Poly(vinyl alcohol) hydrogels in medicine · Poly (vinyl alcohol) • First synthesized by Hermann...
Transcript of 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
Cambridge Polymer Group32015 SCI Rideal Meeting
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
Cambridge Polymer Group42015 SCI Rideal Meeting
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
Cambridge Polymer Group52015 SCI Rideal Meeting
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
Cambridge Polymer Group62015 SCI Rideal Meeting
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)
Cambridge Polymer Group72015 SCI Rideal Meeting
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”
Cambridge Polymer Group82015 SCI Rideal Meeting
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”
Cambridge Polymer Group92015 SCI Rideal Meeting
Bercea et al. 2013
Solvent-driven gelation
Cambridge Polymer Group102015 SCI Rideal Meeting
• 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
Cambridge Polymer Group112015 SCI Rideal Meeting
Theta solvent swelling line
Thetagel structure
Cambridge Polymer Group122015 SCI Rideal Meeting
• 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
Cambridge Polymer Group132015 SCI Rideal Meeting
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
Cambridge Polymer Group142015 SCI Rideal Meeting
0102030405060708090100
10
100
1000
0 20 40 60 80 100 120 140
Pha
se a
ngle
(d) [
deg]
G* [
Pa]
Time after start [min]
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]
Time after start [min]
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
Cambridge Polymer Group152015 SCI Rideal Meeting
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
Cambridge Polymer Group162015 SCI Rideal Meeting
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
Cambridge Polymer Group172015 SCI Rideal Meeting
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
Cambridge Polymer Group182015 SCI Rideal Meeting
Injecting PVA
• Phase separation not instantaneous– “window” of useful time for injections or manipulation
• Allows injection of PVA hydrogel without toxic crosslinkers
Cambridge Polymer Group192015 SCI Rideal Meeting
0
10
20
30
40
50
60
70
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
Cambridge Polymer Group2015 SCI Rideal Meeting
Work performed in collaboration with MGH under NIH 1R01HL092101-01A1
Tissue bulking
Cambridge Polymer Group212015 SCI Rideal Meeting
• 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
Cambridge Polymer Group222015 SCI Rideal Meeting
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
Cambridge Polymer Group232015 SCI Rideal Meeting
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
Cambridge Polymer Group242015 SCI Rideal Meeting
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
Cambridge Polymer Group252015 SCI Rideal Meeting
0
10
20
30
40
50
60
70
80
90
100
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%
Rec
over
ed S
trai
n [%
]Strain Recovery as a % of strain at 10 N
n=5
Cambridge Polymer Group262015 SCI Rideal Meeting
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
Cambridge Polymer Group272015 SCI Rideal Meeting
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
Cambridge Polymer Group282015 SCI Rideal Meeting
Plot: MGH OBBL
Cambridge Polymer Group292015 SCI Rideal Meeting
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
Cambridge Polymer Group302015 SCI Rideal Meeting
Hydrogel coefficient of friction
• CoCr against hydrogel flats
Cambridge Polymer Group312015 SCI Rideal Meeting
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?
Cambridge Polymer Group322015 SCI Rideal Meeting
Functional proteins
• Mucins - Terminal groups cysteine-rich and naturally gel-forming• Lubricin (PRG4) - present in synovial fluid as lubricant
Cambridge Polymer Group332015 SCI Rideal Meeting
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
Cambridge Polymer Group342015 SCI Rideal Meeting
1H NMR of TPVA
• 1H NMR of converted product indicate presence of mercaptopropionic ester fragment– Degree of modification ~3%
Cambridge Polymer Group352015 SCI Rideal Meeting
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
Cambridge Polymer Group362015 SCI Rideal Meeting
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.
Cambridge Polymer Group372015 SCI Rideal Meeting
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
Cambridge Polymer Group382015 SCI Rideal Meeting
Gelation kinetics
Cambridge Polymer Group392015 SCI Rideal Meeting
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
Cambridge Polymer Group402015 SCI Rideal Meeting
• 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
Cambridge Polymer Group412015 SCI Rideal Meeting
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
Cambridge Polymer Group is a contract research laboratory specializing in polymers and their applications. We provide outsourced research and development, consultation and failure analysis as well as routine analytical testing and custom test and instrumentation design.
Cambridge Polymer Group, Inc.56 Roland St., Suite 310Boston, MA 02129(617) 629-4400http://[email protected]
Cambridge Polymer Group422015 SCI Rideal Meeting