Stimuli-Responsive Water Soluble and Amphiphilic Polymers

Chapter 17

Responsive Polymer/Liposome Complexes: Design, Characterization and Application

Downloaded by CORNELL UNIV on May 23, 2012 | http://pubs.acs.org Publication Date: November 28, 2000 | doi: 10.1021/bk-2001-0780.ch017

Françoise M. Winnik* and Tania Principi Department of Chemistry, McMaster University, 1280 Main St W, Hamilton, Ontario Canada L8S 4M1

The interactions between vesicles and hydrophobically -modified copolymers of N-isopropylacrylamide (NIPAM) and N-glycine acrylamide (Gly) have been examined by fluorescence spectroscopy, dynamic light scattering, and dye -release studies. Four different polymer samples were employed: a copolymer of NIPAM and Gly (PNIPAM-Gly), a copolymer of NIPAM, Gly, and N-(1-pyrenylmethyl acrylamide) (PNIPAM-Gly-Py), a copolymer of NIPAM, Gly, and N-(n-octadecylacrylamide) (PNIPAM-Gly-C ), and a copolymer of NIPAM, Gly, and N-[4-(1-pyrenyl)butyl]-N-n -octadecylacrylamide (PNIPAM-Gly-C Py). The polymers form soluble micelles in cold water but their solutions undergo phase separation upon heating above a critical temperature. In the presence of liposomes the polymeric micelles are disrupted and the hydrophobic substituents of the polymer are incorporated within the liposome bilayer to yield polymer/liposome complexes. The binding of PNIPAM-Gly -C Py with liposomes was assessed as a function of the chemical composition of the liposome membrane, the total lipid concentration, and the incubation time. Three factors control the polymer/liposome interactions: (1) hydrophobic interactions driven by the nonpolar side groups of the polymer; (2) hydrogen-bond formation between the amide residues of the NIPAM units and the ethylene glycol head groups of the 18

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© 2001 American Chemical Society

In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

277

278 lipid; and (3) hydrogen-bond formation between the Gly residues of the polymer and those linked to the liposome bilayer via incorporation in the bilayer of the glycine -terminated lipid, 1',3'-dihexadecyl N-[1-(N-glycyl)succinyl] -L-glutamate ((C ) -Glu-C -Gly). The latter interactions are labile and can be disrupted by changes in pH. Thus, efficient release of calcein took place when a pH 7.4 suspension of calcein-loaded liposomes containing (C ) -Glu-C -Gly in their bilayer and coated with PNIPAM-Gly-C Py was acidified to pH 6.0. 16 2

2

16 2

2


18

Introduction Liposomes form spontaneously when lipids are dispersed in aqueous media. They consist of one or several bilayer membranes, which capture a pool of water. The practical value of liposomes is derived from two unique properties: 1) their ability to entrap either water-soluble materials in the internal water reservoir or liposoluble compounds in the lipid bilayer; and 2) their compatibility with natural membranes making them safe for medical use and facilitating their penetration into cells. Current applications range from drug vehicles (1,2) and diagnostics tools to cosmetics formulations and encapsulating media in the food industry. Liposomes are fragile structures created in water as a result of a delicate balance of interacting forces, which arise when amphiphilic compounds are added to water. In most practical situations vesicles have to be stabilized so that they conserve their integrity in hostile environments. They must also deliver the entrapped materials once they have reached their target. Among the various approaches to achieve these goals, one option explored in several laboratories seems particularly promising. It consists in adsorbing securely on the outer surface of the liposome a polymer, which responds, by some controllable physical change, to an external stimulus, such as a pulse of light or a sudden change in pH, temperature, or ionic strength of the suspending medium (J). Extensively studied systems include complexes between phosphatidyl choline membranes and the pH-sensitive polyelectrolyte poly-(2ethylacrylic acid) (4) or the temperature-responsive poly-(Wisopropylacrylamide) (PNIPAM) (5). In most studies of liposome/PNIPAM systems, anchoring of the polymer on the bilayer is effected via a small number of long alkyl chains attached to the polymer backbone, an approach pioneered by Ringsdorf and coworkers (d), and employed effectively by several research



