Hydrogels - American Chemical Society

Chapter 6

Synthesis and Characterization of pH-Responsive Poly(organophosphazene) Hydrogels

Downloaded by UNIV OF MINNESOTA on July 20, 2013 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch006

Harry R. Allcock and Archel Ambrosio Department of Chemistry, The Pennsylvania State University, University Park, PA 16802

Water-soluble polyphosphazenes can be crosslinked either by gamma-radiation or by di- or tri-valent cations. The resultant materials imbibe or extrude water in response to changes in temperature, pH, or cations. These hydrogels offer the prospect of uses in responsive membranes, devices for the controlled delivery of drugs, enzyme immobilization and mediation media, and tissue engineering.

Background and Purpose The long-range purpose of this work is to develop hydrogels that can be used in switchable membranes - ie. that can exist in a gate-open or gate-closed state. Possible applications include pulsed drug delivery, on-off switching of enzymes or cells, substrates for tissue engineering, or as components in sensors. In previous work we have developed two different types of hydrogels based on the polyphosphazene platform. Here we demonstrate that the molecular features of these two earlier systems can be combined to generate a third system that offers some advantages over the earlier two.

82

© 2003 American Chemical Society In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

Switched ^system Amount Released

Membrane diffusion controlled

/

Time

\

Release timed to coincide with bacterial growth cycle

Time

Figure 1. Comparison between normal diffusion-controlling membrane and an on/off switchable hydrogel membrane.

Drug

Membrane

Gasket / £

Amount Released



84

State A Hydrogel Expanded

State B Hydrogel Collapsed

Small Molecules

Gate Open

Gate Closed

Molecules can pass through, or trapped molecules escape Figure 2. Use of controlled expansion and collapse of a hydrogel to control membrane behavior.



Figure 3. On/off membrane switch for control of enzyme or cell activity.


86 The polyphosphazene platform is based in the molecular structure shown as 1.

r — N =

R

I

i

P—


!

These polymers can be prepared by several techniques, but the route utilized most in this work is summarized in Scheme I. It is based on the use of a reactive macromolecular intermediate - poly(dichlorophosphazene) (3), which can be produced either by the ring-opening polymerization of compound 2 or by the living cationic polymerization of 4. Replacement of the chlorine atoms i n 3 by organic side units is accomplished by treatment of 3 with nucleophiles, such as the sodium salts of alcohols or phenols or with primary or secondary amines. Two or more different side groups can be introduced into the same macromolecule by simultaneous or sequential exposure to the two nucleophiles.

Alkyl Ether-Substituted Polymers The use of the sodium salts of alkyl ether alcohols as nucleophiles allows access to a wide range of polymers of the types shown i n Table 1. A l l these polymers are soluble in water, and most show lower critical solution temperature (LCST) behavior, with L C S T ' s that vary with side chain length, with linearity or branching, and with side chain terminating units (1-3). Thus, each polymer is soluble i n water below the L C S T , but becomes insoluble when the solution is heated above that temperature. This behavior is reversible. Particularly important is the polymer with OCH2CH2OCH2CH2OC2H5 side groups, which has an L C S T near human body temperature. A l l of these polymers can be crosslinked by exposure to ^ C o gamma rays or to ultraviolet light i n the presence of a photosensitizer (4-6). The crosslinking occurs by a free-radical mechanism (Figure 4) and by cross-combination of the radicals so-formed. Thin films of the hydrogel can be grafted to the surface of other polymers (Figure 5) (7). Crosslinking converts a water-soluble polymer into one that absorbs water to form a hydrogel, with the degree of swelling being inversely proportional to the degree of crosslinking, and hence to the radiation dose. (Figure 6). The hydrogels exhibit L C S T behavior at approximately the same temperatures as the uncrosslinked polymers. Thus, maximum swelling i n water occurs below the L C S T , but the gels contract and extrude water above that temperature. This phenomenon can be used to open or close the membrane to permit or prevent diffusion of a drug or of nutrients to enzymes or cells. For



2

3

X

p

p

4

Me SiN=PCI

ci '

N

N

3

C I

25°C

Catalyst 3

- Me SiCl

I

L

I

OR

c«

-HCI

NaCl

R'ONa

— i

r

I r. 4 N =

n = 15,000

RNH,

NHR

I

P—

I

OR' -"-in

OR | P-

N =

NHR

Single-Substituent Polymers

Mixed-Substituent Polymers

n

-I 11

NaCl

CI

I

P-

NaCl

OR

CI

I

I

P-

- r N = P-

RONa

N =

RONa

N=

OR

Scheme I. Macromolecular Substitution Synthesis of Polyphosphazenes.


