The Role of Self-Healing Coatings on Soft Polymer Fibers by
Inbar Yamin
Submitted to the
Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science at the
Massachusetts Institute of Technology
June 2015
© 2015 Inbar Yamin
All Rights Reserved
The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created
Signature of Author…………………………………………………………………………
Department of Materials Science and Engineering
May 8, 2015
Certified By…………………………………………………………………………………
Niels Holten-Andersen
Assistant Professor of Materials Science and Engineering
Thesis Supervisor
Accepted By………………………………………………………………………………...
Geoffrey Beach
Associate Professor of Materials Science and Engineering
Chairman, Undergraduate Committee

The Role of Self-Healing Coatings on Soft Polymer Fibers by
Inbar Yamin
Submitted to the Department of Materials Science and Engineering on May 8, 2015 in Partial Fulfillment of the
Requirements for the Degree of Bachelor of Science in
Materials Science and Engineering
ABSTRACT
Mussel byssal threads exhibit unique self-healing mechanical properties. This study designed a synthetic system modeled after the byssal thread structure in order to isolate the origins of their unique self-healing mechanical properties. PDMS fibers were coated with metal-coordination bonds crosslinked PEG gels and their mechanical properties were tested with uniaxial tension tests. The synthetic system achieved a similar behavior to that of the natural mussel fibers, showing that a thin stiff coating on a soft polymer fiber can have a dramatic effect on its mechanical behavior. The coated fibers were much stiffer at small strains than the uncoated PDMS. The linear elastic region was followed by a distinct yield stress, which indicated the coating beginning to fracture. At high strains, when the coating had failed catastrophically, the PDMS behavior dominated.
The coatings were healed though hydration in a humid environment and were then able to recover their stiffness similar to mussel byssal threads.
Thesis Supervisor: Niels Holten-Andersen
Title: Assistant Professor of Materials Science and Engineering
2

Acknowledgements
I am grateful to Professor Niels Holten-Andersen for encouraging me to pursue my own project from start to finish. Your continued support and optimism through the many setbacks helped me see the project through and continue on until we found a system that worked well. Your unrivaled enthusiasm as the pieces started to fall into place showed the depth of your commitment not only to this project but also to my success as a student.
Thank You For Everything.
Thank you, Beth Cholst, for helping bridge the connection with Professor Mathias
Kolle’s Laboratory with whom we collaborated deeply throughout this whole project.
Thank you immensely to Joseph Sandt. Despite having your own Master’s Thesis to complete, you helped to collect all the quantitative data for this project. Thank you for always being patient and encouraging and most of all for donating so much of your time to this project.
Lastly, thank you to my family, my parents and brother. You have always been, not only unfailingly supportive, but also excited to hear about every project that I have pursued.
You taught me how ask questions, and asking the right question is 99% of the work. In many ways, this thesis is a family accomplishment, an outcome of many years of growing and learning together. I could not have done it without you.
3

Table of Contents
List of Figures……………………………………………………………………………..5
1. Background……………………………………………………………………………..6
1.1 Mussel Byssal Fibers and Their Cuticle………………………………………6
1.2 Mechanical Properties of Distal Mussel Byssal Thread………………………9
1.3 Metal Coordination Bonds in Cuticle of Byssal Thread…………………..…11
1.4 Synthetic Gel Inspired by Mussel Thread Metal-Coordination Bonding……13
2. Introduction……………………………………………………………………………13
3. Materials and Methods………………………………………………………………...14
3.1 Coating PDMS Fibers………………………………………………………..14
3.2 Mechanical Testing…………………………………………………………..17
3.3 Optical Microscopy…………………………………………………………..18
4. Results…………………………………………………………………………………18
5. Discussion……………………………………………………………………………..29
6. Conclusions……………………………………………………………………………30
References………………………………………………………………………………..32
4

List of Figures
Fig. 1 Mussel diagram with Byssal Threads ……………………………………………...6
Fig. 2 SEM Images of Byssal Thread Core and Cuticle…………………………………..7
Fig. 3 Annotated Diagram of Mussel byssal thread………………………………....……7
Fig. 4 Salami Structure of the Cuticle of the Mussel Byssal Thread………………….......8
Fig. 5 Cyclic Uniaxial Loading of Distal Byssal Threads……………………………….10
Fig. 6 Cyclic Uniaxial Tension Loading of Byssal Threads with a Time Delay………...11
Fig. 7 Structure of L-DOPA, 3,4 dihydroxyphenyl-L-alanine……………………..……11
Fig. 8 Evolution of the Catechol-Fe Complexes as pH is Increased…………………….12
Fig. 9 Recovery of Cohesiveness Over Time of Metal-Coordination Crosslinked Gel ...13
Fig. 10 Plasma Etcher……………………………………………………………………15
Fig. 11 PDMS Structure Before and After Oxygen Plasma Treatment………………….15
Fig. 12 PEG-Dopa at 200 mg/ml mixed at a 3:1 ratio with FeCl
3
……………………….16
Fig. 13 Coated PDMS Fibers…………………………………………………………….16
Fig. 14 Custom uniaxial tension apparatus………………………………………………17
Fig. 15 Stress vs. Strain of Uncoated PDMS Fiber……………………………………...19
Fig. 16 Stress vs. Strain of Coated PDMS Fiber ………………………………………..20
Fig. 17 Stress vs. Strain of Coated and Uncoated PDMS Fibers………………………...21
Fig. 18 Stress vs. Strain of Healed Coated Fiber………………………………………...22
Fig. 19 Stress vs. Strain of Healed Coated Fiber and Semi-Dry Coated Fiber…………..23
Fig. 20 Stress vs. Strain of Coated PDMS Fiber with pH 6.5 coating…………………...24
Fig. 21 Stress vs. Strain of NaOH and Bis-Tris Coated PDMS Fibers………………….25
Fig. 22 Optical Microscope Image of Dry Coated PDMS Fiber ………………………..26
Fig. 23 Optical Microscope Image of Dry, Broken Dry coated PDMS Fiber…………...27
Fig. 24 Optical Microscope Images of Coating Healing Process ……………………….28
List of Tables
Table 1: Measurement of Fiber Diameters……………………………………………...18
Table 2 : Young’s Moduli (in kPa) of Fibers with Different Coatings…………………..26
5