279 teams (7,8,9,10). In systems using these polymers, termed 'hydrophobicallymodified-PNIPAMs' (HM-PNIPAM), the alkyl substituents serve as anchors and the N-isopropylacrylamide chain provides the thermosensitivity. In water, HM-PNIPAMs, like PNIPAM itself, undergo reversible phase transition when heated above a lower critical solution temperature (32 °C in the case of PNIPAM (11), LCST). Detailed investigations of this phase transition have established that, at the LCST, the PNIPAM chains undergo a collapse from hydrated extended coils to hydrophobic globules, which aggregate and form a separate phase (12,13). It is believed that the extended hydrated form of the polymer contributes to the stabilization of the liposome bilayer. This stabilizing effect can, however, be curtailed dramatically by an increase in the temperature above the LCST, as the collapse of the chain triggers a contraction of the external lipid bilayer. Polyelectrolyte/liposome complexes are sensitive to pH changes. In certain lipid bilayers, polyelectrolytes can promote phase changes, create local defects, or cause aggregation or fusion. Alternatively, as demonstrated in the elegant work of Tirrell and collaborators, the polyelectrolyte induces pH-dependent lateral diffusion of charged lipids incorporated in the bilayer, affecting the permeability of the membrane (4). Particularly effective in promoting such effects are derivatives of poly(2-ethylacrylic acid) anchored into the bilayer through phospholipid residues (4,14). The design of liposomes sensitive to two different stimuli under controlled conditions is a natural extension of these recent achievements. Copolymers of NIPAM and comonomers of varying hydrophilicity have been prepared (15) and a few recent reports describe their interactions with liposomes (16,17). We have designed liposome/polymer complexes based on the thermosensitivity of PNIPAM and the pH-sensitivity of the carboxylic acid substituents of glycine residues incorporated along the polymer backbone. The copolymer employed, PNIPAM-Gly-CjgPy (18), Figure 1, consists of a PNIPAM chain that carries at random approximately 20 mole percent of glycine residues as well as a small number of hydrophobic groups labeled with a fluorescent dye. This unique copolymer combines the phase transition characteristics of PNIPAM with the responsiveness to changes in pH, via protonation/deprotonation of the glycine carboxyl groups. In response to specific changes in temperature, solution pH, or ionic strength, it will change its conformation. The thrust of this article is a demonstration of the controlled release of substances entrapped in liposome/polymer complexes using copolymers of NIPAM and glycine acrylamide anchored onto phospholipid and nonphospholipid liposomes. To appreciate the intricacies of the system, it is necessary to understand the solution properties of the polymers in the absence of liposomes. These will be reviewed briefly in the first section of this manuscript.


280

PNIPAM-Gly-Py

ρ = 1, η = 83, m = 16

PNIPAM-Gly-Cis


η = 80, m = 18, ρ = 2

PNIPAM-Gly-Py

η = 80, m = 18, p = 2

Figure 1. Structure of the polymers used in this study The physicoehemical properties of polymer/liposome complexes will be discussed in a second part, starting with simple HM-PNIPAM's anchored onto phospholipid liposomes. Techniques employed to monitor the interactions include microcalorimetry, fluorescence spectroscopy and dynamic light scattering. The release properties of complexes between PNIPAM-GlyC] Py/liposomes will be described. Systems examined include complexes of PNIPAM-Gly-CjgPy with cationic phospholipid liposomes as well as with nonphospholipid liposomes made up of n-octadecyldiethylene oxide and cholesterol spiked with either the cationic surfactant dioctadecyldimethylammonium bromide (DDAB), the anionic amphiphile dioctadecylphosphate (DP), or a neutral lipid with a glycine-terminated head group, (Ci ) -Glu-C -Gly (Figure 2). Results are interpreted in terms of the relative importance of electrostatic interactions, hydrogen bond formation, and hydrophobic interactions in guiding the formation, stability, and release properties of the various liposome/polymer complexes. 8

