Biointerfacial Characterization rci.rutgers/~moghe/583.html

1. Biointerfacial Characterization www.rci.rutgers.edu/~moghe/583.html BME 125:583 Lecture 1 Sep. 7, 2006 Prof. Prabhas Moghe

2. PROPERTIES OF MATERIALS P. Moghe •  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! e.g. does the application require load-bearing materials? does the application require a soft, resorbable material?

3. BULK PROPERTIES - MATERIAL DISTINCTIONS - 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

4. SURFACE PROPERTIES OF MATERIALS tissue fluid and proteins cell BIOMATERIAL • Surface properties determine biomaterial-tissue interface. • Properties: Biocompatibility Topography/Roughness Wettability Surface Mobility Crystallinity Chemical Composition

5. Methods to Characterize Biomaterial Surfaces http://www.rci.rutgers.edu/~moghe/Bioprop.html

6. 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.

7. lv  sl sv 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 cos  At equilibrium, surface energy = +

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

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

10. q q Wilhelmy Plate Method for Contact Angle Measurement F 1 2 F 3 F P, perimeter, =2(t+w) mg gL gL gL gL mg mg Fb gL where Fb = rL Vimm g Cos q - Fb F = mg + p Both liquid surface tension & L/S/V contact angle can be computed First do experiment with fully wetting plate and find gL. Then mount biomaterial on the recording balance and find cosq.

11. Ramé-Hart Goniometer Drop-Image Program: http://www.ramehart.com/goniometers/dropimagefinn.htm

12. Dynamic Contact Angle Measurements Dynamic contact analysis is done by increasing or decreasing the drop volume until the three-phase boundary moves over the surface. ADVANCING CONTACT ANGLE RECEDING CONTACT ANGLE r a

13. Contact Angle Hysteresis Difference between advancing and receding contact angle is called contact angle hysteresis. recession Buoyancy Slope Low hysteresis is obtained on well cleaned, non-interacting surfaces. advancement Force in out zero depth Immersion Depth

14. HYSTERESIS LOOP FOR POLYMERIC BIOMATERIAL Buoyancy Slope A=>receding contact angle B=>advancing contact angle q receding A out 2 Force second cycle (red) 1 2 in 1 B zero depth q advancing Immersion Depth

15. Zisman Method Critical Surface Tension Critical surface tension, c Complete spreading Cosq=1.0 Various liquids 0 Smaller Contact Angles  poly(ethylene):31 dyn/cm PTFE : 19 dyn/cm PVC : 41 dyn.cm Cosq=0 90 Stable Sessile Drop 10 20 30 40 50 60 lv Dynes/cm

16. Thermodynamics of Spreading/Adhesion on Materials Interfacial free energy of adhesion = Cell-solid interfacial free energy - Cell-liquid interfacial free energy - Solid-liquid interfacial free energy. Fadh = cs - cl - sl If Fadh < 0, adhesion and spreading are energetically favorable Fadh and Substratum free energy (wettability) Fadh 50 100 s [erg. cm-2] Very hydrophobic substrates

17. Biological Interactiveness & Biomaterial Critical Surface Tension Non-adhesive zone Biomaterials with good adhesion Relative biological interaction 20 30 40 80 Critical Surface Tension (dynes/cm) Baier, Adv. Chem. Ser. 145:1, 1975

18. 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)

19. 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

20. Research Paper Discussion

21. Relationship between substrate PEG content and surface hydrophobicityTziampazis, Kohn, and Moghe, Biomaterials 21:511, 2000

22. Discussion