Tuning Wet Adhesion of Weak Polyelectrolyte Multilayers - ACS

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Article

Tuning Wet Adhesion of Weak Polyelectrolyte Multilayers Chao Li, Yuanqing Gu, and Nicole S. Zacharia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18910 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

2

Tuning Wet Adhesion of Weak Polyelectrolyte Multilayers.

3

Chao Li,1,± Yuanqing Gu,1 Nicole S. Zacharia1,*

4

ORCiD: Chao Li 0000-0003-3153-9079; Yuanqing Gu 0000-0001-5943-8523

5 6 7

1

Department of Polymer Engineering, University of Akron, Akron, OH 44325, USA current address: Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210, USA

8

*to whom correspondence should be addressed: [email protected]

9

KEYWORDS: Polyelectrolytes; Layer-by-layer Assembly; Salt; Peel Test; Wet Adhesive

1

,±

10

ABSTRACT. Weak polyelectrolyte multilayers assembled by the layer-by-layer method are

11

known to become tacky upon contact with water and behaves as a viscoelastic fluid, but this wet

12

adhesive property and how it can be modified by external stimuli has not yet been fully explored.

13

We present here a study on the wet adhesive performance of polyelectrolyte multilayers

14

consisting of branched poly(ethylene imine) and poly(acrylic acid) under controlled conditions

15

(e.g., pH, type of salt, ionic strength) using a 90º peel test. The multilayers demonstrate stick-slip

16

behavior and fail cohesively in nearly all cases. The peel force is highest at neutral pH, and it

17

decreases in both acidic/basic environments due to inhibited polyelectrolyte mobility. The

18

addition of salts with various metal ions generally reduces peel force, and this effect tracks with

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 46

1

ionic strength. When transition metals ions are used, their ability to form coordination bonds

2

increases the peel force, with two exceptions (Cu2+ and Zn2+). With a transition metal ion such as

3

Fe3+, the peel force first increases as a function of concentration and then eventually decreases.

4

The peel force increases proportionally to peel rate. The films are also characterized via zeta

5

potential (when assembled onto colloidal particles) and shear rheometry. This work provides

6

insight into both the wet adhesive properties of polyelectrolyte multilayers and also to the

7

interactions between polyelectrolyte multilayers and metal ions.

8 9

INTRODUCTION

10 11

Synthesis of effective synthetic polymeric wet adhesives has been a challenging problem, due

12

at least in part to the fact that the polymers will often interact more strongly with water than the

13

desired substrate.1 While there are many potential biomedical applications2,3 for these materials,

14

such as surgical adhesives, current commercial wet adhesives do have limitations. Surgical

15

adhesives based on natural materials such as fibrin or collagen are difficult to produce and

16

expensive, while synthetic adhesives based on cyanoacrylates or urethanes still suffer from low

17

wet adhesion or the creation of inflammation in the body.4 Hydrogels often exhibit limited

18

adhesive properties.4 These materials should be able to set under wet conditions and materials

19

with different mechanical properties based on applications are desirable. For example, a soft

20

adhesive that is modulus matched to tissue or the desired substrate may have advantages in


2

Page 3 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

resisting failure of the joined interface in comparison to stronger adhesives.5 Self-healing

2

adhesives could be useful by allowing for repositioning, and switchability of the degree of

3

adhesion that could allow for selective adhesion. A number of bioinspired strategies have been

4

proposed, most notably taking inspiration from mussel adhesive or gecko feet,6,7,8 but these

5

materials often require complex syntheses or fabrication. Imitating mussel adhesion specifically

6

has led to a great deal of work on polyelectrolyte coacervation as the basis of wet adhesion.

7

Presented here is a wet adhesive based on the interactions between commercially available,

8

synthetic polyelectrolytes, whose properties can be tuned based on pH and exposure to different

9

ions, even to changing from adhesive to non-adhesive repeatedly. This system has already been

10

demonstrated to have self-healing properties and can be reused by rewetting after being dried.9

11

Polyelectrolyte (PE) complexes and multilayers are materials formed by the complexation of

12

oppositely charged PEs into films, coatings, dispersible particles, or even as bulk material. The

13

layer-by-layer (LbL) technique, based on sequential exposure of a substrate to polycation and

14

polyanion solutions, is a widely studied method to form polyelectrolyte multilayers (PEMs).10

15

The conceptually simple LbL process has various advantages over other thin film forming

16

methods such as water-based processing, controllable thickness, ability to incorporate multiple

17

functionalities, and substrate determined morphology.11,12 Materials based on PE complexation

18

combine the properties of both components, and have been suggested for use as sensors,13

19

actuators,9,14 biomedical devices,15 self-healable electrodes,16 energy storage devices,17 and

20

surfaces with special wettability.18 These materials are generally considered to be well suited to


3


Page 4 of 46

1

use in aqueous environments, where they are well hydrated although more dense than typical

2

hydrogels, and as such are often considered for biomedical applications. One property of some

3

PE complexes and multilayers is their ability to function as a wet adhesive material,19,20,21,22

4

however, much is still unknown about PEM wet adhesives and how to control their properties.

5

One example of a wet adhesive PEM material is the weak PE branched poly(ethylene imine)

6

and poly(acrylic acid) (BPEI/PAA) multilayer.9 Weak PEs are those containing labile functional

7

groups that are environmentally sensitive, such as carboxylic acid or amine groups. Water is an

8

extremely efficient plasticizer for PEMs,23 causing a rheological transition from a rigid, glassy

9

film to a viscous, liquid-like material.9,14 Wet BPEI/PAA multilayers can serve as an adhesive

10

that can be affixed to various types of substrates such as sheets of polystyrene, glass slides,

11

paper, or aluminum foil regardless of surface charge or hydrophobicity. This therefore shows the

12

possibility for BPEI/PAA based materials to act as a new surface sizing and coating agent,

13

instead of commonly used starch19 or proteins24, or to be used to join pieces of paper to enhance

14

the strength of wet papers. Moreover, wet BPEI/PAA multilayers can adapt to surfaces with

15

different topography attributing to the mobility and diffusivity of polyelectrolyte chains, which

16

addresses the issue of poor adhesion between adhesive and rough surface25 and may have

17

potential to be applied in dental treatments.26 When using weak PEs the interpolyelectrolyte

18

forces are readily modulated in response to surrounding conditions such as pH,27 presence of

19

ions,28 and ionic strength.29 This can be used for controlling the adhesion of a BPEI/PAA film,

20

which may be useful for the development of tunable, smart wet adhesives.


4

Page 5 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

In this work, the wet adhesive performance of BPEI/PAA multilayers was investigated by

2

using different types of aqueous solutions. The interactions of different metal ions with PEs can

3

dramatically influence physical properties of the PEM, and although there are studies on the

4

assembly of PEMs with metal ions,30,31,32,33 there is still a great deal to be learned. A number of

5

factors complicate the interactions of various metal ions and PEs, including coordination bond

6

formation, ionic radius, hydration shell, and hydrophobicity of the ion. Our group has previously

7

demonstrated that copper can be used to harden BPEI/PAA PEMs through metal ligand

8

interactions,33,34 and that property was used in this work. Free-standing BPEI/PAA multilayer

9

films were fabricated by the LbL technique, and their wet adhesive behavior was assessed by a

10

90° peel test using a BPEI/PAA film sandwiched between two pieces of aluminum foil and

11

wetted by solutions with various pH and different types of metal salts. The film could repeatedly

12

be modulated from low to high adhesive states by sequential exposure to different solutions. It

13

was demonstrated that the wet multilayers were able to adhere to liver tissue, holding together

14

pieces and closing cuts, showing potential application to wound healing. Also, the wet adhesive

15

property and ability to adhere to tissues can make BPEI/PAA multilayers useful in tissue

16

engineering5,35 and bioadhesive delivery systems.36,37 Furthermore, the wet adhesive property

17

applied to both multilayers and complex coacervates of the PEs, which were even more easily

18

prepared by simply mixing solutions of the two PEs and collecting the precipitate.