6 2

2


281


ο

Figure 2. Structure of lipids used in this study

Experimental Section

Materials Dimethyldioctadecylammonium bromide (DDAB), palmitoleyl phosphatidyl choline (POPC), palmitoleyl phosphatidyl serine (POPS) and dimyristoyl phosphatidyl choline (DMPC) were purchased from Avanti Polar Lipids. Cholesterol, w-octadecyldiethylene oxide (Brij 72, E0 -Ci H 7), and calcein were obtained from Sigma Chemical Co. r,3'-dihexadecyl N-[1-(Nglycyl)succinyl]-L-glutamate ((Ci )2-Glu-C -Gly (Figure 2)) was prepared following the published procedure (19). The copolymer of Nisopropylacrylamide, iV-[4(l-pyrenyl)butyl)]-iV-«-octadecylacrylamide, and Nglycidylacrylamide (PNIPAM-Gly-C Py), Mn 25,000, Mw/Mn 2.1 (from GPC 2

6

8

3

2

18


282 data calibrated against poly(ethylene oxide) standards) was prepared as previously described (18). The copolymers PNIPAM-Gly, PNIPAM-Gly-Py, and PNIPAM-Gly-Ci (Figure 1) were obtained as described (20). A sample of PNIPAM-C Py was synthesized following a procedure reported earlier (21). 8

18


Instrumentation Dynamic light scattering measurements were performed with a Brookhaven Instrument Corporation Particle Sizer Model BI-90. UV spectra were measured with a Hewlett Packard 8452A photodiode array spectrometer, equipped with a Hewlett Packard 89090A temperature controller. Fluorescence spectra were recorded on a SPEX Industries Fluorolog 212 spectrometer equipped with a GRAMS/32 data analysis system. Temperature control of the samples was achieved using a water-jacketed cell holder connected to a Neslab circulating bath. The temperature of the sample fluid was measured with a thermocouple immersed in a water-filled cuvette placed in one of the four cell holders in the sample compartment. The slits setting ranged from 0.5 to 2.5 mm (emission) and 1.0 to 2.0 mm (excitation) depending on the chromophore concentration. The excitation wavelength was 346 nm, unless otherwise stated. The pyrene excimer to monomer ratio (IE/IM) was calculated by taking the ratio of the intensity (peak height) at 480 nm to the half sum of the intensities at 378 and 397 nm. In time-dependent studies, a suspension of liposomes (15 μΐ, 20 g L" ) was added to a solution of PNIPAM-Gly-Ci Py (3 mL, 0.002 g L" ). Spectra were measured immediately prior to liposome addition and every 2 min thereafter. 1

1

8

Cloud Point Determinations Cloud points were determined by spectrophotometric detection of the changes in turbidity (λ =600 nm) of aqueous polymer solutions (1.0 g L" ) heated at a constant rate (0.5 °C min" ) in a magnetically stirred UV cell. The value reported is the temperature corresponding to a decrease of 20 % of the solution transmittance. 1

1

Calorimetric Measurements Calorimetric measurements were performed on a ΝΑΝΟ differential scanning calorimeter N-DSC (Calorimetry Sciences Corp.) at an external pressure of 3.0 atm. The cell volume was 0.368 mL. The heating rate was 1.0 °C min" , unless other wise specified. 1


283 Sample Preparation Polymer solutions for spectroscopic analysis were prepared from stock solutions (5.0 g L" ). Ionic strength was adjusted by the addition of NaCl. Solutions in water were not degassed. Solutions in methanol were degassed by vigorous purging (1 min) with methanol saturated argon. Liposomes were prepared as follows. A solution in chloroform of the desired amounts of lipids was poured into glass test tubes. The solvent was evaporated under a stream of nitrogen. The resulting lipid film was dried under high vacuum for at least two hours. The dry lipid film was hydrated in an aqueous solution of NaCl to give a lipid suspension of concentration 20 g L"\ For experiments with calcein-loaded liposomes, calcein (70 mmol L" ) was added to the hydration. The lipid suspension was subjected to extrusion (60 °C in the case of non-phospholipid liposomes, 30 °C in the case of phospholipid liposomes) using a Lipofast extruder (Avestin, Canada) fitted with 100 or 200 nm polycarbonate filters obtainedfromMillipore. Liposome/polymer mixtures were prepared by addition of an extruded liposome suspension to polymer solutions in the desired proportions. The mixtures were allowed to equilibrate at room temperature for at least 30 min. 1


