Self-Assembly of P oly electrolyte Multilayers : towards engineering bio-compatible surfaces

1. Self-Assembly of Polyelectrolyte Multilayers: towards engineering bio-compatible surfaces The GOAL is to coat surfaces of implants, so that they don’t suffer rejection– contact lenses, sutures, diagnostic probes, drain pipes, stents, artificial limbs, bone grafts, tissue scaffolds, dentistry, hips, plates, pins, organs…

2. The GOAL is to coat surfaces of implants, so that they don’t suffer rejection– contact lenses, sutures, diagnostic probes, drain pipes, stents, artificial limbs, bone grafts, tissue scaffolds, dentistry, hips, plates, pins, organs… The DARK ages: wash it really well and hope for the best. The ENLIGHTENMENT: tailor the chemistry somewhat ( we like to work with metals, inorganics, hard plastics…) The FUTURE: it’s more than just Chemistry– charge, surface ions, protein adsorption/resistance, water content, physical morphology are all important and need tailoring… REQUIREMENTS are: tunable chemistry, hydrophilicity, thermodynamic minimum, stable layers, time stability, controlled water content, ion content, modulus, application to various geometries.

3. - Na+ Polyelectrolytes for Adsorption : pK ~ 9.5 MW ~ 70K pK ~ 5.5 MW ~ 90K poly anion (PAA - ) poly cation (PAH +) Surfaces : cleaned glass, Si, aluminum . . . Multilayers : rinse PAH + PAA - rinse ~ 10 min ~ 10 min

4. Polyelectyrolyte Multilayers : Layering is reproducible Adsorption is irreversible Films are stable, and Surface coverage is good Thin Film formation is: easy, cheap, robust, clean, and versatile ... Polyelectrolytes can readily incorporate any secondary function : for specific chemistry, and to hold water and ions . . . Decher, Thin Solid Films1992, Science1997 Rubner, Macromolecules1995

5. 2) Spinning on a surface flatspherical 3) Flow through cell Layering on 10nm SiO2 colloid demonstrates suitability to high curvature surfaces. To demonstrate suitability to confined dimensions. TEM image of a layered colloid Advantage #1: Versatility of Adsorption Geometry 1) Dipping

6. ? ? ? FCHARGE ~ 0.8 FCHARGE ~ 0.6 Advantage Number 3: the layer thickness depends strongly on pH of assembly: and coils are oddly stretched: FCHARGE 0.01 0.1 0.5 0.9 0.99

7. Model of adsorption to a surface of variable charge : Equilibrium as a balance between coil entropy and enthalpy, and considering the surface sites as chargeless ‘stickers’ then solved for ‘equilibrium’ layer height

8. The penalty for deforming from an ideal coil is the usual configurational entropy: fewerconfigurations : greaterconfigurations :

9. R P sticking a loop H A P A B B sticker greaterconfigurations fewerconfigurations Now, we introduce an entropic penalty for “sticking” : a reduction in configurations available for a stuck coil loop leads to a counter-intuitive lowest energy :

10. Low Charge THINLayers Moderate Charge THICKLayers High Charge THINLayers minimizing the Total Free Energy : FCHARGE 0.01 0.1 0.5 0.9 0.99 rationalizes both the excessiveamount, and the sharptransition

11. Challenges for the Study of Polyelectrolyte Multilayers: 1) New theoretical approaches are required : adsorption is irreversible, layer properties not necess. in equilibrium 2) New experimental techniques are required : Dry Wet (in situ) dried layer structure is not necess. the same as the in situ structure

12. Now the ‘sticky’ experimental questions: The biocompatible properties of these self-assembled layers depend STRONGLY on the film morphology:. a) SWELLING, b) ELASTICITY, and c) CHARGE

13. dry sample sample in liquid cell Using in situellipsometry to measure layer thickness : We can observe the polymer swelling in real time as water is added :

14. in situellipsometry : wet We can then compare wet and dry thickness : layers can swell substantially wet dry dry Layer = but they DON’T seem to care about solution properties like pH or [ion] : 210 190 underwater thickness 170 150 2 4 6 8 10 12 H p

15. dry sample sample in liquid cell Using in situellipsometry to measure layer thickness : The mechanism for the swelling can now be determined as CASE II+ (n  1, not Fickian n = ½), and the swelling depends strongly on the assembly pH, and humidity (not environment)

16. pH 6.5 pH 5.0 pH 3.5 Layer swelling cares greatly about the assembly pH, and the humidity : PAA/PAH in bath pH 4.0, eq’d in ambient relative humidity of 45% Rate constant of growth of swelling (k) can vary by 10 orders of magnitude, over just 3 units of pH of multilayer assembly.

17. + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - Layer swelling cares greatly about the assembly pH, and the humidity : Exactly the same layers (PAA/PAH25 made at pH 3.5, in bath pH 4.0), but: eq’d at 23% humidity and at 45% humidity

18. sample in liquid cell dry sample sample in liquid cell Ellipsometry can only measure average density however, but, we can use variable angle neutron reflectivity : thermal neutrons wavelength 2.4Å. Measure reflected intensity as incident angle is increased. angle (wavevector Q) We can now observe a gradient polymer swelling profile (after fitting):

19. Force-Distance Curves obtained by AFM Elastic Deformation of a sphere touching a flat surface under load (k = 0.12 N/m) We measure . Knowing R (tip radius) and  (poisson ratio  0.5) we can solve for the Young’s Modulus (E), which is related to ‘crosslink’ density

20. Relative Elasticity of PAH/P-Azo films + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + – – – – – + + + pH = 5, 7 exhibit an elastic modulus 50x that of films made at pH = 9, 10.5 assemblypH = 5,7 assembly pH = 9, 10

21. + + + - - - - • PAH/P-Azo coated tip indented into PAH/P-Azo layers on glass (400nm) Measuring Adhesion in Multilayer Films Bare Silicon Nitride AFM tip tip coated with thin layers pH 5 tip coated with thick layers pH 9

22. + + + - - - - • PAH/P-Azo coated tip indented into PAH/P-Azo layers on glass (400nm) Measuring Adhesion in Multilayer Films Adhesion

23. 8 tip / sample 7 pH 5 pH 5 0.5 ± 0.3 nN tip / sample pH 5 pH 9 6 tip / sample pH 9.5 pH 9 5 4 3 2.8 ± 0.5 nN + 2 6.7 ± 2.3 nN + + 1 - - - - 0 • PAH/P-Azo coated tip indented into PAH/P-Azo layers on glass (400nm) 0 2 4 6 8 10 12 Force (nN) Measuring Adhesion in Multilayer Films Event Frequency

24. + + + + + + + + + + PAH / NaCl wash 3x H2O + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + PAA / NaCl wash 3x H2O We can try to measure the ionic surface charge : by layering onto Si nanoparticles with repeated washing, drying 70 nm Similar to that done by Möhwald, Caruso, 1998

25. + + + + + + + + + + + + + + + + + + + + + + + + + with the coated colloid between electrodes the electrophoretic mobility is proportional to the zeta potential of the charged colloid

26. in this way we can map out the acid-base equilibria inside the layers to observe a pK estimate : 2nd PAA layer prepared at pH = 7 The result? solution pka ~ 7.0 multilayer pKa < 4.0

27. This phenomenon appears to be general over different size particles, and pH of multilayer assembly: 1st PAA Layer

28. We have also observed pK shifts for a weak base (PAH) 2nd PAH Layer

29. We have also observed pK shifts for a weak base (PAH) 2nd PAH Layer dilute solution ~8.7

30. These shifts increase with layer number, then converge dilute solution ~8.7

31. These shifts increase with layer number, then converge dilute solution ~8.7 dilute solution ~7.0

32. Relating pKa(app) to PAH/PAA Properties : • Film Thickness and Roughness • Surface Wettability High charge density Low charge density –– COO¯ –– COOH –– COO¯ –– NH3+ –– COO¯ –– COO¯ –– COO¯ –– NH3+ –– COOH –– NH3+ –– COOH –– COOH –– COOH –– NH3+   

33. Perhaps More Interestingly, Biopolymers on Colloid : Poly(L-lysine) (PLL) Hyaluronic Acid (HA) Dilute solution (PLL/HA)3-PLL (PLL/HA)4 pH = 5.0 pH = 7.0 pH = 9.0

34. z Cantilever and Tip x y friction force scan direction Sample Some interesting behaviour at biological pH : 40 30 dilute solution 20 10 Zeta Potential (mV) 0 stuck to surface -10 -20 Using lateral force AFM, the SURFACE FRICTION varies by an order of magnitude depending on the pH -30 -40 2 3 4 5 6 7 8 9 10 11 12 pH at BIOLOGICAL pH, The coils are fully charged in solution (STICKY), but become uncharged (SLICK) on the surface

35. Controlled Release from Multilayer Films • Can we use weak polyelectrolyte multilayer films for controlled loading and release of small molecules? Multilayer System:PAH/HA Small Molecule Probes: Indoine Blue max = 589 nm Chromotrope 2R max = 510 nm

36. Equilibrium Film Swelling (PAH/HA)10 Films Assembled at pH = 3.0 The idea is to: 1) prepare films that will swell greatly at a pH value, 2) load them up with a target molecule, 3) change the pH to ‘trap’ the molecules in the collapsed matrix, then 4) release the molecules on command at a desired pH.

37. Dye Incorporation 24 h exposure to saturated dye solutions (PAH/HA)10 Films from Assembly pH = 3.0

38. Release Profiles for Chromotrope 2R, Indoine Blue Dyes loaded at their max swelling, then placed in various pH baths: pH 3 pH 10 pH 3 pH 10 Remarkably, after a few % of the dye has bled away, the loading is completely stable over arbitrarily long immersion periods.

39. Dye Release Chromotrope 2R Indoine Blue (PAH/HA)10 Films Assembled at pH = 3.0 Susan Burke, Macromolecules2004, 37, 5375.