BME 3500 - Test 1 Reading

Biocompatability

How well a material’s biological response fits it’s INTENDED application

A material is STILL biocompatible if it’s intended purpose causes things like inflammation, calcification (bone implant), etc.

3 Classes of Biomaterials

Metals = Strong, conductive material which is formable into complex shapes

Ceramics = Hard, degradation-resistant material but is BRITTLE

Polymers = Huge range of TUNABLE properties based on a variety of factors (can be synthetic or natural)

What types of bonds do each of the 3 biomaterial classes make?

Metals = Non-directional METALLIC Bonds (electron sea)

Ceramics = Non-directional (mainly ionic, but can be somewhat covalent) bonds

Polymers = Directional and typically COVALENT bonds in long chains (can have some ionic character)

Directional comes from the fact that covalent bonds require a specific bond angle for precise overlap of orbitals while ionic and metallic bonds can form regardless of orientation

Natural vs Synthetic Polymers

Natural Polymers are chemically similar to native tissue

• Pros: Integrates with tissue well

• Cons = Limited supply and weaker mechanics —> failures like calcification are common

Synthetic Polymers are more manufactured from somewhat non-bodily materials

• Pros: Easy to mass reproduce and are more tunable

• Cons: Generally BIOINERT and less regulatory approval

Bioinert

Material has minimal interactions with the biological systems in the body (no reactions at all)

Bioreabsorbable

Material has a temporary function in the body and breaks down after (like a temporary stent)

Bioactive

Material has a specific chemical or biological effect on the body

What are 3 examples of polymer types?

Hydrogels = Swell and retain water

Elastomers = Deform substantially but recover quickly

Composites = Utilize 2 chemically different polymer components

What are 3 properties of biomaterials?

Degradative - Implant location, shape and inflammation-driven local chemistry affect degradation rate

Surface Properties - Outermost atomic layers govern protein absorption which drive biological response

• Hydrophobic vs Hydrophilic

• Rough vs Smooth (physical entrapment of material)

Bulk Properties - Mechanical (strength), physical (crystallization/mp) and chemical composition

• Tissues are often anisotropic = direction-dependent mechanical properties (vary along diff axes like tension vs compression, etc.)

What is the difference between ionic and covalent bonds?

Ionic = Complete transfer of electrons between atoms with LARGE electronegativtiy differences (> 1)

Covalent = Sharing of electrons (typically in polymers)

• Polarity arises if one atom in the bond is more electronegative and holds the electrons closer

Sigma vs pi bond

Sigma bonds create single bonds through head to head overlap

Pi bonds create double and triple bonds via parallel overlap (2 diff pi bonds means p orbitals in two different axes —> overlap occurs both above and below)

Metallic Bonds

Electrons essentially delocalize into a sea around cation cores (non-directional)

What is the difference in lattice structures for ionic + metallic vs covalent

Ionic and Metallically bonded structures typically pack closer together and more efficiently into simple crystal lattices

Covalently bonded polymers are directional which means that it restricts how atoms can be arranged —> more fluid like/ amorphous properties

Crystalline vs Amorphous Structure

Crystalline = Atoms are in a rigid and repeated order

Amorphous = No overall order or pattern —> fluid-like

What biomaterial classes are amorphous vs crystalline?

Metals = Only Crystalline

Ceramics = Crystalline or Amorphous

Polymers = Crystalline or Amorphous

What are the 2 key Metal Unit Cell Structures?

Body Centered Cubic (BCC) = 1/8 atom at each corner with 1 full atom in the center —> 2 full atoms total

Face Centered Cubic (FCC) = 1/8 atom at each corner with ½ atom at each FACE —→ 3 + 1 = 4 full atoms total

What is the CN for FCC and BCC?

FCC has a coordination number of 12

BCC has a coordination number of 8

What is APF and what is it for FCC and BCC?

APF = volume of atoms / total atoms in a unit cell

APF (BCC) = 0.68

APF(FCC) = 0.74

What are the 6 lattice properties?