1

Calcein Release Efflux of calcein from the liposomes was observed using fluorescence spectroscopy (22). Extruded vesicles containing calcein were purified on a Sephadex G-75 column to isolate loaded vesicles from free calcein. An aliquot of dispersion of liposomes encapsulating calcein was added to 2.5 mL of glycine buffer solutions of desired pH (50 mmol glycine, 0.1 M NaCl) in a quartz cell kept at 37 °C. The fluorescence intensity of the solution was monitored for 1100s (k = 450 nm, X = 515 nm). The percent release of calcein from liposomes is defined as: % release = (I-I )/(L-Io) x 100, where I and I are the initial (t = 100 s after addition) and final (t = 1100 s)fluorescenceintensities, respectively and L is thefluorescenceintensity of the liposome suspension after the addition of sodium dodecyl sulfate (SDS, final concentration 1.6 mmol L" ). exc

cm

0

0

1

Results and Discussion

Materials and Spectroscopy Synthesis and Structure of the Copolymers The pyrene-labeled copolymer PNIPAM-Gly-Py was prepared from a copolymer of NIPAM, N-acrylamidoglycine ethyl ester, and N -


284 Table I. Physical Properties and Composition of the Polymers B

0/A Composition (mol %)' Pyrene Content

Polymer

M

n

M (MJM ) w

n

1

(molz )

a

PNIPAM-Gly PNIPAM-Gly-Py PNIPAM-Gly-C PNIPAM-Gly-PyC 18

a 18

NIPAM 80 78 78 83

Gly 20 17 19 19

1.2 χ 10 9.4 χ 10·

30,000 23,000 22,000 25,000

77,000 (2.5) 54,000 (2.2)


a

fromref. 18, fromanalysis of the H NMR spectra fromU V absorbance spectra

b

1

c

acryloxysuccinimide, by treatment first with 1-pyrenylmethylamine to introduce the pyrene chromophore through an amide link, and, second, with a mild base to hydrolyze the ester groups of the protected glycine residues (20). The hydrophobically-modified copolymer, PNIPAM-Gly-C Py, was obtained by copolymerization in dioxane of NIPAM, N- acrylamidoglycine ethyl ester, and N-[4-( 1 pyrenyl)-butyl]-N-n-octadecylacrylamide followed by base-catalyzed hydrolysis of the ethyl ester group (18). The composition and physical properties of the copolymers are listed in Table I, together with those of several HMPNIPAM's needed to carry out control experiments. The chemical structures of the polymers are depicted in Figure 1. 18

Spectroscopy of the Polymers in Solution The emission of pyrene attached to a polymer chain is sensitive to changes in the chromophore separation distance. This feature was used extensively in this study, on the basis of the principles outlined in this section. The emission of locally isolated excited pyrenes ('monomer' emission, intensity I ) is characterized by a well-resolved spectrum with the [0,0] band at 379 nm. The emission of pyrene excimers (intensity I ), centered at 480 nm is broad and featureless. Excimer formation requires that an excited pyrene (Py*) and a pyrene in its ground-state come in close proximity within the Py* lifetime. The process is predominant in concentrated pyrene solutions or under circumstances where microdomains of high local pyrene concentration form even though the total pyrene concentration is very low. This effect is shown for example by comparing the spectra of PNIPAM-Gly-CigPy in water and in methanol (Figure 3). The emission of pyrene from a methanolic solution of PNIPAM-Gly-CigPy M

E



285

Figure 3. Fluorescence spectra ofPNIPAM-Gly-C Py in water and in methanol; polymer concentration: 0.05 g U ; X -341 nm. I8