88

Table I Lower Critical Solution Temperature of Water-Soluble Polyphosphazenes


R I — N = P—

R

LCST (°C)

-OCH CH OCH 2

2

30

3

-OCH CH OCH CH OCH3 2

2

2

65

2

-OCH CH OCH CH OC H

5

38

-OCH CH OCH CH OC H

9

51

2

2

2

2

2

2

2

2

2

4

-OCH CH OCH CH NH 2

2

2

2

CH OCH 2

None

2

3

-OCH2CHOCH3

44

CH OCH CH OCH 2

2

2

-OCH CHOCH CH OCH 2

2

2

3

38

3

CH OCH CH OCH CH OCH 2

2

2

2

2

•OCH CHOCH CH OCH CH OCH 2

2

2

2

2

3

50

3

CH OCH CH OCH CH OCH CH OCH 2

2

2

2

2

2

2

-OCH CHOCH CH OCH CH OCH CH OCH 2

2

2

2

2

2

2

3

3

61

Hydrogels derived from these polymers collapse when heated above the LCST



9H3

P

fH

2

2

fH

P

2

fH

P

Gamma-rays orUV

H,

2

2

2

2

n

Figure 4. Radiation crosslinking of [NP(OCH CH20CH CH OCH3) ] to convert a water-soluble polymer to a hydrogel.

CH

P

3

2

£H

P

2

fH

?

P'


00


MEEP Coated Polymer

Co y-Ray Irradiation

60

MEEP Surface Graft

2

H0

Surface Hydrogel

2

2

2

Figure 5. Grafting of [NP(OCH CH OCH CH20CH3)2]n (methoxyethoxyethoxyphosphazene, MEEP) hydrogels to the surfaces of organic polymers.

Organic Polymer

MEEP Polymer Solution

Surface Grafted MEEP Hydrogels on Organic Polymers



91

T 5

10

15

20

25

Radiation Dose (Mega Rads) Figure 6. Swelling of MEEP hydrogels by water absorption as a function of crosslinking induced by gamma radiation.


92 example, the enzyme urease has been immobilized inside membranes of these polymers during the crosslinking step (8), and the enzymic activity can be promoted or inhibited by opening or closing the membrane by small changes in temperature. Some evidence exists that these polymers and their hydrogels show antibacterial properties (9).


Carboxylate-Bearing Polyphosphazenes The second hydrogel system developed in our earlier program was based on the polymer shown as 5, and known by the acronym PCPP [poly (carboxyphenoxyphosphazene)l.

n 5 It is prepared by the sequence of reactions shown in Scheme II, and involves an alkaline deprotection step to yield the free carboxylic acid groups (10). Polymer 5 is insoluble in water, is soluble as its sodium or potassium salt, and is precipitated as a gel by di- or tri-valent cations such as calcium or aluminum ions which form ionic crosslinks between chains. Decrosslinking and redissolving occurs when sodium or potassium ions displace the mulivalent cations. This sequence of property changes has been used to microencapsulate human cells or proteins (11-16), and to provide a vehicle for the delivery of vaccines (17) (Figure 7). PCPP is more useful than naturally occurring polymers such as alginates because of the high loading of carboxylate groups and its availability with precisely controlled molecular weights.

The Combination Hydrogel System Numerous advantages can be foreseen for combining the structural features of the alkyl ether phosphazene hydrogels with those of the carboxylate type. For example, specific properties can be tuned into the system by varying the ratios of the alkyl ether to carboxylate units over the range of, say, 95:5 to 5:95 in order to emphasize either the LCST behavior or the reversible ionic crosslinking,



Scheme II. Ionic Crosslinking of a Polyphosphazene Polyelectrolyte



Soluble

Microsphere Open

Figure 7. Polyphosphazene microspheres for cell encapsulation or as delivery vehicles.

Calcium Crosslinked Insoluble

Microsphere Closed



95 These polymers are synthesized by the protocol shown in Scheme III (18). In this process, the aryloxy groups were introduced first to define the final side group ratio, followed by the alkyl ether side groups. The ester units were then deprotected by treatment with strong base. Primary crosslinking was then achieved by gamma-ray techniques. Note that the mixed-substituent polymers shown in Scheme III, although represented by an exclusively non-geminal structure, are in fact composed of units which also contain two of the same type of side groups on the same phosphorus atom. The swelling of these gels is dependent on pH, cation charge, and the ionic strength of the medium, as shown in Figure 8. At low pH values, the gels contract, but swell progressively as the pH is raised from 4 to -7.5. The actual degree of swelling depends on the ratio of the two side groups, as shown in Figure 8 A. The higher the ratio of carboxylate groups, the more dramatic is the effect. Replacement of sodium ions by calcium ions contracts the gels as ionic crosslinks now exert an influence, and the contraction is enhanced as Ca is replaced by A l or Fe (Figure 8C). The higher valency cations presumably increase the density of crosslinking. At low ionic strengths (for example, below 1 M NaCl) increases in salt concentration cause gel contraction as the ionic polymer retracts (Figure 8B). 2+

