poly(vinyl alcohol) hydrogels in medicine · poly (vinyl alcohol) • first synthesized by hermann...

Cambridge Polymer Group, 56 Roland Street, Suite 310

Boston, MA 02129

7-17 Presentation (10/1/2010)

Poly(vinyl alcohol) hydrogels in medicineStructure and mechanics

Gavin Braithwaite

Poly (vinyl alcohol)

• First synthesized by Hermann and Haehnel in 1924• First commercialized in US by Dupont in 1939• Not polymerized directly

– Vinyl acetate formed through catalyzed addition of acetic acid to acetylene

– Radical polymerization to PVAc– Converted to PVA by hydrolyzation in alcohol in presence of catalyst

• Conversion rates influence solubility and application

Cambridge Polymer Group22015 SCI Rideal Meeting

PVA uses

• Market (2006) > 1 million metric tons • Precursors - Reacted to create polyvinyl acetals• Solutions - Emulsion polymerization aid and rheology modifier• Fiber production - Clothing, Concrete reinforcement• Paper products - Adhesives, Sizing (preparation)• Barriers - Water soluble and CO2 barrier films• Medicine – Sponges, Eye drops, Embolization, Nerve guides, cartilage• Children’s toys - slime• Sport - fishing


PVA structure

• Solubility and mechanical properties variable– Hydrolysis rates 87-99+%– Naturally atactic but syndiotactic can be obtained– Broad range of molecular weights

• Hydroxyl groups– Crystal structure similar to PE– Water soluble– Does not melt (decomposes)– Hydrogen bonds

• Biocompatible– Cells grow happily– No known toxicity– Not naturally degraded– Generally not persistent


Billmeyer (1984)

Crosslinking of PVA

• Chemical crosslinking– Formaldehyde, Glutaraldehyde etc– Radiation

• Physical crosslinking– Hydrogen bonding between hydroxyls– Mediated by water molecules– Borate ion (“slime”)– Formation of crystallites in water– Solutions not long-term stable


Physical Association

• Freeze-thaw process– “Cryogels” first proposed in 1971 (Peppas)– Repeated freeze-thaw cycles

• Properties influenced by– Cycles, concentration, molecular weight,

hydrolysis, time of freezing

– Freezing process• Liquid-liquid phase separation• Polymer-poor regions freeze first


Holloway et al. 2013

Hydrogen bonded crystals

• Hydrogen bonds form crystals– Onset ~55 °C, Peak ~70 °C, H 1-5 J/g– (solid PVA 161 J/g, Tm ~230 °C)


Holloway et al. 2013

PVA cryogel structure

• Croygel structure driven by formation of ice crystals – Fine pores form during melting of

polymer rich regions– Structure “sharpens” with

increasing freeze-thaw– Phase separation process– Hydrogen bonded “crosslinks”


Scale bar: 10 m

1 FT

2 FT

5 FT

15% PVA 25% PVA10% PVA5% PVAConfocal microscopy. All images and gels MGH.

200 kDa 99.9+% PVA in DI water8 hour freeze, 8 hour thaw

Confocal Images: Jeeyoung Choi (MGH)

Gelation of PVA

• Coarseness of gel depends on– Molecular weight– Number of cycles– Concentration– Age– Hydrolysis

• Crystal junctions– Not a conventional gel– Polymer-rich “bridges”


Bercea et al. 2013

Solvent-driven gelation


• An entangled polymer solution in a good solvent is stable and homogeneous

• If the solvent quality is dropped • The solution enters an unstable condition

where there is coexistence of a polymer rich and polymer poor region

• The polymer can phase separate into two concentration regions as the solvent quality changes

• When the temperature is dropped the hydrogen-bonded crystals can form in the polymer-rich regions

• A physical crosslinked hydrogel is formed

Image: ESEM CPGSimulation unknown

20 m

Theta point

• Freeze-thaw PVA hydrogel - Swell Ratio at equilibrium in mixed solvent (measurements using CPG SRT)

• 0% PEG “good” solvent, 15% PEG “poor” solvent, 28% PEG “bad” solvent• Solvency approximately inversely related to temperature for these systems


Theta solvent swelling line

Thetagel structure


• Names defined as (wt% PVA-wt% PEG 400) relative to water– 200 kDa 99.9+% hydrolyzed PVA

• Distinctive pore structure– Order 10 m diameter– Size and morphology depends on PVA concentration

15-28 DP10-28 DP 25-28 DP

Scale bar 20 m

PVA in water


Theta temperature for PVA in water is 97 °C (Polymer Handbook, Brandrup)

• 15% PVA in DI water is essentially stable up to 93 °C

0

10

20

30

40

50

60

70

80

90

100

1

10

100

1000

0 50 100 150 200 250

Tem

p [

C]

G',

G" [

Pa]

Time after start [min]