Edge lengths are described with: a,b,c

Edge angles are described with: alpha, beta, gamma

BCC and FCC are CUBIC systems with a=b=c and alpha = beta = gamma = 90 degrees

What is the coordination number formula for CERAMICS?

m * CN (cation) = n * CN (anion)

m = Subscript of cation in overall chemical formula

n = subscript of anion in overall chemical formula

What is the repeat unit of a polymer and what 2 types are there?

Monomers:

• Bifunctional = Forms linear chains

• Trifunction = Forms branched networks

How is the Molecular Weight (MW) of a polymer determined?

MW is usually determined by the overall length of the polymer, but there are two methods to quantify it:

• Number Average (Ma) = Each chain treated equally

• Weighted Average (Mw) = Heavy/larger chains weighted higher for overall polymer mass calculation (reflects mechanical properties)

What is the Polydisperity Index?

PI = Weighted Average (Mw) / Number Average (Ma) and is greater than or equal to 1.0 always

Conformation vs Configuration

Conformation = Shape of polymer due to rotation (SOLELY AROUND SINGLE BONDS) —> no chemical changes

Configuration = Polymer arrangement formed via breaking chemical bonds

What are the 3 types of tacticity (configuration)?

1) Isotactic = Side (R) groups are on the same side of the chain

2) Syndiotactic = Side (R) groups alternate sides

3) Atactic = Random Side (R) group arrangement

What are the 4 types of polymer structures?

• Linear = Long end to end chain

• Branched = Side chains BRANCH off a main chain

• Cross-linked = Linear or Branched chains are joined COVALENTLY to form a 3D network

• Network = Can only be formed using trifunctional monomers if from scratch

What are the 3 main routes of polymer synthesis?

1) Addition (chain-reaction) = Uses bifunctional monomers and has the same chemical formula as the monomer.

• Occurs in 3 steps: initiation —> propogation (determines length) —> termination

• Initation can occur via free radicals

2) Condensation (step-reaction) = Can use multiple mer types and eliminates water as a byproduct (not same chemical formula as mer)

3) Genetic Engineering = Uses protein-based monomers and gives precise control over sequence and MW

Copolymers vs Homopolymers

Homopolymer = one repeat unit type (one mer type)

Copolymers = 2 or more mer types:

• Random = no pattern of mers

• Alternating

• Block = mers clustered into distinct blocks

• Graft = side chains of one homopolymer attached to backbone of other (fake copolymer)

Spectroscopy vs Chromatography

Spectroscopy - Measures how a compound absorbs different types of energy (quantized energy usually)

Chromatography - Physically separates molecules based on properties like size and charge (SEC, CEX)

Ordering of Wave types (high to low energy)

1) x-rays: Excites the inner shell electrons

2) UV/Visible Rays: Excites the valence electrons in both bonding and antibonding orbitals

3) Infrared: Induces Bond vibration

4) Radio Waves (NMR): Induces nuclear spin

also in order of shortest —> longest WAVELENGTH

X-ray mainly for crystalline materials (bones) such as metals and ceramics while the others are used for polymers

What is X Ray Diffraction (XRD)

X-rays have tiny wavelengths which is similar to the gaps between ATOMS in a crystal formation.

Thus when X-rays hit a crystal they bounce off the orderly arrangement of the atoms inside it

Since the waves can travel at random total distances, constructive interference only occurs when the difference in path length is a INTEGER multiple of the wavelength (n * lambda) —> a light is only recorded when the light of the XRD bounces off the perfect angle —> can be used to find the distance atoms

What is the use of Bragg’s Law

It allows us to relate wavelength to interatomic spacing

n lambda (wavelength) = 2d * sin (psi)

d = intermolecular spacing

psi = diffraction angle

What is UV-VIS Spectroscopy?

Absorption of wavelengths (energy) promotes electrons from bonding to antibonding orbitals, but different structures absorb at different wavelengths

Allows us to determine concentration of a substance

What is the Beer-Lambert Law

A = sigma * l * C

A = Absorption

Sigma = molar absorption coefficient

l = path length(usually 1)

C = Concentration

Plotted as absorbance vs wavelength

What is Infrared Spectroscopy (IR)?