3 +

3+

Bioerosion The polymers and hydrogels described above are stable in neutral aquous media for long periods of time. Hydrogels derived from 5 have been maintained in water for more than 5 years without evidence of decomposition. However, certain applications may require bioerodibility to satisfy the requirements for long term biocompatibility. This can be accomplished by the introduction of small amounts of hydrolysis sensitizing side groups of the types shown in Table 2 (19-31). When these side groups are the only ones present in a polyphosphazene, hydrolysis occurs to give the side group alcohol or amine, phosphate, and ammonia. When present in small amounts along the chain, these same side groups can induce slow hydrolysis of the polymer or the gel with hydrolysis rates that depend on the loading of these groups and the precise structure of the sensitizing unit.

Conclusions The combination of alkyl ether and carboxylic acid-bearing side groups creates additional opportunities for hydrogel design and behavior over and above those offered by either of the two single-substituent types. Even when not crosslinked through the alkyl ether groups, these polymers offer interesting new possibilities for microencapsulating bioactive molecules or cells. The



N =

-N=

_

2

2

P

4

3

7

2

COOH

2

+

NaOH

2. HCI

2

1. t - B u O K / H 0

2. N a O C H C H 2 0 C H 2 C H O C H 3

6

1. N a O C H C O O C H

OCH2CH OCH CH20CH3

P-

I

CI 2

2

-N =

N== P 7

O

\ /

COO" N a

+

OCH2CH2OCH2CH2OCH3

3

COOC H

OCH CH OCH2CH20CH3

Scheme III. Synthesis of Hybrid Hydrogel System



97

Figure 8. A. Effect of pH on the equilibrium degree of swelling of the hydrogels. B. Influence of sodium chloride concentration on the equilibrium degree of swelling. C. Effect of cation charge on the equilibrium degree of swelling of the hydrogels. Polymer a=poly[(30% oxybenzoate)(70% methoxyethoxyethoxy)phosphazene]. Polymer b=poly[(50% oxybenzoate)(50% methoxyethoxyethoxy)phosphazene]. Polymer c=poly[(76% oxybenzoate)(24% methoxyethoxyethoxy)phosphazene]. Polymer d=poly[(94% oxybenzoate)(6% methoxyethoxyethoxy)phosphazene].

Continued on next page.



98

Figure 8. Continued.


99

Table 2. Bioerodible polyphosphazenes

OC H I P— I OC H 2


N=

2

IJJHCH COOC H; 2

5 5

f- N =

2

2

5

3

2

2

3

N= 5

-J

n

OCH(CH )COOC H5

S

- N — P— I OCH COOC H 2

P— I CH NHCOOC H

S

OCH COOC H " 2

2

n

2

P— I OCH(CH )COOC H 3

2

n

OCH CH(OH)CH OH 2

2

h- N — P —

I

OCH CH(OH)CH Ot 2

2

n _l n

'N' I


100 recent access to phosphazene block copolymers (32-36) provides additional opportunities for combining these two types of side groups i n ways that could be useful for microencapsulation or for incorporation into controlled diffusion membranes.

Acknowledgments


This work was supported by Ethicon Inc. (a Johnson & Johnson Company), and by the U S Army Research Office.

