1. MASE 542/CHEM 442 BIOMATERIALS
Chemical Bonding/Interactions
Spring 2012
H. Funda Yagci Acar

2. UHDPE...used for base bath
Polyester:fabric
Polyurethane...pants
MMA...bath tub

3. What governs materials choice?
1. Bulk properties: matched to those of natural organs
2. Ability to Process
3. Federal Regulations: FDA

4. Commonly Used Biomaterials
Applications
Silicone rubber
Catheters, tubing
Dacron
Vascular grafts
Poly(methyl methacrylate)
Intraocular lenses, bone cement
Polyurethanes
Catheters, pacemaker leads
Stainless steel
Orthopedic devices, stents
Collagen (reprocessed)
Cosmetic surgery, wound dressings
hard-tissue vs soft-tissue
material property-functional requirements (performance)

5. How to choose the material?
Application
Properties
Mechanical (ex., modulus)
Chemical (ex., degradation)
Optical (ex., whiteness, clarity)
Structure

6. Material Properties Bulk properties Surface properties Chemical
Mechanical
Optical
Thermal
Acoustic
Electrical
Magnetic
Surface properties

7. Structure Primary Chemical Structure Atomic & Molecular: 0.1–1 nm
Length scale of bonding – strongly dictates biomaterial performance
Intramolecular bonding
Intermolecular interactions
Biocompatability heavily depend on the primary chemical structure

8. Primary Structure Chemical Bonding
Primary Bonding

9. Bonding 101  • What promotes bonding?
• What types of bonds are there?
• What properties are inferred from bonding?

10. Atomic Structure
Valence electrons determine all of the following properties
Chemical
Electrical
Thermal
Optical
Adapted from, Callister 7e.

11. Electron Configurations
Valence electrons – those in unfilled shells
Filled shells more stable
Valence electrons are most available for bonding and tend to control the chemical properties
example: C (atomic number = 6)
1s2 2s2 2p2
valence electrons

12. Electronic Configurations
valence
electrons
ex: Fe - atomic # = 26
1s2 2s2 2p6 3s2 3p6
3d 6 4s2 1s 2s 2p
K-shell n = 1
L-shell n = 2 3s 3p
M-shell n = 3 3d 4s 4p 4d
Energy
N-shell n = 4
Adapted from Fig. 2.4, Callister & Rethwisch 8e.

13. Electronegativity Smaller electronegativity Larger electronegativity
• Ranges from 0.7 to 4.0,
• Large values: tendency to acquire electrons.
Smaller electronegativity
Larger electronegativity
Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

14. Ionic Bonding metal + nonmetal donates accepts electrons electrons
Dissimilar electronegativities  
ex: MgO Mg 1s2 2s2 2p6 3s O 1s2 2s2 2p4
[Ne] 3s2 
Mg2+ 1s2 2s2 2p O2- 1s2 2s2 2p6
[Ne] [Ne]

15. Ionic Bonding - + • Occurs between + and - ions.
• Requires electron transfer.
• Large difference in electronegativity required.
• Example: NaCl
Na (metal)
unstable
Cl (nonmetal)
electron + -
Coulombic
Attraction
Na (cation)
stable
Cl (anion)

16. Ionic Bonding Energy – minimum energy most stable r A B EN = EA + ER =
Energy balance of attractive and repulsive terms r A n B
EN = EA + ER = + -
Adapted from Fig. 2.8(b), Callister & Rethwisch 8e.

17. Ionic Solids Cation is surrounded by anions Ordered ions
Poor electron conductors
Low Energy
Low chemical reactivity
Hard, brittle
NaCl, MgO

18. Ionic Bonding Predominant bonding in Ceramics NaCl MgO
Give up electrons
Acquire electrons
CaF 2
CsCl
Adapted from Fig. 2.7, Callister & Rethwisch 8e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

19. Covalent Bonding similar electronegativity  share electrons
bonds determined by valence – s & p orbitals dominate bonding
shared electrons
from carbon atom
from hydrogen
atoms H C CH 4
C: has 4 valence
EN: 2.5
H: has 1 valence e-,
EN: 2.1
Adapted from Fig. 2.10, Callister & Rethwisch 8e.

20. Covalent Bonding Most organics (polymers) Silicon Carbon Diamond
Hardest material
Very strong
Poor electron conductor

21. Polar Covalent Bonds Ionic-Covalent Mixed Bonding: % ionic character =
where XA & XB are Pauling electronegativities %)
100 ( x
Ex: MgO where XMg = and XO = 3.5

22. METALIC BONDING Electrons migrating between the atoms
Valance electrons between the atoms are like a glue
Chemically reactive
Electrical and thermal conductivity
Plastic deformation

23. Intermolecular Interactions
Secondary Bonding

24. Intermolecular interactions “Secondary Bonding”
Dipole
H-Bonding
Van Der Waals

25. Dipole-Dipole Interaction
Dipole-Dipole forces are the attractions between the opposite partial charges in polar molecules.
usually weak, ~3-4 kJ/mol, and significant only when molecules are in close contact.
The more polar the molecule, the stronger the dipole-dipole forces and the higher the boiling point.

26. H-bonding
Hydrogen bonds are particularly strong dipole-dipole forces that arise in molecules which have H—N, H—O, or H—F bonds.
The electronegativity difference between O, N, and F vs. H is so large that these bonds are especially polar, and the attractions between unlike partial charges are especially strong.
H-bonds can have energies up to 50 kJ/mol.