IR causes bonds to vibrate BUT only bonds with a permanent dipole (one atom w higher electronegativity causes a dipole —> diatomic elements are immune)

Plotting as % transmittance vs wavenumber

What is Nuclear Magnetic Resonance (NMR)?

Nuclei with spin (unequal # of protons and neutrons) act like tiny magnets and in an external magnetic field they will align wither with or against the field

The fact that they align with (lower energy and more stable) or against (higher energy and less stable) the field creates TWO ENERGY STATES

We can then try to use different radio wave wavelengths to match the energy gap between the two states and cause resonance (a flip from low to high energy orientation)

However, the elements an atom is attached to will affect it’s NMR. If a hydrogen is attached to a carbon (more electron density kept on H since C is not electronegative) it will be shielded more and requires a higher frequency wave (higher E) but if it’s attached to a Fluorine (very electronegative so less electron density on H) it will be less shielded and can resonate at lower frequencies.

How do you read an NMR?

An NMR is plotted as peak intensity vs chemical shift

Chemical Shift (x) in ppm = How much this peak’s frequency differs from a reference standard’s frequency

Peak Intensity (y)

Reference = TMS because it’s hydrogen atoms are very well shielded and shows up reliably at 0 ppm —> all other molecules are measured relative to it

If a H is not shielded well (attached to O) it will show up further from the standard 0 ppm value.

What is Mass Spectroscopy?

This isn’t really spectroscopy since it doesn’t measure absorption of radiation

Here the sample is ionized by being bombarded with electrons and then the ions are separated by mass using a magnetic field

Technically separated by a mass to charge ratio since they are ionized by lighter ions deflect more

Plotted as relative intensity vs mass (mass/charge or m/z)

Often paired with chromatography to distinguish between isotopes

What is HPLC or Size Exclusion Chromatography (SEC)?

Essentially a column is filled with a number of porous beads that only small molecules can wander into and later a liquid solvent is injected that carries your dissolved sample

A big molecule will just pass straight through without entering any of the pores in the beads while smaller ones can enter and will take longer to elute through the column

Thus, the bigger molecules come out first and then the smaller ones come out later

In HPLC what is GFC vs GPC

Gel Filtration Chromatography - A aqueous (water-based) solvent carries the sample through HYDROPHILIC beads → used for hydrophilic/soluble proteins

Gel Permeation Chromatography - An organic solvent carries the sample through HYDROPHOBIC beads → used for non-water soluble things like synthetic polymers

How is HPLC plotted/read?

Plotted as peak intensity vs retention time

Retention time = how long the molecule took to get through

Low retention time = high MW

High retention time = low MW

Helps determine MW of polymers using a standard curve of known MW standards

What is an edge location?

An extra plane of atoms inserted in the crystal but doesn’t go all the way through (squeezed in the middle of two planes)

The line of where that half-plane ends is called the dislocation line

What is Burger’s vector (b)?

If you draw a closed loop around an ideal crystal and redraw the closed loop around a crystal with a defect the loop will no longer reach the same endpoint (which is the startpoint)

Burger’s vector is the distance from the endpoint to the original startpoint in the defect crystal.

What is special about Burger’s vector in an Edge vs Screw dislocation?

In an Edge dislocation, Burger’s vector is PERPINDICULAR to the dislocation line

In a Screw dislocation, Burger’s vector is PARALLEL to the dislocation line

What are some key characteristics of dislocations?

1) Create local lattice strain (compression on one side and tension on the other)

2) Type of dislocation is determined by the relationship between Burger’s vector and the dislocation line

3) Burger’s vector does not change for a dislocation

4) The “slip plane” will contain both the Burger’s vector and dislocation line (since either they are perpindicular or parallel)

5) Dislocation cannot end inside a perfect crystal, it must either end at the surface, loop back on itself or meet another dislocation

How do dislocations let crystals undergo plastic deformation?

Since in an edge dislocation the extra plane does not go all the way through, the first bond after the half-plane ends is very high energy and easy to break —> half plane slides down and process repeats till all atoms have shifted an entire plane over

What is a slip?

Plastic deformation along the slip plane (highest atomic density)

What is a slip system?

