introduction --- porosity and pore geometries

fibers: fiber size and packing density --- porosity and pore geometry

advantages: 1) exchange of fluids and gases, 2) tissue ingrowth & implant anchoring 3) tissue engineering applications disadvantages: 1) decrease in mechanical strength,

2) altering biodegradation and corrosive properties

% porosity must be optimized

3.5 Crystallinity and Polymeric Materials

physical property of polymer ---- % crystallinity

3.5.1. % Crystallinity chemical structure of mer and polymer’s configuration

factors: 1) mer side groups 2) chain branching3) tacticity 4) regularity of mer placement

in copolymer side groups:

large and bulkybranched vs. linear

location of side groupstacticityblock copolymer

% crystallinity : density 비교

3.5.2. Chain-folded model of crystallinity

Basic unit of polymer crystalline structure: Lamella structure cf.) polymeric crystal’s unit cell

Real situation1) several polymer chains per each lamella2) single chain between lamella structure and interface 3) amorphous regions separating lamellae 4) intermingled chains

Spherulite formation three dim. radial arrangement of lamellae impingement upon growth

3.5.3. Defects in Polymer Crystals

(1) Linear defects (2) Planar and Volume defects

planar defects: boundaries between spherulites volume defects: void formation

3.6 Thermal Transition of Crystalline and Non-crystalline Materials

thermal transition of biomaterials ---- viscosity and material deformation

3.6.1. Viscous flow

crystalline materials --- plastic deformation non-crystalline materials --- viscous flow

rate of deformation & applied stress

viscosity: material’s ability to resist deformation (handle-ability)

water; caramel; glass

3.6.2. Thermal transition (1) Metals and crystalline ceramics

T > Tm: liquid and viscous flow T < Tm: solid --- crystal structure and grain boundaries 유지

(2) Amorphous ceramics (Glasses) T>Tm: liquid stateTm: temp with viscosity of 100 PTw: temp with glass viscosity of 104 P T<Tg: solid state (glass)

(3) Polymers liquid (rubbery solid) & glass Tm and Tg

Crystalline polymers T>Tm: random ordering of chains with no repeating structure

[translational motions] Tm>T: highly ordered crystals

secondary bonds and Tm1) degree of branching --- Tm 감소 2) molecular weight --- Tm 증가

Amorphous polymers T>Tg: rubbery elastic materialsT<Tg: glassy and brittle polymer [Tg<Tm] --- 1.4 < Tm/Tg < 2.0 for polymer

chain vibration and rotation1) chain flexibility2) chemical constituents [bulky side groups, polar groups, high mol. wt., X-linking]

Polymers to be crystallizable Tg< Tc <Tmtemp increase --- polymer chains with energy

--- highly ordered crystalline state [exothermic process] --- disruption of the crystal structure

polymer annealing

degree of crystallinity

3.7 Techniques: Introduction to Thermal Analysis

Temp analysis; measurement of the physical properties of a material as a function of temperature

TGA (thermogravimetric analysis) DMA (dynamic mechanical analysis) DSC (differential scanning calorimetry)

3.7.1. Differential Scanning Calorimetry (1) Basic principles

power-compensated DSC heat-flux DSC

(2) Instrumentation

furnace/DSC sensors/ processor

(3) Information provided

Tg: heat capacity Tm: peak temp% crystallinity