1. Biodegradable thiol-modified poly(vinyl alcohol) hydrogels
Yuri Svirkin, Adam Kozak, Gavin Braithwaite
MRS 2013

2. Introduction PVA biocompatible Well respected biomaterial
Not readily injectable and in situ crosslinkable
Not intrinsically biodegradable
Not sufficiently mucoadhesive
This presentation will talk about
New method of making an injectable PVA hydrogel that is mucoadhesive and degradable
MRS 2013

3. Synthesis of Thiolated Poly(vinyl alcohol)
Conversion of some OH groups to thiol groups adds thiol pendant groups directly to the PVA backbone
Pendant thiol groups were introduced into PVA by esterification reaction with mercaptopropionic acid following work of R.S. Parnas and collaborators. The degree of modification was regulated by the ratio of mercaptopropionic acid to PVA, reaction time and temperature. The degree of modification was determined by 1H NMR.
MRS 2013
R.M. Dicharry, et al Biomacromolecules, Vol. 7, N 10, 2006

4. Synthesis of TPVA/PEGDA Hydrogels
TPVA crosslinking with difunctional poly (ethylene glycol) thiol-reactive molecules forms a hydrogel via Michael-Type addition reaction
Thiol groups control crosslink density
PEGDA chain length control molecular weight between crosslinks
TPVA +
They were prepared by Michael-type addition using poly (ethylene glycol) diacryalate as a crosslinker. Crosslinking reaction occurs between pendant thiol group of PVA and acrylate end groups of PEG. The hydrogels were synthesized under physiological conditions (neutral pH around 7, aqueous media: 1xPBS and ambient temperature) with no side product formation. Reaction do not require initiators and irradiation to proceed.
pH 7.4 1xPBS
PEGDA
MRS 2013

5. 1H NMR of TPVA
1H NMR of converted product indicate presence of mercaptopropionic ester fragment
Degree of modification ~3%
1H NMR of modified PVA shows the presence of mercaptopropionic fragments. Signals at ppm are due to methylene protons at ppm of mercaptopropionic ester fragment. In this spectrum, degree of modification is around 3%.
MRS 2013

6. Molecular Weight Distribution of PVA and TPVA
Gel Permeation Chromatography indicates a small fraction of higher molecular weight species
MWD of TPVA closely matches that of parent PVA, with small fraction of HMW present only in modified polymer.
MRS 2013

7. Gelation kinetics of TPVA/PEGDA (concentration)
Rheology provides a sensitive tool for tracking gelation kinetics as a function of concentration
Effect of concentration at 25 °C:
green (3%);
red (4.5%)
Gel formation, crosslinking reaction between TPVA with PEGDA, was monitored by viscometry at total polymer concentration ranging from 3.0 to 4.5 % (w/v) and gelation temperatures 25 °C or 37 °C under dynamic conditions. On this slide is shown storage modulus change with time. The gelation levels off after 1-2 hours, depending on conditions. On the left is shown effect of concentration, with higher gelation rates corresponding to higher concentration. On the right picture is shown effect of temperature with higher temperature corresponding to higher rates.
MRS 2013

8. Gelation kinetics of TPVA/PEGDA (temperature)
Temperature also controls gelation kinetics
Effect of temperature at 3.0%
green (25 °C);
red (37 °C)
Gel formation, crosslinking reaction between TPVA with PEGDA, was monitored by viscometry at total polymer concentration ranging from 3.0 to 4.5 % (w/v) and gelation temperatures 25 °C or 37 °C under dynamic conditions. On this slide is shown storage modulus change with time. The gelation levels off after 1-2 hours, depending on conditions. On the left is shown effect of concentration, with higher gelation rates corresponding to higher concentration. On the right picture is shown effect of temperature with higher temperature corresponding to higher rates.
MRS 2013

9. Kinetics and Properties of TPVA/PEGDA Hydrogels
Gelation point
The gelation time was determined as a cross-over between G’ and G’’. The properties of hydrogels are regulated by concentration of TPVA and PEGDA, degree of modification, temperature of the reaction and shown in table. Two initial components, TPVA and crosslinker PEGDA, are low viscosity liquids that gelate under physiological conditions in an aqueous media in less than 3 minutes. That makes these hydrogels suitable for applications where injectable in situ crosslinkable hydrogels are required.
Polymer concentration, % [w/v]
Temperature, °C
25 °C
37 °C
Gelation time, [min] G’
[Pa]
G’’
[Pa] 
3.0
23.3
803 5
4.2
3607
480
4.5
9.2
6440
133
9860
280
MRS 2013

10. Swelling kinetics of TPVA hydrogels
Tracking unconfined swelling of the hydrogels in 1xPBS suggests no steady state is achieved
Gel
disintegration
onset
After 18 days hydrogel start disintegrating into pieces and after 5 weeks turned into transparent homogeneous solution
Swelling of 4.5 % w/v TPVA/PEGDA hydrogel in 1xPBS as a function of degradation time. Swelling percent=(W-Wo)/Wo;
MRS 2013

11. Degradation of TPVA Hydrogel
The use of the mercaptopropionic and acrylate results in hydrolysable ester bonds within the crosslinks
TPVA hydrogels were designed and synthesized such that crosslinks contain hydrolysable ester bonds . Each crosslink contains four hydrolysable ester bonds. Points of degradation are shown in red. As a result of hydrolysis precursor products would form: PVA and PEGDA that are biocompatible polymers. Degradation was studied by swelling ratio and GPC analysis of degradation products.
MRS 2013

12. TPVA Degradation Products by GPC Analysis
Cleaving of ester bonds yields species with TPVA and PEGDA molecular weights
TPVA
TPVA/PEGDA
Degradation
products
Soluble degradation products of TPVA/PEGDA hydrogel were analyzed by GPC and compared with precursor polymers TPVA and PEGDA from which hydrogels were prepared. The degradation products contain two main peaks: the low molecular weight peak overlays perfectly with PEGDA (A) while the second peak is slightly shifted toward high molecular weight compared to TPVA. This high molecular weight fraction is likely due to the release of PVA molecules containing fragments of partially hydrolyzed crosslinker. The crosslinks contain four ester bonds and if only a subset are hydrolyzed the remaining fragments will be still linked to PVA leading to molecular weight increase. The molecular weight profile of degradation products closely matches that of precursor materials TPVA and PEGDA, indicating them and their derivatives as the main degradation products. The degradation products: PVA and PEGDA are biocompatible and selecting appropriate molecular weight allows their easy elimination by passing through kidney.
PEGDA
MRS 2013

13. TPVA Mucoadhesive Properties
Thiol groups known to be mucoadhesive
Mixing of TPVA with mucin and tracking rheology response proves molecular interactions
Complex viscosity of TPVA (red), mucin (blue) and TPVA combined with mucin (red) measured at 25 °C.
TPVA/Mucin
Mucin
TPVA
MRS 2013

14. Features and applications of TPVA/PEGDA Hydrogels
TPVA/PEGDA hydrogels formed through Michael-Type addition reaction
physiological conditions from low viscosity TPVA and PEGDA
well suited for in situ applications
Hydrolysable ester bonds in crosslinks
inherited biodegradability
degradation results in formation PVA and PEGDA
products are biocompatible and low molecular weight to allow for easy elimination by passing through kidneys
Unreacted thiol groups,
not used in crosslinking reactions result in mucoadhesive properties
Potential applications
percutaneous, temporary tissue bulking and scaffolding
Cambridge Polymer Group,
56 Roland Street, Suite 310
Boston, MA 02129
MRS 2013