19 20

EXPERIMENTAL SECTION


5


Page 6 of 46

1

Materials: Branched polyethylenimine (BPEI, Mw = 25,000 g/mol), hydrochloric acid, sodium

2

hydroxide, lanthanum(III) chloride heptahydrate, iron(III) chloride hexahydrate, silver nitrate,

3

magnesium chloride, zinc chloride, sodium chloride, potassium nitrate, potassium chloride, 2,2-

4

bipyridyl, copper(II) nitrate hemi(pentahydrate), iron(II) chloride tetrahydrate and 1,10-

5

phenanthroline were purchased from Sigma-Aldrich. Poly(acrylic acid) (PAA, MW = 50,000

6

g/mol, 25 wt% aqueous solution), was purchased from Polysciences. DI water with 18.2 MΩ cm

7

resistivity from a Milli-Q filtration system (Millipore, Bedford, MA, USA) was used for all

8

experiments. All materials were used as received without further purification.

9

Piranha solution was prepared by mixing 98% sulfuric acid with 30% hydrogen peroxide at a

10

volume ratio of 7/3. BPEI aqueous solution with the concentration of 80 mM (respect to amine

11

groups) and PAA aqueous solution with the concentration of 40 mM (respect to carboxylic acid

12

groups) were prepared by stirring overnight. Then, 1 M hydrochloric acid was used to adjust the

13

pH of BPEI solution to 9.5 and 1 M sodium hydroxide was used to adjust the pH of PAA

14

solution to 4.5. Both BPEI solution and PAA solution were ﬁltered prior to use.

15

Materials Fabrication: Prior to fabricating BPEI/PAA multilayers coated latex spheres, latex

16

spheres were prepared by using seeded semi-continuous polymerization with methyl

17

methacrylate (MMA), butyl acrylate (BA), and 2-hydroxyethyl methacrylate (HEMA) as

18

monomers applying Optimax (Mettler Toledo) working station as described in previous

19

work.38,39 Briefly, seed latex beads were first synthesized through batch emulsion


6

Page 7 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

polymerization. After dissolution of 4.82 g sodium dodecyl sulfate (SDS), 0.81 g sodium

2

bicarbonate, 112.60 g MMA and 48.10 g BA (molar ratio MMA:BA = 3:1) in 300.00 g DI water

3

at 60 °C under nitrogen protection, polymerization was initiated by adding ammonium persulfate

4

aqueous solution (0.81 g in 10.00 g DI water) and heated to approximately 80 °C. As addition of

5

ammonium persulfate is a heat releasing process, temperature should be raised carefully to avoid

6

overheating. HEMA-rich latex beads with 60 wt.% HEMA were then synthesized using the as-

7

prepared seeds according to previous work.38,39 Seed dispersion was prepared by mixing 13.14 g

8

seeds and 0.12 g sodium bicarbonate in 155.00 g DI water at 80 °C for 30 min. Monomer pre-

9

emulsion for the shell was obtained by adding 20.18 g MMA, 25.82 g BA, and 69.00 g HEMA

10

into 115 mL aqueous mixture of 0.46 g sodium bicarbonate, 0.80 g SDS and 0.58 g ammonium

11

persulfate and stirred for 1 h. The monomer pre-emulsion was slowly fed into the reactor using

12

peristaltic pump in 3.7 h. After completing feeding, the reactor temperature was raised to 90 °C

13

for at least 1.5 h to ensure maximum conversion. All the process was proceeded under nitrogen

14

protection.

15

Film fabrication: The BPEI/PAA multilayer films were fabricated by LbL technique with a Zeiss

16

HMS series programmable MICROM DS 50 slide stainer (Mercer Scientific Inc, USA) at room

17

temperature.9 Clean polystyrene substrates were exposed to BPEI solution for 10 min and then

18

were rinsed in three isolated DI water baths for 1 min each bath. Subsequently, they were

19

immersed in PAA solution for 10 min and rinsed in three DI water baths for 1 min each. One


7


Page 8 of 46

1

BPEI/PAA bilayer was thus formed on polystyrene substrates. This BPEI/PAA deposition cycle

2

was repeated for 50 times to form the desired multilayer films (denoted as (BPEI/PAA)50). After

3

drying these films, they were peeled off the polystyrene substrates.

4

BPEI/PAA multilayers were coated on latex spheres by centrifugation assisted LbL process.

5

Briefly, HEMA-rich latex spheres were first purified by dispersing in DI water, centrifuging at

6

6,500 rpm for 15 min, and removing the supernatant prior to use. The collected latex spheres

7

were redispersed in 30 mL BPEI solution by ultrasonicating for 5 min and vortexing for 15 min.

8

The BPEI modified latex spheres were collected by centrifugation at 6,500 rpm for 15 min. The

9

collected sample was then washed through repeating redispersion in 30 mL DI water and

10

centrifugation at 6,500 rpm for 15 min for 3 times. Subsequently, the sample was modified with

11

PAA layer through identical modification and washing process with PAA solution. After

12

repeating n cycles of BPEI/PAA modification process, n BPEI/PAA bilayers were deposited on

13

each latex sphere surface. BPEI/PAA coacervate was prepared by mixing BPEI solution (pH 9.5,

14

120 mM with respect to amine group) and PAA solution (pH 4.5, 120 mM with respect to

15

carboxylic acid group) at volume ratio 1:1 and stirring overnight.

16

Characterization: The peel force of (BPEI/PAA)50 films were measured using a 90° peel-off test

17

by using a Texture Analyzer (Texture Technologies, MA, USA). Prior to measurement, the

18

(BPEI/PAA)50 film was sandwiched between two smooth aluminum foil substrates (6.75 cm2),

19

wetted with 200 µL selected solution, and equilibrated for 1 h. The excess solution was carefully


8

Page 9 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

removed using a piece of Kimwipe. The top substrate of the sample was fixed to the sample

2

clamp, and the bottom substrate was fixed to a moving stage (shown in Figure S1 (a) and (b)).

3

The peel force against displacement distance at a determined peel rate was recorded by the

4

Texture Exponent 32 software. Stick-slip behavior was observed during the peel test and a

5

maximum force and minimum force could be seen at each stick-slip cycle. Here, the peel force of

6

wet (BPEI/PAA)50 film was calculated in terms of the mean value of maximum forces of several

7

cycles. The energy to separate the two substrates during the peel test was calculated by

8

integrating the peel force-distance curve.

9

The effect of metal ion type, ion concentration and pH on adhesive force was studied by

10

applying different solutions at a constant peel rate of 0.1 mm/s. For metal ion type study, 5mM

11

metal ion solution of Na+, K+, Mg2+, Ca2+, Ba2+, La3+, Zn2+, Ag+, Cu2+, Fe2+, and Fe3+ were used,

12

respectively. For investigation of ion concentration effect, Na+, Cu2+, and Fe3+ solutions with

13

concentration of 1 mM, 5 mM, 10 mM, 20 mM, 40 mM or 100mM were respectively applied.

14

pH effect was tested with DI water adjusted to pH 3, 5, 7 or 9. Solution pH was adjusted with

15

either HCl or NaOH but not both in order to avoid salt formation. For the reusability test, the

16

BPEI/PAA sides of the peeled aluminum foils were wetted with DI water and pressed face-to-

17

face for 1 h. After removal of excess water, the stuck sample was again subjected to 90° peel

18

test.


9


Page 10 of 46

1

The surface charge of (BPEI/PAA)4 coated latex spheres in various environment was measured

2

by re-dispersing in related solutions and analyzed by Zeta-potential Analyzer (Brookhaven

3

Instrument, USA). The particles were dispersed in DI water adjusted to different pH values (pH

4

= 3, 5, 7, 9), and 5 mM metal ion solutions (Na+, K+, Mg2+, La3+, Zn2+, Ag+, Cu2+, Fe2+, Fe3+, pH

5

2.9), respectively.

6

Fourier transform infrared spectroscopy (FTIR) spectra were acquired by an Alpha-P FTIR

7

spectrometer (Bruker Optics, Billerica, MA, USA) with attenuated total reflectance (ATR) mode.

8

The samples were prepared by deposition of (BPEI/PAA)7 on a silicon wafer and analyzed after

9

immersing in DI water with different pH (pH=3, 5, 7, 9) for 1 h and drying overnight.

10

The interaction of copper ions bound to various ligands and PEMs was studied by using

11

solutions containing different molar ratios of copper ion and ligands. Specifically, 2,2-bipyridyl

12

and 1,10-phenanthroline were used as ligands and stirred with copper ions overnight. Molar

13

ratios of ligand to copper ions of 2:1, 1:1, 1:2, and 1:4 were used to test films at a peel rate of

14

0.1mm/s. In all of these ligand-containing solutions the concentration of copper ions was 5mM

15

and the pH of all solutions was adjusted to 2.9.

