3 Physical Properties of Biomaterials CHAPTER 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 Edge dislocations half-plane dislocation 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 branching 3) tacticity 4) regularity of mer placement in copolymer side groups: large and bulky branched vs. linear location of side groups tacticity block 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 situation 1) several polymer chains per each lamella 2) 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 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 state Tm: temp with viscosity of 100 P Tw: 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 Tm 1) degree of branching --- Tm 감소 2) molecular weight --- Tm 증가 Amorphous polymers T>Tg: rubbery elastic materials T<Tg: glassy and brittle polymer [Tg<Tm] --- 1.4 < Tm/Tg < 2.0 for polymer chain vibration and rotation 1) chain flexibility 2) chemical constituents [bulky side groups, polar groups, high mol. wt., X-linking]

Polymers to be crystallizable Tg< Tc <Tm temp 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