Biointerfacial Characterizationwww.rci.rutgers.edu/~moghe/583.html

Lecture 1Sep. 7, 2006

Prof. Prabhas Moghe

BME 125:583

PROPERTIES OF MATERIALS

•  Bulk and Surface Properties Can Control Tissue InterfaceDynamics After In Vivo Implantation of Biomaterials

• Bulk and Surface Material Characteristics are Property Dependent - These Features must be known prior to anyMedical Application!

P. Moghe

e.g. does the application require load-bearing materials? does the application require a soft, resorbable material?

- Interatomic forces

- Atomic structure based material classes- Metals, Ceramics, Glasses, Polymers

-Microstructure-Interatomic bonds-3-D atomic clusters-Crystallite structure-Grain size and phase changes

-Mechanical Properties of Materials

BULK PROPERTIES - MATERIAL DISTINCTIONS

SURFACE PROPERTIES OF MATERIALS

celltissue fluidand proteins

BIOMATERIAL

• Surface properties determine biomaterial-tissue interface.

• Properties: BiocompatibilityTopography/RoughnessWettabilitySurface MobilityCrystallinityChemical Composition

Method Principle Depth analyzed Resolution CostContactangles

Liquid wetting of materialestimates surface energy

3 – 20 Ao 1 mm Moderate

ESCA /XPS

X rays causeElectron emission

10 –250 A 10 –1 50um

V.Expensive

AES Electron beam causesAuger e. emission

50 – 100 A 100 A V.Expensive

SIMS Bombarded ions cause surfacesecondary ion emission

10 A – 1 um 100 A V.Expensive

FTIR-ATR

IR radiation causes excitation 1-5 um 10 um Expensive

STM Quantum tunneling current betnprobe & conducting material

5 A 1 A Expensive

SEM Electron beam causes secondaryelectron emission

5 A 40 A Expensive

CLSM Reflected /fluorescent imagesspatially determined

10 um – 2000um

0.5 um Expensive

Methods to Characterize Biomaterial Surfaces

http://www.rci.rutgers.edu/~moghe/Bioprop.html

Surface Contact Energetics

-Molecules exterior to materials are most accessible to adjacent phases as well as incoming cells (Surface Reaction)

-There is always a positive energy necessary to create a unit area of surface. Systems reach equilibrium by minimizing this surface area/energy. In solids, this happens by changing the nature of interface to one with lower (lowest) energy.

-Energy minimization occurs when groups/chains in polymer rearrange to yield lowest interfacial energy.e.g. hydrogel migrates to/from surface of a graft copolymerexposed to water/dry air.

Determination of Surface Energetics

• Excess free energy per unit surface area is surface tension

• Young’s Theory of the Spreading of Liquid Droplet:

svsl

lv

At equilibrium, surface energy sv sl lv cos = +

Techniques to measure contact angles

air

Static Drop

Capillary air-bubble

DuNouy Ring orWilhelmy plate

Electrobalance Recorder

Lid

MeasuringCell

liquid

MotorizedPlatform Clamp support

Measuring Plate/Rod

Wilhelmy Technique for Contact Angle Analysis

Wilhelmy Plate Method for Contact Angle Measurement

F FF

mg

mgmg Fb

LL

LL

F = mg + p L Cos - Fb where Fb = L Vimm g

1 23

Both liquid surface tension & L/S/V contact angle can be computedFirst do experiment with fully wetting plate and find L. Then mountbiomaterial on the recording balance and find cos.

P, perimeter,=2(t+w)

Ramé-Hart Goniometer

Drop-Image Program: http://www.ramehart.com/goniometers/dropimagefinn.htm

Dynamic Contact Angle Measurements

ra

ADVANCING CONTACT ANGLE

RECEDING CONTACT ANGLE

Dynamic contact analysis is done by increasing ordecreasing the drop volume until the three-phaseboundary moves over the surface.

Contact Angle Hysteresis

Difference between advancing and receding contact angle is called contact angle hysteresis.

Force

Immersion Depth

Buoyancy Slope

in

out

advancement

recession

zerodepth

Low hysteresisis obtained onwell cleaned,non-interactingsurfaces.

Force

Immersion Depth

zerodepth

in

out

second cycle(red)

2

1 21

Buoyancy Slope

receding

advancing

A

B

HYSTERESIS LOOP FOR POLYMERIC BIOMATERIAL

A=>receding contact angleB=>advancing contact angle

Zisman Method Critical Surface Tension

Various liquids

10 20 30 40 50 60

lv Dynes/cm

0

90

Critical surface tension, c

Cos=1.0

Cos=0

Smaller ContactAngles

Completespreading

StableSessileDrop

poly(ethylene):31 dyn/cmPTFE : 19 dyn/cmPVC : 41 dyn.cm

Thermodynamics of Spreading/Adhesion on Materials

Fadh = cs - cl - sl

Interfacial free energy of adhesion = Cell-solid interfacial freeenergy - Cell-liquid interfacial free energy - Solid-liquid interfacial free energy.

If Fadh < 0, adhesion and spreading are energetically favorable

50 100

s [erg. cm-2]

Fadh

and Substratum free energy (wettability)Fadh

Very hydrophobicsubstrates

Biological Interactiveness & Biomaterial Critical Surface Tension

Critical Surface Tension (dynes/cm)Rela

tive b

iolo

gic

al in

tera

ctio

n

Non-adhesive zoneBiomaterials with good adhesion

40 803020

Baier, Adv. Chem. Ser. 145:1, 1975

Industrial Products for Contact Angle Measurement

• Advanced Surface Technology Products, Inc.Computer-interfaced contact angle analysis

• Cahn, Inc.Dynamic Contact Angle Analysis Zisman MethodWilhelmy and DuNuoy Rings

• KSV Limited, FinlandDigital Tensiometer (DR/W)

Concerns in Contact Angle Measurements

•The measurement is subjective

•Surface roughness influences the contact analysis

•Surface unevenness influences the results

•The liquids used can be contaminated (reducing lv)

•Liquids can reorient the surface structure

•Liquids can absorb, swelling the surface

•Liquids can dissolve the surface

•Environment needs to be controlled carefully

•Dynamic measurements have hysteresis

Research Paper Discussion

Relationship between substrate PEG content and surface hydrophobicity

Tziampazis, Kohn, and Moghe, Biomaterials 21:511, 2000

Discussion