l

exc

displays mostly pyrene monomer emission, a common occurrence in dilute solutions ([Py] = 3.0 χ 10" mol L" ). In contrast, the emission of pyrene from an aqueous solution of the polymer in the same concentration exhibits a strong excimer emission that vouches for the presence of hydrophobic microdomains in which pyrene groups come in close contact. Disruption of the hydrophobic microdomains, by added surfactants or co-solvents for example, resulting in the local dilution of the chromophores can be detected easily by a significant decrease in excimer emission and a concomitant increase of the intensity of Py monomer emission. The fluorescence spectra of a pyrene-labeled copolymer of NIPAM and glycine acrylamide, PNIPAM-Gly-Py, which does not carry pendent octadecyl chains, are presented in Figure 4 for solutions in water and in methanol. In methanol, the emission originates almost entirely from locally isolated Py* In water, the relative contribution of excimer emission is larger. Although not apparent from Figures 3 and 4, the overall emission intensity is lower for polymers in water, indicating a large extent of pyrene self-quenching. The excimer in this case originates mostly from pre-formed pyrene dimers or higher aggregates and is not formed via the dynamic mechanism postulated by Birks (23) and exemplified by the pyrene emission from methanolic solutions of PNIPAM-Gly-Py or PNIPAM-Gly-Ci Py. Pyrene dimers have been detected in aqueous solutions of several pyrene-labeled polymers (24). 6

1

8



286

Figure 4. Fluorescence spectra of PNIPAM-Gly-Py in water and in methanol; polymer concentration; 0.05 g L~ ; X = 342 nm, methanol; X = 344 nm, water l

exc


exc

287 Properties of Aqueous Solutions of Copolymers of NIPAM and N-glycine acrylamide Even though PNIPAM exhibits many properties characteristic of amphiphilic polymers, it does not form aggregates in water below the cloud point of the solution. Similarly, PNIPAM-Gly and PNIPAM-Gly-Py do not aggregate even in the case of solutions of low pH as long as solutions remain below their cloud point, as concluded from the absence of signal in dynamic light scattering measurements (DLS). In contrast, a strong signal was detected when the DLS measurements were performed on aqueous solutions of the hydrophobicallymodified copolymer (20 °C, 0.1 g L" , [NaCl] = 0.1 M, pH 2.0 to 9.0). The effective hydrodynamic diameter of the PNIPAM-Gly-Ci Py aggregates was 16 nm ± 2 nm (20). The pH of the solution did not affect the size of the polymeric micelles. Moreover, their size was not altered by changes in salt concentration ([NaCl] =0.1 to 2.0 M) for a PNIPAM-Py-Gly-C Py solution of neutral pH (7.1). Only in solutions near the conditions of ionic strength, pH, and temperature corresponding to the point of macroscopic phase separation did we observe a significant change in the size of the aggregates. Thus, the dynamic light scattering studies point to the fact that in aqueous solutions of PNIPAMGly-CjgPy the structure of the hydrophobic microdomains is determined by the nature of the hydrophobic group and its level of incorporation along the chain, and that it is hardly affected by changes in the degree of protonation of the glycine residues. The hydrophobic core of such micelles is composed of octadecyl groups and pyrene (Py) labels. It is surrounded by a corona consisting of hydrated poly-(N-isopropylacrylamide) moieties and partially neutralized glycine residues. The collapse of the polymeric corona can be effected either by changes in temperature, at constant pH and ionic strength, or by a pH-jump at constant temperature (20). Return to the original conditions of pH and/or temperature restores polymeric micelles in their original solvated form. This conclusion is strengthened by results gathered from a study of the pHdependence of the photophysical properties of PNIPAM-Gly-CigPy in aqueous solutions. The ratio I /I remained constant, independent of pH ranging from 2.5 to 8.0 (20). All the NIPAM/Gly copolymers were soluble in water at or below room temperature, independent of the pH of the solutions. However, depending on pH, their aqueous solutions became turbid when heated above a critical temperature, or cloud point. This temperature was determined for polymer solutions of various pH by the simple spectrophotometric method based on the detection of changes in a solution transmittance at a wavelength of light absorbed by neither the solvent nor the solute. The cloud points of aqueous solutions of all NIPAM/Gly copolymers exhibit a marked dependence on the pH of the solution (Table II). They decrease in value with decreasing pH, reflecting 1


8

18

E

M


288 the increased hydrophobicity of the copolymers as they assume their fully protonated form. Mierocalorimetry offers another view of the phase transition phenomenon. A decrease in pH or progressive protonation of the polymer results in a decrease in the transition temperature and a broadening of the endotherm, as illustrated in Figures 5, which displays the temperature dependence of the partial molar heat capacity for solutions of PNIPAM-Gly-Py and PNIPAM-Gly-Ci Py of various pHs. Each polymer solution exhibits an endotherm near the cloud point. Thus, the collapse of the copolymers is triggered primarily by the response of the NIP AM units to changes in solution temperature, and only mildly affected by the presence of hydrophobic substituents. 8