G' Cool onlyG'' Cool onlyG' Heat and coolG'' Heat and cool93 C 89 C

80 C 80 C

40 C

Effect of solvent quality

• 15-x: 15% PVA in x% PEG in water• 15% PEG behaves qualitatively the same as DI at all temperatures• 28% PEG is unstable below approximately 50 °C


0102030405060708090100

10

100

1000

0 20 40 60 80 100 120 140

Pha

se a

ngle

(d) [

deg]

G* [

Pa]


15-0 G* 15-15 G* 15-28 G*15-0 delta 15-15 delta 15-28 delta

80 ºC Cool to 40 ºC 40 ºC

0102030405060708090100

10

100

1000

10000

100000

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

Pha

se a

ngle

(del

ta) [

deg]

G*

[Pa]


G* 15-28 hold at 60CG* 15-28 cool to 40CG* 15-28 hold at 20Cdelta 15-28 hold at 60Cdelta 15-28 cool to 40Cdelta 15-28 hold at 20C

60C 40C 20CCooling starts

• 15-28 (15% PVA in 28% PEG400/water)• Temperature below 55 °C drives gelation

Importance of temperature


00.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8 10 12 14 16Time [hr]

Stra

in [m

m/m

m]

20-28 250PVA 20-28 100PVA15-28 250PVA 15-28 100PVA25-28 250PVA 25-28 100PVA

PVA hydrogel creep properties

• “Diurnal cycle” 0.5 MPa on, 0.05 MPa off• Higher molecular weight implies stiffer• High creep strain and poor creep recovery


20%-65%* Loading Curve E

0

0.5

1

1.5

2

2.5

3

DP DP DP AG RSA3 AG RSA3 AG RSA3-PEG

AG RSA3-PEG

1FT 5FT 5FT

10-28 15-28 25-28 15-28 25-28 15-28 25-28 15-28 15-28 15%

Elas

tic M

odul

us [M

Pa]

Elastic Modulus

n=5


Cartilage ~ 10MPa

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

EWC [mass water/mass hydrogel]

Max

imum

cre

ep s

trai

n af

ter 8

hrs

at 0

.5 M

Pa [m

m/m

m]

10-28 DP15-28 DP25-28 DP15-28 1FT15-28 5FT15% 5FT15-28 AG RSA315-28 AG RSA3-PEG25-28 AG RSA325-28 AG RSA3-PEG

EWC vs. max creep strain

n=3 in all cases. Scale bar 20 m


Injecting PVA

• Phase separation not instantaneous– “window” of useful time for injections or manipulation

• Allows injection of PVA hydrogel without toxic crosslinkers


0

10

20

30

40

50

60

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80

90

10

100

1000

-50 -30 -10 10 30 50

Pha

se a

ngle

(del

ta) [

deg]

G*

[Pa]

Time after reached 40C [min]

G* 15-28 10C/minG* 15-28 30C/mindelta 15-28 10C/mindelta 15-28 30C/min

45 degree phase angle

Mitral Regurgitation

• Common complication of coronary artery disease – Doubles risk of late death

• Heart attack results in compromised muscle• Over time changes geometry of chamber

– Wall thins and distorts• Deforms mitral valve tethers• Results in reversed flow through valve• Chronic problem, usually fatal

• Current solutions inadequate– Ring annuloplasty

• Invasive and complex


Work performed in collaboration with MGH under NIH 1R01HL092101-01A1

Tissue bulking


• Mitigation of Mitral Regurgitation (MR)– Inject bulking agent to displace muscle wall

AO

Restored Leaflet

Coaptation

AO

PM

Leaflet Tenting

MR

Biomaterial

Echo Transducer

Apex

Ischemic LV

LA

Coapting Surface

Biomaterial

AO

Restored Leaflet

Coaptation

AO

PM

Leaflet Tenting

MR

Biomaterial

Echo Transducer

Apex

Ischemic LV

LA

Coapting Surface

Biomaterial

MR reduction in ovine model


Figure 2

Load-bearing applications

• Applications– Spine– Mosaicplasty– Interpositional Devices– Resurfacing

• Requirements– High fatigue resistance– Good recovery– Loads over 3 kN intermittently

• Thetagels (and cryogels) do not have sufficient properties – Cartilage ~ 10MPa compressive modulus– Chemical crosslinking– “Toughening” by annealing


Toughening of thetagels – “annealing”

• Annealing of PVA gels (dehydration) changes morphology

• For 15-28, annealing procedure results in thicker struts and a more closed pore structure.