1. Background Information
1.1 Mussel Byssal Fibers and Their Cuticle
Marine mussels must adhere to rocks for long periods of time under the high repeated lift and drag forces of waves 1 . They are able to do this with many thin threads, which comprise the byssus (see Figure 1).
Fig. 1 Mussel diagram with byssal threads shown 2 .
The underlying design of these threads are of great interest because of their unique materials properties. The fibers are great examples of a strong underwater adhesive, they have self-healing properties, and they are tough (not brittle) while withstanding high stress before yielding leading to great shock-absorbing ability 1,3 .
As a biological material the proteins that the fibers are made of have unique and complex amino acid sequences and structures, however, one important property that has been show to govern many of these properties is metal-coordination bonds between amino acid functional groups and metal ions. A byssus consists of a bundle of 50 -100 individual threads which are extensible to greater than 100% strain 3 . Threads are several centimeters long and have a diameter of 200 microns 1 . The foot of the mussel injection molds soluble precursors that self-assemble into functional fibers made mostly of a modified fibrous collagen. Afterwards, a 2 to 5 micron thick coating is applied in a
6

separate secretion 4 . The coating is 4 to 6 times stiffer than the core of the fiber and can undergo 100% strain before tearing.
Fig. 2 SEM images of byssal thread core and cuticle 3,4
The thread has four distinct regions: the stem, the plaque, the proximal, and the distal (see
Figure 3). The stem attaches the thread to the mussel tissue; the plaque has the adhesive that sticks the thread to the rock; the proximal region is corrugated, extensible up to
200%, and has low stiffness; and the distal region has high initial stiffness followed by a yield point at approximately 15% strain and then stress stiffening behavior. This region is self-healing, as the damage due to yield is reversible in a time-dependent manner.
Fig. 3 Diagram of the four morphologically and mechanically differing regions of the mussel byssal thread.
7

The self-healing mechanical properties of the distal region make mussel byssus threads of special interest for bio-inspired materials.
The highest strain capacity for an engineered coating of synthetic fibers is around
10%, yet deformable coatings are common in biological systems 5 . For the mussel fiber, the ability to deform extensively before catastrophic crack propagation is a result of its inhomogeneous coating structure (see Figures 2 and 4). Similar to High Impact
Polystyrene, the structure of the cuticle for intertidal mussel fibers that must withstand the strong forces of repeated waves contain microgranules that resist crack propagation.
The domains are approximately 0.8 microns in diameter and comprise 50% of the cuticle volume 5 . Strain in the fibers lead to microcrack formation and granule deformation, but since the granules resist crack propagation, it is not until much higher strains that the cuticle fails catastrophically.
A)                B)              C)
Fig. 4 High resolution images of salami structure of the cuticle of the mussel byssal thread. A) Transmission electron micrograph (TEM) of longitudinal section of the coating. B) AFM image of mussel thread stretched by 50% showing microtears developing and deformation of granules. C) SEM image showing thread stretched by
70% with extensive microtearing before catastrophic failure of cuticle 5 .
8

1.2 Mechanical Properties of Distal Mussel Byssal Thread
When under uniaxial tension, the distal byssal thread exhibits linear elastic behavior and then yielding. If a fiber is put under tension to a percent extension before yielding (under 20%) and allowed to relax, there is little energy dissipated in the process.
In other words, it is considered resilient because much of the elastic strain energy stored during deformation is recovered in elastic recoil 6 . If the fiber is pulled on again, immediately, the fiber exhibits the same mechanical properties as the first cycle. When strained passed the yield point the resilience drops dramatically as more elastic strain energy is dissipated in this cycle. If pulled on immediately again, the Young’s Modulus drops dramatically and there is no longer any yield point (see Figure 5). This mechanical behavior is much more similar to the mechanical properties of collagen which might imply that the stiffness and then yielding stems from the mechanical behavior of the cuticle.
9

Fig. 5 Force vs. strain plots for cyclic uniaxial loading of distal byssal threads at various
percent extensions. Arrows indicate direction of loading while double arrows indicate second cycle extensions. Extension rate is 5 millimeters per minute for A and B and 10 millimeters per minute for C and D 6 .
When the distal region is put under cyclical loading with a time delay there is recovery in the stiffness and resilience of the material. Figure 6 shows two cycles conducted at t = 0
(denoted by the solid lines) followed by two cycles after a 30 minute delay (denoted by the dotted lines). The mechanical properties of distal region fibers are recoverable given enough time indicating that the fibers are in fact self-healing. From a natural selection perspective, it is advantageous for the mussel fibers to exhibit this behavior because of the repetitive stress that the fibers feel from ocean waves.
10