References (1) Allcock, H. R.; Austin, P. E.; Neenan, T. X.; Sisko, J. T.; Blonsky, P. M.; Shriver, D . F., Macromolecules 1986, 19, 1508-1512. (2) Allcock, H . R.; Pucher, S. R.; Turner, M. L.; Fitzpatrick, R. J., Macromolecules 1992, 25, 5573-7. (3) Allcock, H. R.; Dudley, G. K., Macromolecules 1996, 29, 1313-19. (4) Allcock, H. R.; Kwon, S.; Riding, G . H.; Fitzpatrick, R. J.; Bennett, J. L., Biomaterials 1988, 9, 509-13. (5) Bennett, J. L.; Dembek, A . A.; Allcock, H. R.; Heyen, B . J.; Shriver, D . F., Polym. Prepr. 1989, 30, 437-8. (6) Nelson, C. J.; Coggio, W. D . ; Allcock, H . R., Chem. Mater. 1991, 3, 786-7. (7) Allcock, H. R.; Fitzpatrick, R. J.; Visscher, K., Chem. Mater. 1992, 4, 77580. (8) Allcock, H . R.; Pucher, S. R.; Visscher, K. B . , Biomaterials 1994, 15, 5026. (9) Allcock, H . R.; Pucher, S. R.; Fitzpatrick, R. J.; Rashid, K . , Biomaterials 1992, 13, 857-62. (10) Allcock, H . R.; Kwon, S., Macromolecules 1989, 22, 75-9. (11) Cohen, S.; Bano, M. C.; Visscher, K. B.; Chow, M.; Allcock, H. R.; Langer, R., J. Am. Chem. Soc. 1990, 112, 7832-3. (12) Bano, M. C.; Cohen, S.; Visscher, K . B.; Allcock, H . R.; Langer, R., BioTechnology 1991, 9, 468-71. (13) Cohen, S.; Bano, M. C.; Cima, L . G . ; Allcock, H . R.; Vacanti, J. P.; Vacanti, C. A.; Langer, R., Clin. Mater 1993, 13, 3-10. (14) Andrianov, A . K.; Cohen, S.; Visscher, K. B.; Payne, L. G . ; Allcock, H . R.; Langer, R., J. Controlled Release 1993, 27, 69-77. (15) Cohen, S.; Allcock, H . R.; Langer, R., Recent Adv. Pharm. Ind. Biotechnol., Minutes Int. Pharm. Technol. Symp., 6th, Meeting Date 1993, 36-48. (16) Andrianov, A. K.; Payne, L . G.; Visscher, K . B.; Allcock, H. R.; Langer, R., J. Appl. Polym. Sci. 1994, 53, 1573-8.



101 (17) Payne, L . G.; Jenkins, S. A . ; Woods, A . L.; Grund, E. M.; Geribo, W . E.; Loebelenz, J. R.; Andrianov, A . K.; Roberts, B . E . , Vaccine 1998, 16, 9298. (18) Allcock, H . R.; Ambrosio, A . M. A . , Biomaterials 1996, 17, 2295-2302. (19) Allcock, H . R.; Fuller, T. J.; Mack, D . P.; Matsumura, K.; Smeltz, K. M., Macromolecules 1977, 10, 824-30. (20) Allcock, H . R.; Pucher, S. R.; Scopelianos, A . G., Biomaterials 1994, 15, 563-9. (21) Allcock, H . R.; Pucher, S. R.; Scopelianos, A . G., Macromolecules 1994, 27, 1071-5. (22)Ibim,S. M.; Ambrosio, A . A.; Larrier, D . ; Allcock, H. R.; Laurencin, C. T., J. Controlled Release 1996, 40, 31-39. (23) Laurencin, C. T.; El-Amin, S. F.; Ibim, S. E . ; Willoughby, D . A.; Attawia, M.; Allcock, H. R.; Ambrosio, A . A., J. Biomed. Mater. Res. 1996, 30, 1338. (24) Crommen, J. H. L.; Schacht, E . H.; Mense, E. H. G., Biomaterials 1992, 13, 601-11. (25) Crommen, J.; Vandorpe, J.; Schacht, E . , J. Controlled Release 1993, 24, 167-80. (26) Vandorpe, J.; Schacht, E . ; Dunn, S.; Hawley, A.; Stolnik, S.; Davis, S. S.; Gamett, M. C.; Davies, M. C.; Illum, L., Biomaterials 1997, 18, 1147-1152. (27) Allcock, H . R.; Scopelianos, A . G., Macromolecules 1983, 16, 715-19. (28) Allcock, H . R.; Pucher, S. R., Macromolecules 1991, 24, 23-34. (29) Allcock, H . R.; Kwon, S., Macromolecules 1988, 21, 1980-5. (30) Allcock, H . R.; Pucher, S. R.; Scopelianos, A . G., Macromolecules 1994, 27, 1-4. (31) Allcock, H . R.; Fuller, T. J.; Matsumura, K., Inorg. Chem. 1982, 21, 51521. (32) Honeyman, C. H.; Manners, I.; Morrissey, C. T.; Allcock, H. R., J. Am. Chem. Soc. 1995, 117, 7035-6. (33) Allcock, H . R.; Crane, C. A . ; Morrissey, C. T.; Nelson, J. M.; Reeves, S. D . ; Honeyman, C. H.; Manners, I., Macromolecules 1996, 29, 7740-7747. (34) Nelson, J. M.; Primrose, A . P.; Hartle, T. J.; Allcock, H. R.; Manners, I., Macromolecules 1998, 31, 947-949. (35) Allcock, H . R.; Nelson, J. M.; Prange, R.; Crane, C. A . ; deDenus, C. R . Macromolecules 1999, 32, 5736-43. (36) Prange, R.; Allcock, H. R., Macromolecules 1999, 32, 6390-6392.


Hydrogels - American Chemical Society

Recommend Documents