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slide1

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…

slide2

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.

slide3

- 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

slide4

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

slide5

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

slide6

? ? ?

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

slide7

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

slide8

The penalty for deforming from an ideal coil is the usual configurational entropy:

fewerconfigurations :

greaterconfigurations :

slide9

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 :

slide10

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

slide11

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

slide12

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

slide13

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 :

slide14

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

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H

p

slide15

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)

slide16

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.

slide17

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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

slide18

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):

slide19

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

slide20

Relative Elasticity of PAH/P-Azo films

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pH = 5, 7 exhibit an elastic modulus 50x that of films made at pH = 9, 10.5

assemblypH = 5,7

assembly pH = 9, 10

slide21

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  • 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

slide22

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  • PAH/P-Azo coated tip indented into PAH/P-Azo layers on glass (400nm)

Measuring Adhesion in Multilayer Films

Adhesion

slide23

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

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2.8

±

0.5

nN

+

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±

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nN

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  • PAH/P-Azo coated tip indented into PAH/P-Azo layers on glass (400nm)

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Force (nN)

Measuring Adhesion in Multilayer Films

Event Frequency

slide24

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PAH / NaCl

wash 3x H2O

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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

slide25

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with the coated colloid between electrodes the electrophoretic mobility

is proportional to the zeta potential of the charged colloid

slide26

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

slide27

This phenomenon appears to be general over different size particles, and pH of multilayer assembly:

1st PAA Layer

slide31

These shifts increase with layer number, then converge

dilute solution ~8.7

dilute solution ~7.0

slide32

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+

slide33

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

slide34

z

Cantilever and Tip

x

y

friction force

scan direction

Sample

Some interesting behaviour at biological pH :

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30

dilute solution

20

10

Zeta Potential (mV)

0

stuck to surface

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-20

Using lateral force AFM, the SURFACE FRICTION varies by an order of magnitude depending on the pH

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-40

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pH

at BIOLOGICAL pH,

The coils are fully charged in solution (STICKY), but become uncharged (SLICK) on the surface

slide35

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

slide36

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.

slide37

Dye Incorporation

24 h exposure to saturated dye solutions

(PAH/HA)10 Films from Assembly pH = 3.0

slide38

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.

slide39

Dye Release

Chromotrope 2R

Indoine Blue

(PAH/HA)10 Films Assembled at pH = 3.0

Susan Burke, Macromolecules2004, 37, 5375.