Fig. 6 Force vs. strain plot showing cyclic uniaxial tension loading with a time delay of a
distal byssal thread up to a 35% extension. Solid line shows two cycles of loading at t=0 and the dotted line shows two cycles of loading after 30 minutes 6 .
1.3 Metal Coordination Bonds in Cuticle of Byssal Thread
As discussed previously, the cuticle of the byssal thread is 4 to 6 times harder and stiffer than the core of the mussel thread. The thread is also extensible up to 70% strain before the cuticle breaks catastrophically. This extensibility is explained partly by deformable microphase granules that resist crack propagation. After yielding, it was shown (Figure 6) that the mechanical integrity of the threads returns after a time delay.
This behavior leads to the hypothesis that yielding is caused by the breaking of reversible bonds. The cuticle has a high content of 3,4 dihydroxyphenyl-L-alanine (dopa) (see
Figure 7).
Fig. 7 Structure of L-DOPA, 3,4 dihydroxyphenyl-L-alanine. The alcohol groups on the phenyl ring exhibit affinity for many metal ions, with especially high affinity for Fe III 7 .
11

Holten-Andersen et. al. reports that the cuticle has high concentrations of iron and calcium ions 8 . By incubating the threads with EDTA, which chelate strongly with the metal ions, there was a 50% reduction in the cuticle hardness, which leads to the conclusion that metal coordination bonds play a strong role in the mechanically integrity of the cuticle. The catecholic side chain of dopa exhibits a high affinity for transition metal ions, for example the monocatechol- Fe III complex has log K s equal to 18. The metal coordination bonds are as strong as covalent bonds, (at pH 8 it takes 0.8nN to break a single metal-catechol bond) 8 , but they are also completely reversible, which can help explain the self-healing properties of the thread. The stoichiometry of the catechol-Fe III is controlled by pH since it depends of the percent of catechol hydroxyls that are deprotonated (see Figure 8). As the pH increases, the complexes go from mono to bis to tris. The metal-coordination dependence on pH is consistent with the theory that the precursors of the cuticle are secreted by the mussel at a low pH. Since the precursors and metal-ions are mostly in mono-complexes at low pH, they are able to flow. When the cuticle hits the seawater, which is approximately at pH 8, the catechol alcohol groups are deprotonated moving to the tris-complexes, crosslinking the precursors, and therefore, stiffening the cuticle 9 .
Fig. 8 Evolution of the catechol-Fe complexes as pH is increased from mono to bis to tris complexes.
12
Fig. 1.
Mussel-inspired Dopa-Fe 3 þ cross-linking. ( A ) The pH-dependent stoichiometry of Fe 3 þ -catechol complexes. ( B ) Schematic of proposed crosslinking mechanism of byssal thread cuticle: ( i ) Production and storage of mfp-1 and Fe 3 þ in specialized cells of the epithelium lining the ventral groove of the mussel foot. Low pH ( ≤ 5 ) ensures mono-catechol-Fe 3 þ complexes
(no cross-linking), ( ii ) Secretion and self-assembly of cuticle with whole mussel thread in the ventral groove (outline of part of ventral groove indicated with dashed white line), ( iii ) Seawater exposure (pH ∼ 8 ) of nascent byssal thread drives immediate cuticle cross-linking via bis- and/or tris-catechol-
Fe 3 þ complexes (insert shows scanning electron micrograph of mussel thread with partial cuticle on top of fibrous core) (Scale bar, 20 μ m).
when raising pH to initiate cross-linking. The concentrated polymer-FeCl
3 mixture remained a green/blue fluid upon raising pH to ∼
5
, whereas raising pH to ∼
8 resulted in the instant formation of a sticky purple gel. At pH ∼
12 a red elastomeric gel immediately formed (Fig. 2 C and Fig. S2 A – C , G , I , and Movie S1 ).
UV-Vis absorbance spectroscopy confirmed the dominance of mono-, bis-, and tris-catechol-Fe 3 þ complexes, in the pH ∼ 5
, ∼ 8
, and ∼ 12 adjusted polymer networks, respectively ( Fig. S1 ). Raman microspectroscopy performed with a near-infrared (785 nm) laser furthermore demonstrated resonance Raman spectra characteristic of Fe 3 þ -catechol coordination in the polymer networks after
FeCl
3 was added (Fig. 2 D ). Clear spectral differences exist between samples, particularly in the Raman band originating specifically from the chelation of the Fe 3 þ ion by the oxygen atoms of the catechol (470 – 670 cm -1 ). This band consists of three major peaks, which transform significantly with changing pH. The peaks at ∼
590 cm − 1 and ∼
633 cm − 1 between the Fe 3 þ ion and the C
3 are assigned to the interaction and C
4 oxygens of the catechol, respectively, while the peak at
528 cm − 1 is assigned to charge transfer (CT) interactions of the bidentate chelate (see Fig. 1 A )
(18). The area of the CT peak increases relative to the other two peaks upon increasing pH from about 6% at pH ∼ 5 to ∼ 30% at pH ∼ 8 and almost 40% at pH ∼ 12
. This peak progression suggests an increase in bidentate complexation with increasing pH consistent with the transition from mono- to tris-coordinated Fe 3 þ species. Moreover, the resonance signals from the tris-catechol-
Fe 3 þ cross-linked gels at pH ∼
12 and reconstituted Fe 3 þ mfp-1 complexes (7) were found to be remarkably similar to the native thread cuticle ( Table S1 and Fig. 2 D ) (4).
Fig. 2.
PEG-dopa
4
(PEG-dopa
4 with FeCl
3 tris-catechol-Fe 3 þ
-Fe 3 þ crosslinking. (
, 10 kDa PEG core). ( bis- (blue), and tris-catechol-Fe state and color of PEG-dopa
4
3 þ
B
A ) Dopa-modified polyethylene glycol
) Relative fractions of mono- (green),
(red) complexes in solutions of PEG-dopa
4 at a dopa ∶ Fe molar ratio of 3 ∶ 1 as a function of pH. ( C ) Physical gels in mono- (green/blue), bis- (purple), and
(red) complexation (dopa ∶ Fe 3 ∶ 1 ). Insets highlight color of bis- and tris-catechol-Fe 3 þ cross-linked gels. ( D ) Resonance Raman spectroscopy of the same samples as in ( C ). Spectra of PEG-dopa
4 black) and with (dashed green) FeCl
3 without (dashed
(unadjusted pH ∼ 3 – 4 ) are shown for comparison. Additionally, the spectrum from the native mussel thread cuticle is included (dashed red). For each spectra N ¼ 3 .
In agreement with their lack of cross-linking, concentrated polymer-FeCl
3 mixtures at pH ∼ 5 displayed a viscous response in dynamic oscillatory rheology, whereas the bis- and tris-catechol-Fe 3 þ cross-linked gels at pH ∼ 8 and pH ∼ 12
, respectively, behaved increasingly elastically (elastic modulus G 0 > viscous modulus G ″ ) (Fig. 3 A ). For comparison, a covalently cross-linked polymer gel was prepared using NaIO
PEG-dopa
4
(19) ( Fig. S2 D – H
4
-induced oxidation of
). When a tris-catechol-Fe 3 þ crosslinked gel and a covalent gel were exposed to a concentrated
2652 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1015862108
Holten-Andersen et al.