• For 25-28, pores appear to shrink or collapse

25-28

15-28 15-28 annealed

25-28 annealed


20%-65%* Loading Curve E

0

0.5

1

1.5

2

2.5

3


AG RSA3-PEG

1FT 5FT 5FT

10-28 15-28 25-28 15-28 25-28 15-28 25-28 15-28 15-28 15%

Elas

tic M

odul

us [M

Pa]

Elastic Modulus

n=5


0

10

20

30

40

50

60

70

80

90

100


AG RSA3-PEG

1FT 5FT 5FT

10-28 15-28 25-28 15-28 25-28 15-28 25-28 15-28 15-28 15%

Rec

over

ed S

trai

n [%

]Strain Recovery as a % of strain at 10 N

n=5


0

10

20

30

40

50

60

70

80

90

100

DP DP DP AG RSA3 AG RSA3 AGRSA3-PEG

AGRSA3-PEG

1FT 5FT 5FT

10-28 15-28 25-28 15-28 25-28 15-28 25-28 15-28 15-28 15%

Tota

l cre

ep s

trai

n af

ter 8

hr a

t 0.5

Mpa

[%]

Total Creep Strain

n=5


0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

EWC [mass water/mass hydrogel]

Max

imum

cre

ep s

trai

n af

ter 8

hrs

at 0

.5 M

Pa [m

m/m

m]

10-28 DP15-28 DP25-28 DP15-28 1FT15-28 5FT15% 5FT15-28 AG RSA315-28 AG RSA3-PEG25-28 AG RSA325-28 AG RSA3-PEG

EWC vs. max creep strain

n=3 in all cases

Scale bar 20 m


Plot: MGH OBBL


CoF on a rheometer

1 Kavehpour and McKinley “Tribo-rheometry: from gap-dependent rheology to tribology” Tribology Letters (2004) 17(2) 327-335

Spring

Annulus

Surface


Hydrogel coefficient of friction

• CoCr against hydrogel flats


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 10.0 20.0 30.0 40.0 50.0Nominal contact pressure [kPa]

CO

F

15-28 DP, CPG15-28 AG RSA3, CPG15-28 DP, MGH15-28 AG RSA3, MGH

Where next?

• PVA appealing– Readily available– Biocompatible– Already in use

• PVA thetagels– injectable

• Limited application – Poor mechanicals– Tissue bulking?– Degradables?– Mucoadhesives?


Functional proteins

• Mucins - Terminal groups cysteine-rich and naturally gel-forming• Lubricin (PRG4) - present in synovial fluid as lubricant


Thiolation of PVA

• Conversion of some OH groups to thiol groups adds thiol pendant groups directly to the PVA backbone– Mercaptopropionic acid reacted with PVA– Thiol groups react with cysteine residues in proteins to form disulfide

bonds


1H NMR of TPVA

• 1H NMR of converted product indicate presence of mercaptopropionic ester fragment– Degree of modification ~3%


Molecular Weight Distribution of PVA and TPVA

• Gel Permeation Chromatography indicates a small fraction of higher molecular weight species

0

1

2

3

4

5

6

7

8

9

2 3 4 5 6 7

Wn

(Log

M) *

10-

1

Log M

TPVA

PVA


T-PVA/Mucin reaction

• Mixing of TPVA with mucin and tracking rheology response proves molecular interactions– Complex viscosity of TPVA (green), mucin (blue) and TPVA combined

with mucin (red) measured at 25 °C.


0 10.0 20.0 30.0 40.0 50.0 60.0time (min)

0.01000

0.1000

1.000

|n*|

(Pa.

s)

Synthesis of TPVA/PEGDA Hydrogels

• TPVA crosslinking with difunctional poly (ethylene glycol) – thiol-reaction forms hydrogel through Michael-Type addition reaction– Thiol groups control crosslink density– PEGDA chain length control molecular weight between crosslinks– Physiologically benign reaction

b+

TPVA

PEGDApH 7.4 1xPBS


Gelation kinetics


Polymer

concentration,

% [w/v]

Temperature, °C

25 °C 37 °C

Gelation

time, [min]

G’

[Pa]

G’’

[Pa]

Gelation

time, [min]

G’

[Pa]

G’’

[Pa]

3.0 23.3 803 5 4.2 3607 480

4.5 9.2 6440 133 3.0 9860 280

0 2.5 5.0 7.5 10.0 12.5 15.0time global (min)

1.000E-3

0.01000

0.1000

1.000

10.00

100.0

1000

10000G

' (Pa

)

1.000E-3

0.01000

0.1000

1.000

10.00

100.0

1000

10000

G'' (Pa)

Degradability


• Esters are relatively unstable bonds– hydrolyzable

b

0

10

20

30

40

50

60

0 5 10 15

Swel

ling,

%

Time, days

Gel disintegration

onset

TPVA Degradation Products by GPC Analysis

• Cleaving of ester bonds yields species with TPVA and PEGDA molecular weights


10

11

12

13

14

15

3 5 7 9 11 13 15

Sign

al M

V * 1

0

Elution Volume (mL)

TPVA/PEGDADegradation products

TPVA

PEGDA

Thank you

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poly(vinyl alcohol) hydrogels in medicine · poly (vinyl alcohol) • first synthesized by hermann...

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