16

The swelling behavior of (BPEI/PAA)6 film was monitored in situ by a variable angle

17

spectroscopic ellipsometer (VASE, M-2000 UV−visible−NIR [240−1700 nm] J. A. Woollam

18

Co., Inc., Lincoln, NE, USA) equipped with a temperature controlled liquid cell. The cell

19

geometry dictated the angle of incidence to be 75°, and wavelength range was limited from 300


10

Page 11 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

nm to 1050 nm for the recursive fits because of the absorption of solvents in the ultraviolet and

2

near-infrared light region. Standard window correction protocols with a silicon wafer with

3

thermal oxide (25.0 nm) as the reference were used prior to each measurement.40 The

4

ellipsometry data were fitted by applying a four-layer model consisting of a silicon substrate

5

layer, a fixed 1.0 nm Si-SiO2 interface layer, a 0.8 nm thermal oxide layer, and an effective

6

medium approximation layer (with thickness, A and B free fit parameters) in Complete EASE (J.

7

A. Woollam, Co., Inc., Lincoln, NE, USA).41 The PEM film was measured in situ from the dry

8

state. After 1 min, a related solution was carefully charged to the cell by using a syringe to avoid

9

possible air bubbles, and ambient index for the related solution model was meanwhile applied to

10

fit the ellipsometric data. All measurements were carried out at 25 °C.

11

Oscillatory shear measurements were proceeded on a strain-controlled Advanced Rheometric

12

Expansion System G2 rheometer (TA Instruments, USA) equipped with an 8 mm parallel steel

13

plate and a bath trap at a gap of ca. 0.030 mm at 25 °C.9 BPEI/PAA multilayers were placed in

14

the bath trap, containing DI water with varied pH or metal ion solutions. Angular frequency

15

sweeps were acquired from 0.1 rad s–1 to 100.0 rad s–1 at strain amplitude of 0.50%, which

16

locates in the stable strain% region.

17

To adhere the PEM to pieces of pork liver, the pork liver was first cut into equal size pieces.

18

BPEI/PAA film was placed on one pork liver surface, and another pork liver piece was stuck

19

onto the BPEI/PAA surface for 10 min. To close the cut notch in the tissue, BPEI/PAA film was


11


Page 12 of 46

1

wetted and then stuck onto one cut in the liver while the other notch in the same piece of liver

2

was left uncovered for comparison.

3 4

RESULTS AND DISCUSSION

5

Free-standing BPEI/PAA multilayers were prepared through the LbL method as illustrated in

6

Scheme 1. By alternatively dipping a polystyrene substrate in cationic BPEI and anionic PAA

7

solutions with intermediate DI water rinsing steps, BPEI/PAA multilayers with a thickness of ~

8

30 um9 were assembled on a polystyrene substrate, and the resulting film was readily peeled off

9

from the substrate due to the large thickness of BPEI/PAA multilayers and low interaction forces

10

between

hydrophilic

PEs

and

the

hydrophobic

substrate.

When

wet

with

11 12

Scheme 1. Schematic illustration of the preparation process of the free-standing BPEI/PAA multilayers

13

and performance as adhesive upon wetting with water. The film is assembled onto a hydrophobic

14

polystyrene substrate and then peeled off. For the peel test the film is sandwiched between two substrates

15

(polystyrene shown here) and wet with an aqueous solution.

16


12

Page 13 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

various aqueous solutions, the swollen, plasticized film becomes a tacky, viscoelastic material,21

2

allowing the film to behave as an adhesive that can be affixed to various substrates. As a

3

demonstration of these wet adhesive properties, the wet PEM was used to “glue” together two

4

pieces of pork liver, similar to demonstrations by several other groups.42,43 Biological tissues

5

such as liver have complex, charged surfaces, and charge-charge interactions between adhesive

6

and liver tissue allow for the formation of a bond.44 Figure 1 (a) shows one piece of liver with

7

two notches. The notches are formed by cutting and then enlarged by the gravity of the liver

8

hanging from a clip. Figure 1 (a) presents the initial state of the pork liver before any treatment.

9

Figure 1(b) demonstrates the notch on the left being covered over by a wet BPEI/PAA PEM.

10

The adhesive force provided by the PEM is sufficient to draw the sides of the notch together,

11

closing the hole. The right notch remains untreated. Figure 1(c) shows two pieces of liver being

12

glued together end to end; the PEM is strong enough to hold the weight of the bottom piece of

13

liver tissue. Biological tissues such as liver have complex, charged surfaces, and the charged

14

nature of the PEM allows for


13


Page 14 of 46

1 2

Figure 1. (a) Pictures of freshly cut pork liver with two notches cut into the tissue. (b) a wet BPEI/PAA

3

film was used to cover the left notch, allowing the hole to be closed while the right notch was left

4

untreated. (c) Two pieces of pork liver are stuck to one another end to end using a wet BPEI/PAA film.

5

The PEM is strong enough to hold the weight of the bottom piece of liver.

6 7

interactions between the PEM wet adhesive material and the liver tissue, meaning the PEM wet

8

adhesive is a simple and effective way to assemble tissues. Supplementary videos S1 and S2

9

show pieces of freshly cut pork liver suspended from a clamp. In video S1, a second piece of

10

liver is adhered with the wet multilayer and it can be seen that the adhesion is strong enough to

11

withstand the force of gravity, whereas in video S2 shows two pieces of liver being pulled apart

12

after having been affixed together with a wet multilayer. This shows the resistance provided by

13

the adhesive force of the multilayer. The fact that weak polyelectrolytes are components of this

14

wet adhesive system means that the charge of the adhesive will change in response to proximity


14

Page 15 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

of the charged liver surface. Another point of note is that application of the PEM does not

2

discolor the piece of liver tissue at room temperature over an hour. It has been shown that the

3

application of some types of charged hydrogels to the surface of such a piece of tissue will

4

discolor it, implying denaturing of the surface.43 This does not happen with BPEI/PAA materials,

5

indicating a potential for BPEI/PAA materials in the biomedical applications

6

The effect of wetting the BPEI/PAA film with different aqueous solutions was investigated

7

using a peel test that measures the force required to pull apart two substrates affixed with the

8

adhesive in question.44,24 A 90° peel test, which measures the force required to pull apart two

9

substrates affixed with the adhesive and is shown in Figure S1,43,24 was used to probe the

10

adhesive strength as a function of variation in pH and the ionic content of the solution used to

11

wet the PEM. Wet BPEI/PAA films generally demonstrate stick-slip behavior during the peel

12

test, examples of which are shown in Figure S2, and failure typically occurs within the film with

13

the film splitting internally (cohesive failure)24 instead of detaching from the film/substrate

14

interface. Scheme 1 shows the fingering of the wet adhesive which typically occurred during the

15

peel test. Because failure occurs cohesively in almost all cases (except for exposure to Cu2+

16

which will be discussed in more detail below) the peel force can also be taken as a measure of

17

strength of the wet adhesive material itself.

18

To measure variation with pH, DI water adjusted to pH 3.0 to 9.0 was used. More extreme

19

values of pH, either basic or acidic, were avoided for fear of dissolving the films.45,46 When wet


15


Page 16 of 46

1

with neutral water (pH 7.0), the film shows a maximum peel force of approximately 0.13 N

2

(Figure 2(a)). The peel force decreases to ~0.10 N at pH 3.0 and ~0.087 N at pH 9.0. The

3

calculated energies to separate the two adhered substrates are 0.71 mJ, 0.98 mJ, and 0.67 mJ at

4

pH 3.0, 7.0, and 9.0 respectively. Therefore, the adhesive property of the film declines by either

5

reducing or increasing pH of aqueous solution used to wet the film. This is similar to the pH

6

dependent wet adhesion seen in polymethacrylic acid gels, where the ionization of methacrylic

7

acid at various pH values plays an important role in controlling adhesive properties by changing

8

the charge density and network within gels,47 and PAH based adhesives have similar adhesive

9

property as a result of the degree of ionization changing for the weak polyelectrolyte components

10

(i.e. PAH) with different pH to affect the crosslink within PAH adhesives.48 In addition, the peel

11

force and the energy of BPEI/PAA multilayers at their optimal condition show an adhesive

12

strength similar in order of magnitude with other reported adhesives, 44,24,49 although a direct and

13

more precise comparison of measurements made from the different methods used is difficult.