1.4 Synthetic Gel Inspired by Mussel Byssal Thread Metal-Coordination Bonding
Holten-Andersen et. al. developed a synthetic gel based on these metal-coordination bond crosslinks. First a dopa modified polyethylene glycol polymer is mixed with FeCl
3 forming a green liquid where there mostly mono-complexes are present 9 . When the pH is raised to about 7 the liquid turns into a dark blue/purple sticky gel state. At pH 12, the system forms a deep red elastomeric gel. A covalently crosslinked PEG-Dopa gel was created via oxidation by NaIO
4 for comparison with the metal-coordination crosslinked gel. When tested for cohesiveness recovery after simple tearing, the metal-coordinated gel showed recovery in 3 min while the covalently crosslinked gel showed no such recovery (see Figure 9).
Fig. 3.
and pH
Mechanical properties of polymer gels. (
∼ 12
Fig. 9
′ and G
′
″
A ) Frequency sweep of PEG-dopa
, circles) and loss modulus (G simple tearing with the tip of a set of tweezers of tris-catechol-Fe 3 þ
″ , triangles). (
, circles and triangles, respectively). ( C
4
B
.
Fe-gels (dopa ∶ Fe molar ratio of 3 ∶ 1 ) adjusted to pH ∼ 5 (green), pH ∼ 8 (blue),
3 þ complexes (red) or gels (red curve and top row of images) and covalent gels (black curve and bottom row of images). Gels were shear failed under increasing oscillatory strain immediately followed by recovery under 1% strain (switch-over indicated by dashed line) while monitoring the storage modulus (G ′ is normalized to values from linear regime, < 60% strain, to allow easier comparison). For each data point (mean)
N ¼ 4
2. Introduction
N ¼ 2 ).
solution of the Fe 3 While the interesting self-healing behavior of the mussel threads has been studied former dissolved ( Fig. S3 ). This result supports the notion that the redox activity of Fe 3 þ does not lead to significant covalent cross-linking via oxidation of catechol within the time frame of our experiments (< 4 h) as has been reported in other systems isolating the origin of the self-healing behavior has been difficult. In literature there is a from Fe 3 þ catechol-Fe redox activity is under current investigation. The tris-
3 þ cross-linked gel was observed to dissipate > 10-fold more energy (G ″ ) than the covalent gel at low strain rates, even though the elastic moduli (G ′ ) were similar at high strain rates
(Fig. 3 B ). Finally, following failure induced by shear strain or simple tearing, tris-catechol-Fe cohesiveness within minutes whereas the covalently cross-linked
C ).
3 þ cross-linked gels recovered G ′ and
13
Discussion
We demonstrate that load-bearing catecholato-Fe 3 þ cross-links can be established in bulk materials by mimicking the proposed catechol-Fe
≈ 14 .
3
3 þ polymer networks can be easily controlled because quite extensively, due to the difficulty of separating the cuticle from the core of the fiber, catechol-Fe 3 þ cross-links set by final pH (20). The pH-induced shifts of the frequency of maximum viscous dissipation ( observed in shear rheometry demonstrates this effect (21). As discussion about whether the unique mechanical properties stem from the unique
∼
≈ 0
8
.
to
39
∼
12 , f max f max
) shifts from
Hz (mean, SE 0.01,
N ¼ 6 ) in agreement with a decrease in cross-link dissociation rate (see Fig. 3 A ). Our data suggest average relaxation times
τ ( 1 ∕ f max
) of τ ≈ 0 .
070 s and τ ≈ 2 .
56 s, respectively, for bisand tris-catechol-Fe 3 þ cross-linked networks. The near covalent stiffness (G ′ ) of tris-catechol-Fe 3 þ cross-linked networks at high strain rates (see Fig. 3 B ) supports the hypothesis that catecholato-Fe 3 þ coordinate bonds can provide significant strength to bulk materials despite their transient nature, given that the pH is high enough to ensure cross-link stability on relevant time scales.
Holten-Andersen et al.
PNAS ∣ February 15, 2011 ∣ vol. 108 ∣ no. 7 ∣ 2653