Table II. Cloud Points and Temperatures of Maximum Heat Capacity (Tmax) of Aqueous Solutions of N-isopropylacrylamide-N-Glycine Acrylamide Copolymers Polymer

PH

PNIPAM-Gly

PNIPAM-Gly-Py

PNIPAM-Gly-C Py 18

a

Cloud point where (I /I )t=o and (I /I )t=4o min represent the values of the ratio in liposome-free polymer solution and in the mixed system 40 min after addition of liposomes, respectively. E

E


E

M

M

M

E

m

E

M

E

M

M

Figure 9.

Relative changes in the ratio of pyrene excimer to monomer emission intensity ΙΕ/ΙΜΦ^ a polymer/liposomes contact time of 40 min for various 1

liposome composition; polymer concentration: 0.002 g L" ; lipid concentration: ; % change in IE/IM

1

O.lgL

=

100x[ (W^ornin^W^oJ/iW/=0 text).

(see

Next, we performed a fluorescence analysis of polymer/liposomes systems which had been kept at room temperature for 24 h to ensure equilibration. The ratio I /I decreased sharply upon addition of NPL's containing 0 to 10 % Glyterminated lipid to a solution of PNIPAM-Gly-C, Py (Figure 10). The value of I /I reached at saturation (0.15 ± 0.2) is nearly identical to the value reached at identical lipid concentration in mixed system of the polymer and cationic NPL's (0.1). Interactions of the polymer with DMPC-based liposomes containing the Gly-terminated lipid are much weaker than those between the polymer and E

M

8

E

M


295 NPL's, as judged by the rather modest decrease in the ratio I /I in the mixed systems. It is important to note, however, that the ratio IE/IM (0-6) recorded in the case of mixed systems of polymer and DMPC liposomes containing 10 % Glylipid, is identical, within experimental error, to the value obtained previously in systems of PNIPAM-Gly-C] Py and cationic phospholipid liposomes containing 10 % DDAB (Figure 8). A mild dependence of the interactions on the Gly-lipid content is also detected: I /I at saturation decreases from 0.8 (0 % Gly-lipid) to 0.65 (10 % Gly-lipid) (Figure 10). E

M

8


E

M

Figure 10. Changes in the ratio of excimer to monomer emission intensities I /I as a function of lipid concentration in mixtures of PNIPMA-Gly-C Py with liposomes of various compositions. E

M

]8

Release Properties of Polymer/Liposome Systems When PNIPAM-Gly is mixed with liposomes at conditions far from the cloud point, the adsorption of copolymer onto liposomes surface does not disrupt the liposome integrity and, consequently, any encapsulated material will remain confined in the liposome interior. As the liposome-polymer complexes approach the conditions of cloud point, leakage of the encapsulated material may occur as a consequence of the partial or complete disruption of the liposome bilayer (16). We assessed the release properties of mixed PNIPAM-Gly-Ci Py/liposome systems with liposomes containing increasing amounts of Gly-lipid. Leakage was initiated by acidification of a solution of polymer-coated liposomes containing 8


296


calcein in their aqueous pool (see experimental). The release of calcein became more efficient as the magnitude of the drop in pH increased and as the Gly-lipid content increased (Figure 11). Even a relatively mild acidification (pH 7.4 to 6.0) resulted in leakage of 40% of the vesicle content in less than 5 minutes in the case of polymer coated-NPL's containing 10 % Gly-lipid. Naked liposomes of identical composition did not respond to such a mild change in pH (Figure 11).

Figure 11.