14

Building BPEI/PAA multilayers is facile and the process only involves the electrostatic

15

interaction instead of the covalent bonding commonly seen in other adhesives.6,20,49 Compared

16

with other adhesives whose properties is minimally tunable, the adhesive property of BPEI/PAA

17

multilayers can be easily triggered by adding water and controlled by many factors (e.g. pH,

18

ionic strength and the type of metal ions), providing a desired adhesive strength and being easily

19

eliminated.


16

Page 17 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 2. (a) Peel force and energy as a function of pH at peel rate 0.1 mm/s. There is a maximum in both

3

at pH 7 (b) FTIR spectra of (BPEI/PAA)6 films treated by different pH water. It can be seen that

4

carboxylic acid ionization changes only a small amount over this pH range.

5

6

Changes seen here in the peel force and energy at varied pH are due to the pH dependent

7

ionization of both BPEI and PAA. The pKa of PAA is ca. 6.5,50 while BPEI possesses three pKa

8

values (4.5, 6.7, and 11.6) due to the presence of primary, secondary, and tertiary amine

9

groups.34 In the as-prepared dry film, BPEI and PAA are both partially charged with a moderate

10

ionic crosslink density that allows the film to swell and for a good degree of chain mobility. At

11

lower pH, the BPEI amine groups are more charged and the PAA carboxylic acid groups become

12

protonated and therefore neutral. At higher pH, the reverse is true: the BPEI is less charged and

13

the PAA is more charged. As a result, the total number of ionic cross-link points are reduced and

14

adhesive force is weakened. To measure the surface charge of BPEI/PAA under different pH


17


Page 18 of 46

1

conditions, BPEI/PAA multilayers were assembled on negatively charged HEMA-rich latex

2

spheres that served as the substrate, by the sequential alternation of negative and positive surface

3

charges (Figure S3(a)). The zeta potential of the BPEI/PAA coated latex spheres is –25 mV in

4

neutral water, and it becomes more negative after incubated in pH 9.0 water (Figure S3(b)). In

5

lower pH water, the zeta potential in turn appears as positive, and the lower the pH is, the more

6

positive the zeta potential. This indicates that the PAA and BPEI chains in BPEI/PAA

7

multilayers are more charged in basic and acidic environments, compared to neutral.

8

The ionization degree of carboxyl groups in the as-prepared BPEI/PAA film under different

9

pH environment was calculated based on asymmetric stretching band of COO- at ν = 1565–1542

10

cm-1 and C=O stretching of COOH at ν ~ 1710 cm-1 in the related FTIR spectra (Figure 2(b)).50

11

As these two bands from the same sample are assumed to have the same extinction coefficients,

12

the degree of ionization of PAA at a determined pH was calculated as following equation:50

13

(1)

14

where A indicates absorption band intensity. By increasing pH value, the degree of ionization of

15

PAA increases from 56.5% at pH 3.0 to 68.3% at pH 7.0, but it remains essentially unchanged at

16

higher pH condition. Additionally, the degree of ionization of PAA is more than 50% at pH 3.0,

17

showing the pKa of PAA is shifted to a lower value. A similar result has been reported by

18

Rubner and his co-workers and explained by the local environment changes in PEM compared

19

with that in the solution.50 Although one might expect a larger change in carboxylic acid


18

Page 19 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

ionization over the range of pH 3.0–9.0, the PAA charge density is strongly influenced by its

2

surrounding environment such as the PEM. It has been shown that over this pH range the

3

carboxylic acid ionization for a polyelectrolyte multilayer containing PAA is relatively

4

constant,50 showing that the PEM environment acts as a buffer over this range. Accompanying

5

the change of PAA charge density, BPEI becomes more charged at low pH and less charged at

6

high pH,51 but it is difficult to quantify the degree of ionization of amine groups in BPEI from

7

the FTIR spectra because the adsorption bands of the various amine groups overlap with those of

8

PAA and bands associated with the polymer backbones.52

9

10


19


Page 20 of 46

1

Figure 3. Peel force and energy as a function of 5 mM different metal ions at peel rate 0.1 mm/s. The

2

blue line represents the peel force when using DI water and is meant as a guide to the eye to easily

3

identify which ions reduce the force and which ions increase the force.

4

Various metal salts were added to the solution used to wet the multilayer, and the effect of

5

these different metal ions on the peel force and energy is summarized in Figure 3. To avoid

6

hydroxide formation the pH for all 5 mM metal salt solutions was fixed at 2.9, which is the same

7

as that of the as prepared FeCl2 solution. Compared with DI water also adjusted to pH 2.9, the

8

peel force and energy for the wet adhesive BPEI/PAA films decrease by adding Na+, K+, Mg2+,

9

Ca2+, Ba2+, Zn2+ or Cu2+, while these values increase upon exposure to Ag+, La3+, Fe3+ and Fe2+

10

ions. This observation agrees with reports of increased adhesive forces for biopolymer

11

containing PEMs in the presence of Fe3+.20 Although different anions are reported to impact on

12

PEM properties differently,53 the same peel force and energy were measured within experimental

13

error for the samples applied with potassium and copper chloride and nitrate salts (Figure S4).

14

For alkali and alkaline earth metals (Na+, K+, Mg2+, Ca2+, and Ba2+), the trend in peel force

15

matches with the zeta potential of BPEI/PAA PEMS exposed to the ion solutions (Figure S3c,

16

SI), while it is not well-correlated with hydration radius. These alkaline metal ions primarily

17

interact with PAA through electrostatics in the film, which partially weakens or dissolves ionic

18

cross-links through charge screening, increasing chain mobility. While monovalent ions interact

19

with only a single anionic carboxylic acid group, divalent ions are able to form bridges across

20

two carboxylic acid groups which in turn restricts the chain mobility.54 It can be said that


20

Page 21 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

divalent alkaline metal ions reduce the peel force to a greater degree than the monovalent ions

2

do.

3

The situation for transition metal ions is more complex. As well as interacting through

4

electrostatics, these ions can form coordination bonds with the PEs having a covalent nature.

5

Zn2+ behaves similarly to the alkali ions (reducing adhesion), while the other transition metals -

6

with the exception of copper - increase the strength of the wet adhesion. In the cases where the

7

peel force is increased, it can be inferred that the formation of coordination bonds increases the

8

internal forces within the PEM, increasing the amount of force required to pull the PEM apart,

9

further implied by the deswelling behavior of PEM caused by the introduction of transition metal

10

ions capable of bridging polyelectrolyte chains (Figure S5b). For most cases described (both

11

changing pH or metal ions), the wet BPEI/PAA PEMs fail within the film during the peel test

12

(cohesive fracture),24 but when exposed to copper ions failure occurs at the interface of the

13

aluminum foil substrate and the film (adhesive fracture). Copper exposed films have the lowest

14

peel force, at roughly half the peel force of water treated films, and become much stiffer. Also,

15

there is no fingering of the PEM wet with copper containing solution when the substrates are

16

pulled apart. The change in mechanical properties upon exposure to copper ions is different even

17

than for the other metal ions that reduce the peel force. It is only in the copper case that the film

18

becomes stiff. This indicates that the internal forces in the Cu2+ treated film become stronger

19

than the adhesive force between the film and the foil substrate, in contrast to the PEMs treated


21


Page 22 of 46

1

with other metal ions or pH values. It is known that Cu2+ ions have an extremely high affinity

2

with both amine and carboxylic acid groups,31,53 and can coordinate with the amine and

3

carboxylic acid groups. The cross-links restrict the relaxation and mobility of PE chains,

4

stiffening the film, making it not a good adhesive. At this concentration, only exposure to Cu2+

5

creates this phenomenon.

6

7 8

Figure 4. Angular frequency sweep of storage modulus and loss modulus of (BPEI/PAA)50 films soaked

9

in different pH water (a), aqueous solutions containing alkali metal ions (b) and transition metal ions (c).

10

Closed symbols are the storage modulus and open symbols are the loss modulus. (d) Angular frequency

11

sweep of free-standing (BPEI/PAA)50 film in pH 2.9, 5 mM ions. (e) Complex viscosity of films wet with

12

various metal ions at angular frequency 0.5 rad/s at 25 ºC as a function of metal ion.