collagen in the core or the metal-coordinated gel found in the cuticle. In this project, we created a comparable synthetic system to isolate the contribution of a stiff coating that can self-heal to the overall mechanical properties of a soft polymeric fiber. While we can still make no absolute conclusions about the origin of the mechanical behavior in mussel fibers, we have shown that a thin, stiff coating can dramatically change the mechanical properties of a soft polymeric fiber. Additionally, the qualitative stress-strain behavior is very similar to that of the mussel fiber indicating that this is most likely a major factor in the unique mechanical properties of the mussel fiber.
3. Materials and Methods
3.1 Coating PDMS Fibers
Prepare Fibers for Coating
PDMS fibers (approximately 300 microns in diameter) were acquired from Joseph
Sandt in the Mathias Kolle group in the Mechanical Engineering Department at MIT.
PDMS fibers were prepared through extrusion in the Lewis Lab at Harvard University.
Oxygen Plasma etching via the PlasmaEtch etcher was used to make the surface of the
PDMS fibers more hydrophilic (see Figure 10). Fibers were set-up so that they were not in contact with the etcher surface. All sides were therefore exposed to oxygen plasma, and the whole surface got oxidized (methyl side groups converted to hydroxyls) making the fibers more hydrophilic (see Figure 11).
14

View Article Online obtain homogeneous, hydrophilic PDMS surfaces, whereas longer oxidation times yield a thin surface layer of silica.
7
Olander et al.
analyzed the mechanism of PDMS oxidation by oxygen plasma.
8 By X-ray photoelectron spectroscopy (XPS) measurements they revealed that one methyl group of the dimethylsiloxane units is first substituted by an oxygen atom.
This reaction has a half life time of 5 s. After prolonged oxidation, silica like structures formed at the PDMS surface.
It should be emphasized that the hydrophilicity of oxidized stamps was that the polarity of the surface was preserved for at least 20 days.
Delamarche et al.
reacted a PEG silane with an oxidized
PDMS stamp.
12 The polarity of the hydroxyl-terminated PEG longer oxidation times yield a thin surface layer of silica.
7 layer was stable for 7 days and the stamps were used to pattern
Pd/Sn colloids for ELD of NiB. Furthermore, flat stamps with oxygen plasma.
8 By X-ray photoelectron spectroscopy (XPS) protein repellent PEG patterns were used for the transfer of
PDMS is subject to a phenomenon referred to as ‘‘hydrophobic recovery’’, which describes the decrease of polarity of PDMS polar PDMS stamps. He et al.
H
2 exposed a PDMS stamp to Ar/
It should be emphasized that the hydrophilicity of oxidized plasma, which causes homolysis of a methyl group of the surfaces after oxidation with time. Hydrophobic recovery is due to the tendency to minimize the surface energy of the oxidized polymer and is caused by the flexibility of polymer chains.
Hydrophobic recovery occurs on briefly (<30 min) oxidized
This functionalization was especially useful for biological polymer and is caused by the flexibility of polymer chains.
applications, as the coated stamps showed optimal wetting by
PDMS as well as on ‘‘silica coated’’ PDMS surfaces, although the process is slower and less homogeneous in the case of prolonged oxidation times.
7 Importantly, hydrophobic recovery is fast when oxidized PDMS stamps are exposed to air, and much slower when oxidized PDMS stamps are stored in water.
m
The transfer of low molecular weight PDMS oligomers during
CP and the effect of PDMS oxidation on this phenomenon has been investigated in detail.
2.1.2.
9,10 XPS studies on the plasma oxidized PDMS clearly showed that oxygen plasma pre-treatment of the stamps significantly reduces the transfer of PDMS residues on the substrates after printing.
Surface coating of PDMS stamps.
In addition to oxidation, the surface polarity of PDMS can also be increased by applying a coating that is not subject to hydrophobic recovery
PDMS stamps showed significantly decreased WCA (30–35 and the coating was stable for more than one month. A similar way to make amino terminated PDMS stamps was introduced by
Sadhu et al.
oxidation times.
7 when oxidized PDMS stamps are exposed to air, and much slower when oxidized PDMS stamps are stored in water.
in 2007.
14 By means of plasma polymerization of
) been investigated in detail.
9,10 on PDMS (as well as on other polymer stamps). This hydrophilic oxidized PDMS clearly showed that oxygen plasma pre-treatcoating was stable for several months and could be used to ment of the stamps significantly reduces the transfer of PDMS transfer very polar molecules such as poly(propylene imine)
(PPI) dendrimers from aqueous solution onto various substrates.
A major advance in the resolution of m CP involves a radically oxidation, the surface polarity of PDMS can also be increased by altered design of stamps: instead of exploiting the voids in the applying a coating that is not subject to hydrophobic recovery microrelief pattern as an ink diffusion barrier, it is possible to impose an ink diffusion barrier on a flat m PDMS stamp.