27. Ion-Dipole
Dipole forces are the result of electrical interactions between an ion and the partial charges on a polar molecule.
The magnitude of the interaction depends on the charge on the ion (z), the strength of the dipole (µ), and the inverse square of the distance r between the ion and dipole: E = zµ/r2
These forces are responsible for the ability of polar solvents like water to dissolve ionic compounds.
10-50 kj/mol

28. Van Der Waals (London)
All atoms and molecules experience London dispersion forces
but they are the only forces that exist between nonpolar molecules.
instantaneous dipoles created by the random movements of electrons.
Generally weak: 1-10 kJ/mol.
Depends on polarizability and size
A smaller molecule or lighter atom is less polarizable and has smaller London forces because it has only a few tightly held electrons.
A larger molecule or heavier atom is more polarizable and has larger London forces because it has many electrons, some of which are less tightly held and are farther from the nucleus.
asymmetric electron
clouds + -
secondary
bonding H 2
ex: liquid H

29. Propene
GAS
Monomer
million tonnes
90 billion US dollars

30. Poly(propylene) Solid, Plastic Hydrophobic Stiff, melt strength,
impact resistance
high resistance to fatigue
Dishwasher safe cups
mp> 130 °C
Outdoor carpets (hydrohobic-no water absorption)
form-fitting, moisture-wicking clothing

31. fig_02_16
Bonding strengths

32. Summary: Bonding Type Bond Energy Comments Ionic Large!
Nondirectional (ceramics)
Covalent
Variable
Directional
(semiconductors, ceramics
polymer chains)
large-Diamond
small-Bismuth
Metallic
Variable
large-Tungsten
Nondirectional (metals)
small-Mercury
Secondary
smallest
Directional
inter-chain (polymer)
inter-molecular

33. Properties From Bonding: Tm
• Bond length, r
• Melting Temperature, Tm r o
Energy r
• Bond energy, Eo Eo =
“bond energy”
Energy r o
unstretched length
smaller Tm
larger Tm
Tm is larger if Eo is larger.

34. Properties From Bonding : a
• Coefficient of thermal expansion, a D L
length, o
unheated, T 1
heated, T 2
coeff. thermal expansion D L = a ( T - T ) 2 1 L o
• a ~ symmetric at ro r o
smaller a
larger a
Energy
unstretched length Eo
a is larger if Eo is smaller

35. Summary: primary bonds
Ceramics
Large bond energy
large Tm
large E
small a
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Variable bond energy
moderate Tm
moderate E
moderate a
Polymers
(Covalent & Secondary):
Directional Properties
Secondary bonding dominates
small Tm
small E
large a
secondary bonding

36. Primary Chemical Structure
Intramolecular bonding
• Ionic: e- donor, e- acceptor ceramics, glasses (inorganic)
• Covalent: e- sharing glasses, polymers
• Metallic: e- “gas” around lattice of + nuclei
Intermolecular bonding
• Dipole-diople
H-bonding
• Van der Waals
Ion-dipole
• Electrostatic
Hydrophobic Interactions (entropy-driven clustering of nonpolar groups in H2O)
• Physical Entanglement (high MW polymers)

37. Secondary Structure
Higher order

38. Higher Order/Secondary structure
(1 – 100 nm)
The Structure of Crystalline Solids
• How do atoms assemble into solid structures?
• How does the density of a material depend on
its structure?
• When do material properties vary with the
sample (i.e., part) orientation?

39. 1.Crystals (1 – 100 nm) 3D periodic arrays of atoms or molecules
metals, ceramics, polymers (semicrystalline)
crystallinity decreases solubility and bioerosion

40. Energy and Packing • Non dense, random packing
typical neighbor
bond length
bond energy
Energy r
typical neighbor
bond length
bond energy
• Dense, ordered packing
Dense, ordered packed structures tend to have lower energies.

41. Materials and Packing Si Oxygen Crystalline materials...
• atoms pack in periodic, 3D arrays
• typical of:
-metals
-many ceramics
-some polymers
crystalline SiO2 Si
Oxygen
Noncrystalline materials...
• atoms have no periodic packing
• occurs for:
-complex structures
-rapid cooling
"Amorphous" = Noncrystalline
noncrystalline SiO2
Adapted from Fig. 3.23(b),Callister & Rethwisch 8e.

42. 2. Networks
(1 – 100 nm)
Networks: exhibit short range order & characteristic lengths
inorganic glasses, hydrogels , micelles, liposomes

44. Hydrogels Crosslinked- swollen polymer network
Entrap large amounts of water (upto99%)

45. Hydrogels Shape retaining Flexible Slow release of entrapped molecules
Biodegradable or not
Stimuli responsive
Contact lenses, drug delivery, tissue engineering, implants, etc

46. Self-assemblies Liposomes, micelles

48. Microstructure

49. Microstructure 1mm+ Crystal “grains” Spherulites: Precipitates:
Fine grains are stronger
1mm+
Crystal “grains”
Crystallite of varying orientation
Spherulites:
Semicrystalline polymers
Glass-ceramics
Precipitates:
Metals, ceramics, polymers
Stainless steel Fe-Ni-Cr
“fracture fixation plates, angioplasty stents, etc
Spherılites: radially oriented crystallites dispers in amorph pase
Cr: depletes at grain booundries caueing corrosion

50. MATERIALS Metals Ceramics Polymers Polymers Silicone Fibers-textiles
Hydrogels
Smart polymers
Bioresorbable-biodegradable materials
Natural materials
Metals
Ceramics-glasses
Pyrolytic carbon
Composites
Nonfouling surfaces
Nanomaterials

51. fig_01_03
density
fig_01_03

52. fig_01_04
Stiffness
fig_01_04

53. fig_01_05
Strength
fig_01_05

54. Resistance to fracture
fig_01_06
Resistance to fracture
fig_01_06

55. Electrical conductivity
fig_01_07
Electrical conductivity
fig_01_07