13


22

Page 23 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

In response to immersion in water, the BPEI/PAA film quickly swells. The uptaken water

2

serves as plasticizer and facilitate relaxation of PE chains, which turns the initially glassy solid

3

into a viscoelastic gel.9 The rheological behavior of free-standing BPEI/PAA films in pH

4

controlled water or metal ion aqueous solution (pH 2.9, 5 mM) is presented in Figure 4. Upon

5

exposure to different pH water, the BPEI/PAA film exhibits loss modulus (G”) generally higher

6

than storage modulus (G’) over the tested angular frequency region (0.1~100.0 rad/s), although

7

at pH 5 this difference is small (Figure 4(a)). Both moduli are proportional to angular frequency

8

without reaching a plateau value, demonstrating liquid-like flowable nature of the wet

9

BPEI/PAA film. The film has the highest moduli (both G’ and G”) when exposed to pH 7.0,

10

lesser at pH 5.0, and the least at pH 3.0 and 9.0, which corresponds to the trend in adhesive

11

property. It can be concluded that the films with lower moduli have weaker internal forces and

12

fail more readily during the peel test.

13

Upon addition of alkali metal ions, a slight decrease is observed in both storage and loss

14

moduli compared with that in pH 2.9 water, but the loss modulus still dominates over the

15

monitored angular frequency region. While this result indicates the wet film flows, the film

16

becomes mechanically weak, and it collapses and deswells in these alkali metal ion solutions.

17

This can be seen by ellipsometry (Figure S5), which has been also reported in a previous study

18

where monovalent and divalent ions induce the contraction of PEM thickness.55 This

19

phenomenon originates from the charge screening effect by alkali metal ions, which compensate


23


Page 24 of 46

1

negatively charged carboxylic acid groups and partially break the initial ionic bonds between PE

2

chains. This is consistent with previously observed salt softening effect on PEMs,56 and the

3

adhesive property is also weakened as a result.

4

For transition metal ions, again multiple effects are observed. Storage and loss moduli of the

5

PEM drop when in Ag+ solution, remain similar or are even slightly higher compared to those in

6

water when exposed to Fe3+ and Fe2+ solutions, and increase significantly in Cu2+ solution. With

7

the presence of Cu2+, the storage modulus dominates from 0.1 rad/s to 100.0 rad/s, showing a

8

stiff, solid-like nature. This is a result of the additional bonding and the strength of that bonding

9

between both PEs and the copper ions. This transition to a rigid material (tan δ 0.19 at angular

10

frequency 0.5 rad/s) that occurs in the presence of Cu2+ explains the low adhesion of the PEM in

11

that environment. Relaxation of the PE chains in this case is highly restricted and failure in the

12

presence of Cu2+ occurs at the foil/PEM interface instead of within the PEM as with the other

13

cases. While silver treated films show lower moduli, the Ag+ treated films are lossier (tan δ 2.08

14

at 0.5 rad/s) than films wetted by water (tan δ 1.64 at 0.5 rad/s).

15

Figures 4(d) and (e) represent the magnitude of the complex viscosity of the PEM soaked in

16

different metal ion solutions over the entire frequency range and at 0.5 rad/s, respectively.

17

Although it is also not a perfect predictor of peel force, one can make several observations.

18

Complex viscosity contains both viscous and elastic components. The very large value of

19

complex viscosity for the Cu2+ treated films reflects the elastic, solid-like nature of that film. The


24

Page 25 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

significantly high complex viscosity shows that the Cu2+ treated film is no longer compliant,

2

resulting in early failure at the interface between the PEM and the substrates. With a couple of

3

exceptions, for exposure to the ions other than copper the more viscous the higher the peel force.

4

For example, Fe2+ and Fe3+ have the highest peel force values and the highest complex viscosity

5

values. Zinc ions result in the lowest peel force and the lowest complex viscosity. All of these

6

materials have a much lower complex viscosity value than the copper ion treated films. Barium

7

and calcium ions, however, do not follow the trend but overall complex viscosity has a better

8

correlation to peel force for the metal ion treated films than either loss or storage modulus do.

9

This shows that adhesiveness depends on not only the density of ionic cross-links but also PE

10

chain mobility, and rheological data gives some insight into the adhesive behavior of viscoelastic

11

films.


25


Page 26 of 46

1 2

Figure 5. Peel force and energy as function of salt concentration of pH 2.9 aqueous solution containing

3

(a) Na+, (b) Cu2+ and (c) Fe3+ at peel rate 0.1 mm/s.

4

5

Figure 5 presents the correlation between peel force as well as energy with the concentration

6

of alkali ion (Na+, figure 5(a)) and transition metal ions (Cu2+ figure 5(b), Fe3+ figure 5(c)). By

7

increasing the concentration of Cu2+, the peel force as well as energy of the film further

8

decreases, being decreased from 0.073 N and 0.56 mJ in 1 mM Cu2+ to 0.046 N and 0.32 mJ in

9

100 mM Cu2+. With an increased density of Cu2+ containing coordination bonds, the BPEI/PAA


26

Page 27 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

film becomes even stiffer resulting in even earlier failure during peel test. A similar decrease in

2

peel force of BPEI/PAA film is observed by increasing Na+ concentration in the applied aqueous

3

solution, but the Na+ treated films are always more adhesive than Cu2+ treated ones. Starting

4

from ca. 0.12 N in 1 mM Na+ solution, the peel force gradually decreases to 0.063 N in 100 mM

5

Na+ solution. The charge screening effect of NaCl has been widely studied and the high

6

concentration of salts will greatly decrease the ionic strength within PEM and even cause the

7

dissolution of PEM leading to the reduction in the adhesion strength of PEM.28,33,48 For Fe3+

8

treated films, the peel force and energy increase from 0.07 N and 0.54 mJ in 1 mM Fe3+ to 0.136

9

N and 1.02 mJ in 10 mM Fe3+, which is consistent with the idea that coordination bonds can

10

enhance internal forces within the PEM. A similar result has been earlier reported, where

11

complexing appropriate amounts of Fe3+ improves the adhesion property of PEI-based

12

multilayers.20 Building on the previous study, the concentration of Fe3+ continuously increases to

13

a high value and an interesting result has been observed in our work. By further increasing the

14

concentration of Fe3+, however, the peel force and energy decline to 0.054 N and 0.40 mJ in 100

15

mM Fe3+. At these higher concentrations, the Fe3+ treated films become stiff in a manner similar

16

to the Cu2+ treated films. The failure mode also changes in this case, with the film failing at the

17

substrate/PEM interface (as in the Cu2+ treated case) and no longer within the PEM. This shows

18

that these multivalent transition metal ions behave in a similar manner, but with a lower

19

concentration of Cu2+ being able to create this stiffness in the PEM. It can be inferred that Cu2+ is


27


Page 28 of 46

1

able to bind to the amine and carboxylic acid groups in a qualitatively different way, perhaps

2

forming less labile, stiffer, or stronger coordination bonds.

3

As the metal ion interactions are reversible,57 it is possible to control the cross-link density and

4

adhesive behavior by adding chelators. Small molecule ligands, 2,2-bipyridyl and 1,10-

5

phenanthroline, are added to the aqueous solutions of Cu2+ ions. These molecules are expected to

6

bind with two of the six available copper sites. When either chelator is added, both peel force

7

and energy increase monotonically with the addition of chelator molecules (Figure 6(a) and (b)).

8

When an excess of chelator molecules is added (chelator: Cu2+ ratio = 2:1), the peel force

9

becomes almost comparable to that measured by using DI water alone to wet the film. This same

10

result is observed at both pH 7.0 and 2.9. These data show that small molecule chelators can be

11

used to further tune the wet adhesive properties of PEMs. Additionally, the wet adhesive

12

property of a single BPEI/PAA film can be turned on and off by alternately using copper and

13

chelator solutions. Exposing a film to copper ions makes the film stiff and a poor adhesive,

14

which can be reversed by exposure to a solution containing

15


28

Page 29 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 6. The peel force and energy of BPEI/PAA multilayers as function of molar ratio of (a) 2,2-

3

bipyridyl/Cu2+ (b) and 1,10-phenanthroline/Cu2+ at a constant peel rate of 0.1 mm/s. The pH of

4

chelator/Cu2+ solution was fixed at 2.9, the concentration of Cu2+ was fixed at 5 mM. Image (c) shows a

5

film was wetted by DI water, then sequentially exposed to (d) copper ions (e) and chelator solution. One

6

can see the characteristic fingering of the adhesive material in part (e) but not in part (d) when the film

7

was exposed to copper ions. Part (f) again shows a sticky film that was wet with copper ion solution, then

8

the ability to toggle the adhesive property from high to low adhesion with exposure to (g) copper ions and

9

(h) iron ions once again.