For
11 example, by oxidation of the PDMS surface, a thin silicon oxide propyltriethoxysilane (APTES) at the surface of an oxidized film is created, which is essentially impermeable to apolar inks. If
PDMS stamp. The APTES layer was reacted with bis(sulfothe oxidation is directed by a mask, a flat stamp with a surface
(Fig. 3). A first method to chemically render the surface of
PDMS stamps polar for the use in m CP has been reported by
Donzel et al.
in 2001.
11 They condensed aminopropyltriethoxysilane (APTES) at the surface of an oxidized
PDMS stamp. The APTES layer was reacted with bis(sulfosuccinimidyl)suberate to obtain an active ester on the polymer used for m CP of n -alkyl thiols, which are transferred exclusively transfer Pd 2+ complexes as nucleation patterns for electroless in the non-oxidized area. The properties of the diffusion barrier deposition (ELD) of Cu. The major advantage of the coated and the stability of the stamp may be improved by coating the surface which was subsequently used to attach amino-terminated polyethylene glycol (PEG). The hydrophilic stamps were used to transfer Pd 2+ complexes as nucleation patterns for electroless
silicon oxide film with a fluorinated silane SAM. Even volatile, low molecular weight inks can be printed with such chemically patterned flat stamps. Moreover, because the stamp is flat, all oxygen plasma that is generated to oxidize the surface. problems due to deformation of the microrelief surface structures
0
2
plasma
Fig. 11 PDMS Structure before and after oxygen plasma treatment. Oxygen plasma oxidizes the PDMS and converts some of the side groups from methyl to hydroxides making the surface of the PDMS more hydrophilic 10 .
Two Step Coating Process
4-armed PEG-Dopa
4
in methanol at a concentration of 200 mg/ml was mixed with
80 mM FeCl
3
in a 3:1 volume ratio of polymer to metal salt to create a green liquid solution (see Figure 12). PDMS fibers that had been plasma etched recently (within 2 hours) were dipped in the green solution for 30 seconds, then pulled out slowly and allowed to dry for 1 minute. This process was repeated 10 times in order to get a considerable amount of polymer-metal mixture onto the fibers. After the last drying step,
View Article Online stamps was that the polarity of the surface was preserved for at least 20 days.
Delamarche et al.
reacted a PEG silane with an oxidized
PDMS stamp.
12 The polarity of the hydroxyl-terminated PEG layer was stable for 7 days and the stamps were used to pattern
Pd/Sn colloids for ELD of NiB. Furthermore, flat stamps with protein repellent PEG patterns were used for the transfer of proteins from the non-coated (hydrophobic) areas.
Plasma polymerization is another surface treatment to prepare polar PDMS stamps. He et al.
13 exposed a PDMS stamp to Ar/
H
2 plasma, which causes homolysis of a methyl group of the
PDMS. The surface radicals can initiate the polymerization of acetonitrile, resulting in a cyano-terminated polymer adlayer.
This functionalization was especially useful for biological applications, as the coated stamps showed optimal wetting by acetonitrile, which is a standard DNA solvent. The modified
PDMS stamps showed significantly decreased WCA (30–35
!
) and the coating was stable for more than one month. A similar way to make amino terminated PDMS stamps was introduced by
Sadhu et al.
in 2007.
14 By means of plasma polymerization of allylamine, they deposited a 5 nm thick poly(allylamine) coating on PDMS (as well as on other polymer stamps). This hydrophilic coating was stable for several months and could be used to transfer very polar molecules such as poly(propylene imine)
(PPI) dendrimers from aqueous solution onto various substrates.
A major advance in the resolution of m CP involves a radically altered design of stamps: instead of exploiting the voids in the microrelief pattern as an ink diffusion barrier, it is possible to impose an ink diffusion barrier on a flat PDMS stamp.
15 For example, by oxidation of the PDMS surface, a thin silicon oxide film is created, which is essentially impermeable to apolar inks. If the oxidation is directed by a mask, a flat stamp with a surface pattern of silicon oxide on PDMS results. This flat stamp can be used for m CP of n -alkyl thiols, which are transferred exclusively in the non-oxidized area. The properties of the diffusion barrier and the stability of the stamp may be improved by coating the silicon oxide film with a fluorinated silane SAM. Even volatile, low molecular weight inks can be printed with such chemically patterned flat stamps. Moreover, because the stamp is flat, all problems due to deformation of the microrelief surface structures
Fig. 3 Polar coatings for PDMS stamps.
This journal is
ª
The Royal Society of Chemistry 2010
374 | Polym. Chem.
, 2010, 1 , 371–387
Fig. 3 Polar coatings for PDMS stamps.
15
This journal is
ª
The Royal Society of Chemistry 2010