This includes the slip plane and the directions a slip can occur on it

More slip systems = ductile crystal

Fewr slip systems = brittle crystal

How are dislocations and deformations different for ceramics?

The dislocation concepts are the same but ceramics are constricted by needing to maintain electroneutrality

→ You cannot put like-charges next to each other

Leads to larger Burger’s vectors and less slip → ceramics are more brittle than metals

What instability occurs at the external surface of a crystal?

Surface atoms lack the full complement of nearest neighbors which means they are at a higher energy state → surface free energy or surface tension

What are grains and why are they formed?

Most metals and ceramics are polycrystalline which means that they are made of many small randomly oriented crystals which are called grains.

What is a grain boundary?

This is the interface between different small crystals or grains

What instability occurs at grain boundaries?

Since grain boundary atoms don’t have optimal coordination (not completely packed like on the interior of a crystal) they are at higher energy states

→ More chemically reactive and susceptible to corrosion

Smaller grains = Increased boundary energy = Increased interfacial energy → more susceptible overall to corrosion

What are the two degrees of misalignment at the grain boundaries?

Small-angle: Slight misorientation including both tilt boundaries (aligned edge dislocations) and twist boundaries (aligned screw dislocations)

High-angle: Severe misorientation leading to higher energy states

What is a twin boundary?

Special case where a mirror image of atomic arrangement across the boundary → crystal structure is the same on both sides

What are volume defects and what are the 2 types?

Volume defects are 3D regions where long-range crystal order is lost:

1) Precipitates = clusters of subtitutional/interstitial impurities

2) Voids = 3D clusters of vacancies (like a pore)

What is the tradeoff of allowing for voids or pores in 3D structure?

Pores allow for fluid and gas exchange leading to support of tissue growth but can reduce mechanical strength.

→ Must optimize porosity

What is the Chain-Folded Model?

The basic polymer crystal unit cell is called a LAMELLA formed by a chain folding back on itself at lamellar faces.

When polymers crystallize the aggregate into spherulites which are analogous to grains in ceramics/metals

What linear defects occur in polymers?

Dislocations can occur but they have larger burger vectors than in metals/ceramics.

Slip occurs along the axis of the chain since covalent bonds are stronger than secondary bonds between chains.

What planar/volume defects occur in polymers?

Spherulites are analogous to grains and voids (pores) are volume defects and can be important for tissue ingrowth purposes.

What type of materials deform via viscous flow?

Noncrystalline materials deform via viscous flow rather than dislocation motion

What are the thermal transitions for each material type?

Metals/crystalline ceramics = Sharp melting point (Tm) → above it is liquid/viscous flow and below it is an ordered solid

Amorphous Ceramics (glasses) = No sharp Tm and instead defined by viscosity landmarks. Glass Transition Temp (Tg) = acts like rigid glass below this temp and then becomes soft and rubbery above

Polymers: Can be both

Tm (crystalline regions) = Increases with molecular weight → fewer chains to “melt” interactions between

Tg (amorphous regions) = governed more by chain flexibility → C-O bonds rotate more easily (lower Tg) than C-C bonds (higher Tg)

Tc (Crystallization temp) = above Tg where chains gain enough mobility to organize into crystals

What is the use of the avrami equation?

Describes the degree of crystallinity of a material developing over time at a given temperature.

What does percent crystallinity depend on (for polymers)?

Density since crystalline regions pack more densely than amorphous ones

What is Differential Scanning Calorimetry (DSC) and what are the two types?

Used to measure heat flow difference between a sample and reference as both undergo a temperature ramp up

Power-compensated DSC: sample and refernce have separate heaters but delta T = 0 usually

Heat-flux DSC: Share one heater and delta T is measured and converted to heat flow

Good to determine % crystallinity of polymers since we can compare the melting endothermic area of the semicrystalline sample to that of a fully crystaline sample.

How do you read a DSC thermogram?

Tg = step like increase in heat capacity (baseline shift essentially)

Tc = Exothermic peak (actual temp at peak)

Tm = endothermic peak (either temp at peak or start of peak) → could be upside down so start of drop