29


Page 30 of 46

1

only one of the chelators mentioned above. One can cycle between on or high adhesion and off

2

or low adhesion states, as represented in Figures 6(c) – 6(h) showing the PEM sandwiched

3

between two pieces of aluminum foil and those pieces of foil then being pulled apart. When the

4

film is an effective adhesive, one can see the ‟fingering” between the two substrates, when

5

stiffened with Cu2+ there is no fingering. Figure 6(c) shows the sticky film wet with DI water,

6

figure 6(d) shows the film wet with copper ion solution and no fingering, and figure 6(e) shows

7

that the same film has been soaked in a 2,2-bipyridyl solution and again behaves as a wet

8

adhesive. Furthermore, adhesion can also be toggled between sticky, high adhesion and stiff, low

9

adhesion by sequential exposure to iron and copper ions. Figure 6(f) shows the sticky property

10

of the film when wetted with an iron ion solution, then 6(g) shows the film no longer being an

11

adhesive after exposure to copper ions, and 6(h) shows the film becoming sticky by once again

12

being exposed to iron ion solution. This shows that copper ions can displace iron ions that have

13

been bound within the film, but also that iron can again displace the copper bonds. This change

14

of environment toggles between low and high adhesion states as well as mechanical properties,

15

going from stiff to sticky and back. There are reports of using pH, light, and temperature to

16

control wet adhesion,45 but response to metal ion more unusual, with previous reports using

17

polymers with specific chelating moieties incorporated into them.58

18


30

Page 31 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 7. The peel force (left column) and energy (right column) of BPEI/PAA films as function of the

3

peel rate at two different pH conditions:(a) pH 7.0 and (b) pH 2.9.

4 5

The peel force and energy measured using wet BPEI/PAA films in pH 7.0 water at different

6

peel rates are summarized in Figure 7. At low peel rate (0.02 mm/s), the peel force and energy

7

appear to be 0.05 N and 0.29 mJ, respectively. Both of these parameters increase steadily upon

8

increasing the peel rate, reaching 0.44 N and 2.44 mJ at 1.00 mm/s, which is roughly about 10

9

times of those at 0.02 mm/s. This result agrees with previous observations, in which peel force

10

increases with accelerated peel rate.59,60 The overall peel force can be thought to involve two

11

factors: bending force and adhesive force.61 When the peel rate increases, the radius of curvature


31


1

of the aluminum foil substrate decreases, and the resulting larger bending moment of the

2

substrate contributes in increasing the overall force.

0.16

1.4

Peel Force Energy

0.14

1.2

0.12

1.0

0.10

0.8

0.08 0.6

0.06

0.4

0.04

0.2

0.02 0.00

3

Energy (mJ)

Peel Force (N)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 46

1

2

3

4

5

0.0

Peel Cycle

4

Figure 8. Peel force and energy of (BPEI/PAA)50 film through repeated peel test at 0.1 mm/s and DI

5

water treatment. By rewetting the film, it is possible for it to be reused as a wet adhesive. Reduction in

6

peel force of less than 10% is attributed to minor loss of material through the repetition of the experiment.

7

Once the aluminum foil substrates attached by BPEI/PAA multilayers are separated from each

8

other, the PEM covered sides are no longer adhesive. However, by simply re-wetting with DI

9

water they can be made to be sticky once again. As shown in Figure 8, the first peel force

10

required to separate the attached pieces aluminum foil is 0.106 N, and it remains similar in value

11

at 0.096 N to separate them a second time after rewetting the adhesive. After 4 times of

12

peel/recovering treatment, the peel force is kept as 0.084 N. A slight decrease in both peel force


32

Page 33 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

and energy is observed, which is most likely due to the loss of some of the wet adhesive material

2

through the repeated wetting, pulling, and re-wetting.

3

This recoverable wet adhesive nature originates from the water triggered migration of weak

4

PE chains.9 The water triggered sticky behavior of BPEI/PAA can be imagined for use as an

5

environmentally benign adhesive that is reusable. BPEI/PAA multilayers are classified as

6

“exponentially” growing multilayers, meaning that there is a good deal of chain mobility and

7

diffusion throughout the PEM, leading to a homogeneous structure throughout, somewhat

8

different than that created by strong PEs.62 This uniformity is similar within the BPEI/PAA

9

complex coacervate material, explaining why they basically have the same wet adhesive

10

properties.

11

CONCLUSIONS:

12

BPEI/PAA PEMs and complex coacervates are shown to be effective wet adhesives, for

13

example being able to stick together pieces of biological tissue. These materials have previously

14

been shown to have self-healing properties. It is demonstrated that although the films lose

15

stickiness when dried, they can be re-wetted and reused as an adhesive a number of times. The

16

adhesive nature can repeatedly be turned on and off from low to high by sequentially exposing

17

the BPEI/PAA material to solutions of copper ions and small molecule chelators or even copper

18

ion and then iron ion solutions. This switching also changes mechanical state, from a stiff film to

19

a soft, sticky material.


33


Page 34 of 46

1

The wet adhesive performance of BPEI/PAA PEMs wetted with aqueous solutions of different

2

pH as well as containing different metal ions is studied here using a 90° peel test. The films

3

demonstrate a stick-slip behavior and failure occurs within the adhesive and not at the interfaces

4

in almost every case. The peel force is highest when the film is wet with neutral pH water, and it

5

declines under either acidic or basic conditions. One can conclude that there is an optimal

6

amount of crosslink density and PE chain mobility that preserves chain mobility but also gives

7

high enough internal forces that then results in maximum wet adhesive force. Interactions with

8

various metal ions have a strong influence on the adhesiveness of the BPEI/PAA material, in

9

some cases creating a more adhesive material and in some cases reducing the adhesive nature.

10

Transition metal ions interact very differently with PEMs than alkali metals or alkaline earths.

11

Exposure to Cu2+ ions greatly stiffens the BPEI/PAA materials at concentrations much lower

12

than any other metal ion. Rheological measurements show that films that are stiff or have very

13

high complex viscosities are not good adhesives, and some amount of compliance/viscoelasticity

14

is required. However, rheology is not a perfect predictor for metal ion exposed films, showing

15

that electrostatics and other interactions complicate the physical properties of these films. Zeta

16

potential is a good (but also not perfect) predictor of peel force for the metal ion cases.

17

The currently developed BPEI/PAA adhesive is superior to previously reported adhesive

18

containing strong PEs,63 as the wet sticky BPEI/PAA is capable of adhering various substrate

19

regardless of the surface wettability and surface charge,9 while the application of the strong PEs


34

Page 35 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

based adhesives is limited to negatively charged hydrophilic surfaces.42 One can envision further

2

functionalization of these films by loading drugs into them, which is already well established

3

using polyelectrolyte multilayers.64 BPEI/PAA materials have great potential for practical

4

applications in such areas as wound healing, controlled drug delivery, and other biomedical uses.

5

ASSOCIATED CONTENT

6

Supporting Information. Zeta potentials as functions of BPEI/PAA bilayer number, pH of

7

aqueous solution, metal ion types, and in situ swelling ratio change of (BPEI/PAA)6 film in

8

metal ion solutions. This material is available free of charge via the Internet at

9

http://pubs.acs.org.”

10 11

AUTHOR INFORMATION

12

Corresponding Author

13

*[email protected]

14 15

Author Contributions

16

The experiments were performed by and the manuscript was written through contributions of all

17

authors. All authors have given approval to the final version of the manuscript.

18


35


Page 36 of 46

1

Funding Sources

2

All authors received funding from NSF grant DMR 1425187, ARO STIR Contract No.

3

W911NF-14-1-0587 and the Department of Polymer Engineering at University of Akron.

4 5

Notes

6

The authors declare no competing financial interest.

7 8

ACKNOWLEDGMENT

9

The authors would like to thank Dr. Mark Soucek of Akron’s Department of Polymer

10

Engineering for providing latex particles used as a template for zeta potential measurements.

11 12

REFERENCES

13

(1)

14 15

850. (2)

16 17 18

Wilker, J. J. Self-Healing Polymers: Sticky When Wet. Nature Materials. 2014, pp 849–

Ryu, J. H.; Hong, S.; Lee, H. Bio-Inspired Adhesive Catechol-Conjugated Chitosan for Biomedical Applications: A Mini Review. Acta Biomaterialia. 2015, pp 101–115.