the fibers were dipped in 0.1 M NaOH. This dramatically raised the pH and the catecholmetal complexes went from mono to tris, stiffening the coating dramatically and turning it a deep red color (see Figure 13). Fibers were allowed to dry again for a few hours before mechanical tests were performed.
Fig. 12 PEG-Dopa at 200 mg/ml mixed at a 3:1 ratio with FeCl
3 low pH, mono-complexation of catechols with Fe 3+
. Green color indicates
cations as shown in Figure 8.
Fig. 13 Coated PDMS fibers after being dipped in NaOH, which causes them to turn a deep red and indicates tris metal-catechol complexation.
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For different types of coatings, after dipping in the polymer-metal mixtures, the fibers were dipped in Bis-Tris buffer at a pH of 6.57 to create a deep blue coating that was less stiff than a high pH coating.
3.2 Mechanical Testing
Uniaxial tension tests were performed using a custom apparatus in the Kolle Lab,
Joseph Sandt set-up (see Figure 14). The uniaxial tension tester strained the fiber by a preset strain step and then took a force measurement. While not continuous like an
Instron, this apparatus was able to take small enough strain steps to gather useful data.
The fibers are placed between two grips in the set-up, they were pulled to a maximum true strain of 0.8 with steps of 0.01 strain.
Fig. 14 Custom uniaxial tension test apparatus. The fiber is held between grips as shown.
The grips move to strain the fiber to a set amount while the force is measured by a load cell. Force measurements for the fiber are on the order of 1 mN.
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3.3 Optical Microscopy
A Leica optical microscope in the Laboratory for Engineering Materials was used to capture images of the coated fibers in different states (hydrated, dried, broken). The microscope was also used to measure the approximate diameter of the fibers and approximate thickness of the coating.
4. Results
Table 1: Measurements of fiber diameters across multiple fibers. Measurements were attained through optical microscopy.
Sample Diameter +/- Standard Deviation (Microns)
Coated (NaOH dipped), dried PDMS 379 +/ 36
Uncoated, Dried PDMS 363 +/- 55
The diameter of 2 uncoated and 2 coated fibers were measured in multiple places on each fiber. All measurements were averaged to yield a diameter and a standard deviation. Based on measurements from the optical microscopy the thickness of the coating is measured to be approximately 8 microns. By observation, the thickness of the coatings varied slightly from fiber to fiber as well as within the same fiber.
Uniaxial tension tests on uncoated PDMS fibers showed the strain stiffening behavior that is expected of PDMS (see Figure 15). The uniaxial tests also showed a slight hysteresis (when the tension is relaxed the PDMS does not follow the same path as when it is put under increasing tension).
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Stress ( Pa )
12 000
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
Fig. 15 Uniaxial tension engineering stress vs. engineering strain of uncoated PDMS
Fiber. Blue indicates the first cycle and red indicates the second cycle.
1.2
Strain
At high strains there was extra strain stiffening for the first cycle of PDMS. This was not seen for successive cycles, rather, successive cycles behave exactly as the second cycle.
The coated PDMS stress-strain behavior was significantly different than the uncoated fibers and it looked similar to the mussel fiber behavior as shown in Figure 5.
The first cycle showed a stiff linear elastic region followed by yielding as the coating breaks. The second cycle, while the coating is broken, looked just like the regular PDMS second cycle (see Figure 16).
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2 Cycle Stress Strain Curve of Coated PDMS Fiber
8000
6000
4000
2000
Stress ( Pa )
14 000
12 000
10 000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 16 Uniaxial tension engineering stress vs. engineering strain of coated PDMS fiber with high pH coating. Blue indicates the first cycle and red indicates second cycle. The
first cycle shows the stiffness of the coating and then yielding as the coating breaks. The second cycle, when the coating is broken resembles the behavior of uncoated PDMS.
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Stress ( Pa )
2 Cycle Stress Strain Curve of Uncoated and Coated PDMS Fiber
14 000
12 000
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 17 Uniaxial tension engineering stress vs. engineering strain of coated and uncoated
PDMS fibers. Blue indicates the first cycle and red indicates the second cycle. The faded dots are the uncoated fiber, while the bolded are the coated fiber. The first cycle shows the stiffness of the coating and then the yielding as the coating breaks, the shoulder and stiffness are not present in the first cycle of uncoated PDMS. The second cycle, when the coating is broken, resembles the behavior of uncoated PDMS.
After the dried coating was broken in the uniaxial tension test, it was hydrated in a humid atmosphere for an hour and then pulled on again to show healing and the recovery of the stiffness (see Figure 18).
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Stress ( Pa )
2 Cycle Stress Strain Curve of Rehealed Coated PDMS Fiber
12 000
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 18 Uniaxial tension engineering Stress vs. engineering strain of healed coated fiber.
The blue indicates the first cycle and the red indicates the second cycle. The first cycle shows stiffness of the coating and then yielding as the coating breaks. The healing was not consistent as shown by the sharp peaks as the coating breaks in different places. The second cycle when the coating is broken resembles the behavior of the uncoated PDMS.
Unlike the dried fibers, after the healed fibers were removed from the humid environment they were only dried for approximately 10 minutes. The mechanical behavior of a semi-dry coating (that also had only dried for approximately 10 minutes) was compared with that of the re-healed coating (see Figure 19), both show the same yielding stress, implying full recovery of the stiffness of the coating after healing.
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2 Cycle Stress Strain Curve of Rehealed and Semi Dry Coated PDMS Fiber