Plots of the percent release of calcein upon change in pHfrom unmodified liposomes of various compositions (light gray bars) andfrom liposome coated with PNlPAM-Gly-C Py (dark gray bars) in a buffer ofpH 7.4; I 1 polymer concentration: 0.04gL~ ; lipid concentration: O.lgL' ; temperature: l8

3fC. The driving forces for the leakage are believed to be the pH-induced disruption of the polymer/lipid Gly-Gly hydrogen bonds and the conformational change of the adsorbed PNIPAM-Gly chains as they reached conditions close to the cloud point of the polymer (20). As the polymer contracts, due to the protonation of the glycine residues, it brings about severe distortions of the lipid bilayer with subsequent leakage of the liposome content. Such leakage efficiencies were not achieved in control experiments using phospholipid liposomes coated with PNIPAM-Gly-C Py (leakage efficiency: less then 10 %). 18

Acknowledgments This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. Ms T. Principi thanks the Natural Sciences and Engineering Research Council of Canada and McMaster University


297 for summer research fellowships. FMW thanks her co-workers over several years who contributed to this research program: Dr. A. Polozova who carried out the key initial experiments, as well as Dr. Gangadhara, Ms. M . Spafford, and Ms E. Goh.

References 1. Handbook of Non-Medical Applications of Liposomes; Barenholz, Y . Lasic,

D.D., Eds.; CRC Press: Boca Raton, FL, 1996.


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D. D. Liposomes: From Physics to Applications; Elsevier:

Amsterdam, 1993. 3. Tirrell, D.A.; Takigawa, D. Y.; Seki, K.; Ann. N.Y. Acad. Sci. 1985, 446, 237. 4. Thomas, J. L.; Tirrell, D. A. Acc. Chem. Res. 1992, 25, 336. 5. Schild, H. G. Prog. Polym. Sci. 1992, 17, 163.

6. Ringsdorf, H.; Venzmer, J.; Winnik, F. M . Angew. Chem. , Int. Ed. Engl. 1991,30,315. 7. Kono, K.; Hayashi, H.; Takagishi, J. Controlled Release 1994, 30, 69. 8. Hayashi, H.; Kono, K.; Takagishi, T. Biochim. Biophys. Acta 1996, 1280, 127. 9. Kim, J. C.; Bae, S. K.; Kim, J. D. J. Biochem. 1997, 121, 15.

10. Meyer, O.; Papahadjopoulos, D.; Leroux, J. C. FEBS Lett. 1997, 42, 61. 11. Heskins, M ; Guillet, J. E. J. Macromol. Sci. A2 1968, 1441-1455. 12. Binkert, Th.; Oberreich, J.; Meeves, M . ; Nyffenegger, R.; Ricka, J. Macromolecules 1991, 24, 5806-5810. 13. Schild, H. G. Prog. Polym. Sci. 1992, 17, 163-249. 14. Thomas, J. L.; Borden, Κ. Α.; Tirrell, D. A . Macromolecules 1996, 29, 2570. 15. Feil, H.; Bae, y. H.; Feijen, J.; Kim, S. W. Macromolecules, 1993, 26, 2496. 16. Hayashi, H.; Kono, K.; Takagoshi, T. Bioconugate Chem. 1999, 10, 412. 17. Kono, K.; Nakai, R.; Morimoto, K . Takashita, Y.; Biochim. Biophys. Acta 1999, 1416, 234. 18. Spafford, M . ; Polozova, Α.; Winnik, F.M.Macromolecules 1998, 31, 7099. 19. Berndt, P.; Fields, G. B.; Tirrell, M . J. Am. Chem. Soc. 1995, 117, 9515. 20. Principi, T.; C. C. E. Goh; C. W. R. Liu, Winnik, F. M . Macromolecules (in Press, 2000). 21. Ringsdorf, H.; Venzmer, J. ; Winnik, F. M . Macromolecules 1991, 24, 1678. 22. Allen, T,. N . ; Cleland, L. G.; Biochim. Biophys. Acta 1980, 597, 418.

23. Birks, J. B. Photophysics of Aromatic Molecules Wiley Interscience : London, 1979; chapter 7. 24. Winnik, F. M . Chem. Rev. 1993, 93, 587. 25. Winnik, F. M . ; Adranov, Α.; Kitano, H. Can. J. Chem. 1995, 73, 2030. 26. Polozova, Α.; Winnik. F. M . Langmuir 1999, 15, 4222.


Stimuli-Responsive Water Soluble and Amphiphilic Polymers

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