(3)

Moulay, S. Dopa/catechol-Tethered Polymers: Dioadhesives and Biomimetic Adhesive Materials. Polym. Rev. 2014, 54 (3), 436–513.


36

Page 37 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(4)

Duarte, A. P.; Coelho, J. F.; Bordado, J. C.; Cidade, M. T.; Gil, M. H. Surgical Adhesives:

2

Systematic Review of the Main Types and Development Forecast. Progress in Polymer

3

Science. 2012, pp 1031–1050.

4

(5)

5 6

and Polymer Science. 2013, pp 2031–2047. (6)

7 8

(7)

Inspired Synthetic Peptide Films. Adv. Funct. Mater. 2010, 20 (23), 4196–4205. (8)

12 (9)

(10)

20

Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science (80-. ). 1997, 277 (5330), 1232–1237.

(11)

18 19

Gu, Y.; Zacharia, N. S. Self-Healing Actuating Adhesive Based on Polyelectrolyte Multilayers. Adv. Funct. Mater. 2015, 25 (24), 3785–3792.

16 17

Burke, S. A.; Ritter-Jones, M.; Lee, B. P.; Messersmith, P. B. Thermal Gelation and Tissue Adhesion of Biomimetic Hydrogels. Biomed. Mater. 2007, 2 (4), 203–210.

14 15

Anderson, T. H.; Yu, J.; Estrada, A.; Hammer, M. U.; Waite, J. H.; Israelachvili, J. N. The Contribution of DOPA to Substrate-Peptide Adhesion and Internal Cohesion of Mussel-

10

13

Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448 (7151), 338–341.

9

11

Peak, C. W.; Wilker, J. J.; Schmidt, G. A Review on Tough and Sticky Hydrogels. Colloid

Hammond, P. T. Form and Function in Multilayer Assembly: New Applications at the Nanoscale. Advanced Materials. 2004, pp 1271–1293.

(12)

Lutkenhaus, J. L.; Hammond, P. T. Electrochemically Enabled Polyelectrolyte Multilayer Devices: From Fuel Cells to Sensors. Soft Matter 2007, 3 (7), 804.


37


1

(13)

2 3

Sukhishvili, S. A. Responsive Polymer Films and Capsules via Layer-by-Layer Assembly. Current Opinion in Colloid and Interface Science. 2005, pp 37–44.

(14)

Gu, Y.; Huang, X.; Wiener, C. G.; Vogt, B. D.; Zacharia, N. S. Large-Scale Solvent

4

Driven Actuation of Polyelectrolyte Multilayers Based on Modulation of Dynamic

5

Secondary Interactions. ACS Appl. Mater. Interfaces 2015, 7 (3), 1848–1858.

6

(15)

Hoshi, T.; Saiki, H.; Kuwazawa, S.; Tsuchiya, C.; Chen, Q.; Anzai, J. Selective

7

Permeation of Hydrogen Peroxide through Polyelectrolyte Multilayer Films and Its Use

8

for Amperometric Biosensors Multilayer ( PEM ) Films to Prepare an Amperometric.

9

Anal. Chem. 2001, 73 (21), 5310–5315.

10

(16)

11 12

Page 38 of 46

Li, Y.; Chen, S.; Wu, M.; Sun, J. Polyelectrolyte Multilayers Impart Healability to Highly Electrically Conductive Films. Adv. Mater. 2012, 24 (33), 4578–4582.

(17)

Hyder, M. N.; Lee, S. W.; Cebeci, F. Ç.; Schmidt, D. J.; Shao-Horn, Y.; Hammond, P. T.

13

Layer-by-Layer Assembled Polyaniline Nanofiber/multiwall Carbon Nanotube Thin Film

14

Electrodes for High-Power and High-Energy Storage Applications. ACS Nano 2011, 5

15

(11), 8552–8561.

16

(18)

Huang, X.; Chrisman, J. D.; Zacharia, N. S. Omniphobic Slippery Coatings Based on

17

Lubricant-Infused Porous Polyelectrolyte Multilayers. ACS Macro Lett. 2013, 2 (9), 826–

18

829.


38

Page 39 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(19)

Johansson, E.; Lundström, L.; Norgren, M.; Wågberg, L. Adsorption Behavior and

2

Adhesive Properties of Biopolyelectrolyte Multilayers Formed from Cationic and Anionic

3

Starch. Biomacromolecules 2009, 10 (7), 1768–1776.

4

(20)

Karabulut, E.; Pettersson, T.; Ankerfors, M.; Wågberg, L. Adhesive Layer-by-Layer Films

5

of Carboxymethylated Cellulose Nanofibril-Dopamine Covalent Bioconjugates Inspired

6

by Marine Mussel Threads. ACS Nano 2012, 6 (6), 4731–4739.

7

(21)

Pettersson, T.; Pendergraph, S. A.; Utsel, S.; Marais, A.; Gustafsson, E.; W??gberg, L.

8

Robust and Tailored Wet Adhesion in Biopolymer Thin Films. Biomacromolecules 2014,

9

15 (12), 4420–4428.

10

(22)

Han, L.; Wang, Z. ming; Lu, X.; Dong, L.; Xie, C. ming; Wang, K. feng; Chen, X. lang;

11

Ding, Y. hui; Weng, L. tao. Mussel-Inspired Adhesive and Transferable Free-Standing

12

Films by Self-Assembling Dexamethasone Encapsulated BSA Nanoparticles and

13

Vancomycin Immobilized Oxidized Alginate. Colloids Surfaces B Biointerfaces 2015,

14

126, 452–458.

15

(23)

Hariri, H. H.; Lehaf, A. M.; Schlenoff, J. B. Mechanical Properties of Osmotically

16

Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer.

17

Macromolecules 2012, 45 (23), 9364–9372.

18 19

(24)

Li, X.; Pelton, R. Enhancing Wet Cellulose Adhesion with Proteins. Ind. Eng. Chem. Res. 2005, 44 (19), 7398–7404.


39


1

(25)

2 3

Evans, J. R. G.; Packham, D. E. Adhesion of Polyethylene to Metals: The Role of Surface Topography. J. Adhes. 1979, 10 (3), 177–191.

(26)

Eick, J. D.; Johnson, L. N.; Fromer, J. R.; Good, R. J.; Neumann, A. W. Surface

4

Topography: Its Influence on Wetting and Adhesion in a Dental Adhesive System. J.

5

Dent. Res. 1972, 51 (3), 780–788.

6

(27)

7 8

Sui, Z.; Schlenoff, J. B. Phase Separations in pH-Responsive Polyelectrolyte Multilayers: Charge Extrusion versus Charge Expulsion. Langmuir 2004, 20 (14), 6026–6031.

(28)

9

Clark, S. L.; Montague, M. F.; Hammond, P. T. Ionic Effects of Sodium Chloride on the Templated Deposition of Polyelectrolytes Using Layer-by-Layer Ionic Assembly.

10 11

Macromolecules 1997, 30 (23), 7237–7244. (29)

Sung, Y. Y.; Rubner, M. F. Micropatterning of Polymer Thin Films with pH-Sensitive and

12

Cross-Linkable Hydrogen-Bonded Polyelectrolyte Multilayers. J. Am. Chem. Soc. 2002,

13

124 (10), 2100–2101.

14

(30)

Krass, H.; Papastavrou, G.; Kurth, D. G. Layer-by-Layer Self-Assembly of a

15

Polyelectrolyte Bearing Metal Ion Coordination and Electrostatic Functionality. Chem.

16

Mater. 2003, 15 (1), 196–203.

17

Page 40 of 46

(31)

Mentbayeva, A.; Ospanova, A.; Tashmuhambetova, Z.; Sokolova, V.; Sukhishvili, S.

18

Polymer-Metal Complexes in Polyelectrolyte Multilayer Films as Catalysts for Oxidation

19

of Toluene. Langmuir 2012, 28 (32), 11948–11955.


40

Page 41 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(32)

Rahim, M. A.; Islam, M. S.; Bae, T. S.; Choi, W. S.; Noh, Y. Y.; Lee, H. J. Metal Ion-

2

Enriched Polyelectrolyte Complexes and Their Utilization in Multilayer Assembly and

3

Catalytic Nanocomposite Films. Langmuir 2012, 28 (22), 8486–8495.

