Biointerfacial Characterization Lecture 1 Sep. 7, 2006 Prof. Prabhas Moghe BME 125:583

PROPERTIES OF MATERIALS Bulk and Surface Properties Can Control Tissue Interface Dynamics After In Vivo Implantation of Biomaterials Bulk and Surface Material Characteristics are Property Dependent - These Features must be known prior to any Medical 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 cell tissue fluid and proteins BIOMATERIAL Surface properties determine biomaterial-tissue interface. Properties:Biocompatibility Topography/Roughness Wettability Surface Mobility Crystallinity Chemical Composition

Methods to Characterize Biomaterial Surfaces

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 copolymer exposed 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:  sv  sl  lv  At equilibrium, surface energy  sv  sl  lv cos  = +

Techniques to measure contact angles  air  Static Drop Capillary air-bubble DuNouy Ring or Wilhelmy plate

Electrobalance Recorder Lid Measuring Cell liquid Motorized Platform Clamp support Measuring Plate/Rod Wilhelmy Technique for Contact Angle Analysis

Wilhelmy Plate Method for Contact Angle Measurement F F F mg Fb LL LL LL LL  F = mg + p LL Cos  - F b where F b =  L V imm g 12 3 Both liquid surface tension & L/S/V contact angle can be computed First do experiment with fully wetting plate and find  L. Then mount biomaterial on the recording balance and find cos . P, perimeter, =2(t+w)

Ramé-Hart Goniometer Drop-Image Program:

Dynamic Contact Angle Measurements rr aa ADVANCING CONTACT ANGLE RECEDING CONTACT ANGLE Dynamic contact analysis is done by increasing or decreasing the drop volume until the three-phase boundary 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 zero depth Low hysteresis is obtained on well cleaned, non-interacting surfaces.

Force Immersion Depth zero depth in out second cycle (red) Buoyancy Slope  receding  advancing A B HYSTERESIS LOOP FOR POLYMERIC BIOMATERIAL A=>receding contact angle B=>advancing contact angle

Zisman Method Critical Surface Tension Various liquids  lv Dynes/cm 0 90  Critical surface tension,  c Cos  =1.0 Cos  =0 Smaller Contact Angles Complete spreading Stable Sessile Drop poly(ethylene):31 dyn/cm PTFE : 19 dyn/cm PVC : 41 dyn.cm

Thermodynamics of Spreading/Adhesion on Materials  F adh =  cs -  cl -  sl Interfacial free energy of adhesion = Cell-solid interfacial free energy - Cell-liquid interfacial free energy - Solid-liquid interfacial free energy. If  F adh < 0, adhesion and spreading are energetically favorable  s [erg. cm -2 ]  F adh and Substratum free energy (wettability)  F adh Very hydrophobic substrates

Biological Interactiveness & Biomaterial Critical Surface Tension Critical Surface Tension (dynes/cm) Relative biological interaction Non-adhesive zone Biomaterials with good adhesion 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 Method Wilhelmy and DuNuoy Rings KSV Limited, Finland Digital 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