4

(33)

Huang, X.; Schubert, A. B.; Chrisman, J. D.; Zacharia, N. S. Formation and Tunable

5

Disassembly of Polyelectrolyte-cu2+ Layer-by-Layer Complex Film. Langmuir 2013, 29

6

(42), 12959–12968.

7

(34)

Zhang, H.; Wang, C.; Zhu, G.; Zacharia, N. S. Self-Healing of Bulk Polyelectrolyte

8

Complex Material as a Function of pH and Salt. ACS Appl. Mater. Interfaces 2016, 8 (39),

9

26258–26265.

10

(35)

11 12

Journal of Chemical Technology and Biotechnology. 2014, pp 1793–1810. (36)

13 14

Ozdil, D.; Aydin, H. M. Polymers for Medical and Tissue Engineering Applications.

Smart, J. D. An M Vitro Assessment of Some Mucosa-Adhesive Dosage Forms. Int. J. Pharm. 1991, 73 (1), 69–74.

(37)

Smart, J. D.; Kellaway, I. W.; Worthington, H. E. An in-Vitro Investigation of Mucosa-

15

Adhesive Materials for Use in Controlled Drug Delivery. J. Pharm. Pharmacol. 1984, 36

16

(5), 295–299.

17

(38)

Teng, G.; Soucek, M. D.; Yang, X. F.; Tallman, D. E. Effect of the Addition Mode of

18

Cycloaliphatic Diepoxide on the Morphology and Film Properties of Crosslinkable Core-

19

Shell Latex. J. Appl. Polym. Sci. 2003, 88 (2), 245–257.


41


1

(39)

Ocepek, M.; Soucek, M. D.; Berce, P.; Meng, L. Comparison of Particle Size Techniques

2

to Investigate Secondary Nucleation in HEMA-Rich Latexes. Macromol. Chem. Phys.

3

2015, 216 (4), 400–416.

4

(40)

5 6

Page 42 of 46

Qiang, Z.; Xue, J.; Cavicchi, K. A.; Vogt, B. D. Morphology Control in Mesoporous Carbon Films Using Solvent Vapor Annealing. Langmuir 2013, 29 (10), 3428–3438.

(41)

Herzinger, C. M.; Johs, B.; McGahan, W. A.; Woollam, J. A.; Paulson, W. Ellipsometric

7

Determination of Optical Constants for Silicon and Thermally Grown Silicon Dioxide via

8

a Multi-Sample, Multi-Wavelength, Multi-Angle Investigation. J. Appl. Phys. 1998, 83

9

(6), 3323–3336.

10

(42)

Roy, C. K.; Guo, H. L.; Sun, T. L.; Ihsan, A. Bin; Kurokawa, T.; Takahata, M.;

11

Nonoyama, T.; Nakajima, T.; Gong, J. P. Self-Adjustable Adhesion of Polyampholyte

12

Hydrogels. Adv. Mater. 2015, 27 (45), 7344–7348.

13

(43)

14 15

Ben-Zion, O.; Nussinovitch, A. Physical Properties of Hydrocolloid Wet Glues. Food Hydrocoll. 1997, 11 (4), 429–442.

(44)

Rose, S.; Prevoteau, A.; Elzière, P.; Hourdet, D.; Marcellan, A.; Leibler, L. Nanoparticle

16

Solutions as Adhesives for Gels and Biological Tissues. Nature 2014, 505 (7483), 382–

17

385.

18 19

(45)

Kamperman, M.; Synytska, A. Switchable Adhesion by Chemical Functionality and Topography. J. Mater. Chem. 2012, 22 (37), 19390.


42

Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(46)

2 3

1940, 12 (3), 175–214. (47)

4 5

Dennison, D. M. The Infra-Red Spectra of Polyatomic Molecules. Part II. Rev. Mod. Phys.

Bogue, R. Recent Developments in Adhesive Technology: A Review. Assem. Autom. 2011, 31, 207–211.

(48)

Lawrence, P. G.; Lapitsky, Y. Ionically Cross-Linked Poly(allylamine) as a Stimulus-

6

Responsive Underwater Adhesive: Ionic Strength and pH Effects. Langmuir 2015, 31 (4),

7

1564–1574.

8

(49)

9 10

Modified Methacrylic Triblock Hydrogels. Biomacromolecules 2008, 9 (1), 122–128. (50)

11 12

Guvendiren, M.; Messersmith, P. B.; Shull, K. R. Self-Assembly and Adhesion of DOPA-

Choi, J.; Rubner, M. F. Influence of the Degree of Ionization on Weak Polyelectrolyte Multilayer Assembly. Macromolecules 2005, 38 (1), 116–124.

(51)

Cho, C.; Zacharia, N. S. Film Stability during Postassembly Morphological Changes in

13

Polyelectrolyte Multilayers due to Acid and Base Exposure. Langmuir 2012, 28 (1), 841–

14

848.

15

(52)

Lutkenhaus, J. L.; McEnnis, K.; Hammond, P. T. Nano- And Microporous Layer-by-

16

Layer Assemblies Containing Linear Poly(ethylenimine) and Poly(acrylic Acid).

17

Macromolecules 2008, 41 (16), 6047–6054.

18 19

(53)

Epa, N.; Mass, N. I. H.; Library, S.; Ei, V. Z.; Sparkman, J. a. NIST Standard Reference Database 1A. Natl. Inst. Stand. Technol. NIST 2004, No. June 2008, 1–49.


43


1

(54)

2 3

Shao, H.; Stewart, R. J. Biomimetic Underwater Adhesives with Environmentally Triggered Setting Mechanisms. Adv. Mater. 2010, 22 (6), 729–733.

(55)

Reid, D. K.; Summers, A.; O’Neal, J.; Kavarthapu, A. V.; Lutkenhaus, J. L. Swelling and

4

Thermal Transitions of Polyelectrolyte Multilayers in the Presence of Divalent Ions.

5

Macromolecules 2016, 49 (16), 5921–5930.

6

(56)

7 8

Heuvingh, J.; Zappa, M.; Fery, A. Salt Softening of Polyelectrolyte Multilayer Capsules. Langmuir 2005, 21 (7), 3165–3171.

(57)

9

Cencer, M.; Liu, Y.; Winter, A.; Murley, M.; Meng, H.; Lee, B. P. Effect of pH on the Rate of Curing and Bioadhesive Properties of Dopamine Functionalized Poly(ethylene

10 11

Glycol) Hydrogels. Biomacromolecules 2014, 15 (8), 2861–2869. (58)

Nakamura, T.; Takashima, Y.; Hashidzume, A.; Yamaguchi, H.; Harada, A. A Metal-Ion-

12

Responsive Adhesive Material via Switching of Molecular Recognition Properties. Nat.

13

Commun. 2014, 5.

14

(59)

15 16

Brown, H. R. The Adhesion Between Polymers. Annu. Rev. Mater. Sci. 1991, 21 (1), 463– 489.

(60)

Gent, A. N. Adhesion and Strength of Viscoelastic Solids. Is There a Relationship between Adhesion and Bulk Properties? †. Langmuir 1996, 12 (19), 4492–4496.

17 18

Page 44 of 46

(61)

Derail, C.; Allal, A.; Marin, G.; Tordjeman, P. Relationship Between Viscoelastic and

19

Peeling Properties of Model Adhesives. Part 2. The Interfacial Fracture Domains. J.

20

Adhes. 1998, 68 (3–4), 203–228.


44

Page 45 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


(62)

Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.;

2

Lavalle, P. Molecular Basis for the Explanation of the Exponential Growth of

3

Polyelectrolyte Multilayers. Proc. Natl. Acad. Sci. 2002, 99 (20), 12531–12535.

4

(63)

Caldwell, J. W.; Arsura, E. L.; Kilgore, W. B.; Garcia, A. L.; Reddy, V.; Johnson, R. H.

5

Layer-by-Layer Assembly of Collagen Thin Films: Controlled Thickness and

6

Biocompatibility. Biomed. Microdevices 2001, 3 (4), 301–306.

7

(64)

Monge, C.; Almodóvar, J.; Boudou, T.; Picart, C. Spatio-Temporal Control of LbL Films

8

for Biomedical Applications: From 2D to 3D. Advanced Healthcare Materials. 2015, pp

9

811–830.

10 11


45


Page 46 of 46

1 2

Table of Contents Graphic and Synopsis

3 4 5

6

7

8

9

10


46

Tuning Wet Adhesion of Weak Polyelectrolyte Multilayers - ACS

Recommend Documents