Stress ( Pa )
12 000
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 19 Uniaxial tension engineering stress vs. engineering strain of healed coated fiber and semi-dry coated fiber. Blue indicates the first cycle and red indicates the second cycle. Faded dots are the semi-dry coated fiber, while bolded are the re-healed coated fiber. First cycle of both show the stiffness of the coating and then yielding as the coating breaks, the stiff region and shoulder are not present in the first cycle of the uncoated
PDMS. The second cycle when the coating is broken, resembles the behavior of the uncoated PDMS.
In order to help isolate the effect of the coating, the stiffness of the coating was varied by dipping the fiber in Bis-Tris buffer (pH 6.57) instead of NaOH. This turned the fiber a deep blue color and when hydrated, the coating was sticky as compared with the
NaOH dipped coating. The Bis-Tris coated fiber was only dried for approximately an hour before mechanical testing. Figure 20 shows the mechanical behavior of the Bis-Tris coating where the first cycle shows the same stiff region and then yielding, while the second cycle looks similar to uncoated PDMS behavior.
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Stress ( Pa )
2 Cycle Stress Strain Curve of Bis Tris Coated PDMS Fiber
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 20 Uniaxial tension engineering stress vs. engineering strain of coated PDMS fiber with pH 6.57 coating. Blue indicates the first cycle and red indicates the second cycle.
The first cycle shows the stiffness of the coating and then yielding as the coating breaks.
The second cycle, when the coating is broken, resembles the behavior of the uncoated
PDMS.
A comparison of the mechanical properties of the NaOH dipped coating and Bis-
Tris dipped coating are shown in Figure 21.
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2 Cycle Stress Strain Curve of NaOH and Bis Tris Coated PDMS Fibers
Stress ( Pa )
14 000
12 000
10 000
8000
6000
4000
2000
0.2
0.4
0.6
0.8
1.0
1.2
Strain
Fig. 21 Uniaxial tension engineering stress vs. engineering strain of NaOH and Bis-Tris coated PDMS fibers. Red indicates the NaOH fiber and blue indicates the Bis-Tris fiber.
The bolded, darker dots indicate first cycle and lighter, faded dots indicate the second cycle. The difference in the yielding stress and slope of linear elastic region in the first cycle shows the relative stiffness of the coating. For both, the first cycle shows stiffness
of coating and then yielding as the coating breaks. The second cycle when the coating is broken resembles the behavior of the uncoated PDMS.
The slopes of the linear-elastic regions in the first cycles of each coating were calculated using Mathematica by finding the slope of the best-fit line. Table 2 shows the summary of the results. The magnitudes of the Young’s Moduli depend on the absolute diameter of the fibers, which is suspected to vary by as much as 75 microns from fiber to fiber. While the absolute values of the Young’s Moduli may not be precise, the relative difference between them is the focus to show how varying the stiffness of the coating varies with stiffness of the fiber.
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Table 2 : Young’s Moduli (in kPa) of Fibers with Different Coatings.
Fiber Coating Type Young’s Modulus First Cycle (kPa)
Tris-Bis Coating
NaOH Coating
Semi-Dry NaOH Coating
34.9 +/- 1.63
269 +/- 29.7
112 +/- 5.28
Rehealed NaOH Coating (semi-dry) 121 +/- 5.49
Optical Microscopy was performed on the dry, broken, hydrated, and re-healed fibers as shown in Figures 22, 23, and 24.
NaOH  Coating
NaOH  Coating   Uncoated
Fig. 22 Dry NaOH dipped coated PDMS fiber at two different magnifications. Uncoated fiber is shown (at lower magnification) for comparison.
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Fig. 23 Dry, Broken NaOH dipped coated PDMS fiber at two different magnifications.
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Hydrated,
Broken  Coating
After  10  Min.   After  30  Min.
Fig. 24 Broken NaOH coated fiber was hydrated by dipping in deionized water. Images show healing progression, from immediately after hydration, to 10 minutes with mostly healed gel that has just begun to dry, to 30 minutes, which shows fully healed and mostly dried gel.
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5. Discussion
Coating PDMS fibers with the metal-coordinated PEG gels developed in the Holten-
Andersen group led to a significant change in mechanical behavior of the fibers. This mechanical behavior, as hypothesized, mimicked that of the mussel threads, which also contain a soft core and stiff cuticle (Figure 5 and Figure 17). When the coating is intact, the fibers show a stiff, linear elastic region, followed by a distinctive yield point (Figure
17). The bumps after the first initial yield point are believed to indicate the further breakdown of the coating. The re-healed coatings can regain full stiffness but are often less even and this is shown by the increase in the number and amplitude of the bumps after the initial yielding of the coating (Figure 18). Hydrated gels and coatings are less stiff and this can be seen by the stiffness of the fibers when measured in the semi-dry state as opposed to being fully dry (Table 2). Because the coatings are pulled on in a dry atmosphere they did not exhibit self-healing without being put back in a humid atmosphere.
When the fibers were dipped in a medium pH buffer (Bis-Tris buffer) after polymer/FeCl
3
adsorption, the fibers turned a deep blue and exhibited the same qualitative mechanical behavior as that of the high pH coated fibers (Figure 20). As expected, the stiffness was smaller than the high pH coated fibers (Figure 21). This is further evidence of the mechanical properties of the coating contributing significantly to the mechanical behavior of the fibers.
Future considerations include the fact that the custom set-up did not allow varying the strain rate for the tension test. As we are working with a coating whose mechanical
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properties change as a function of strain rate, in the future we will like to do multiple tests at various strain rates.
Measuring the diameter of the fibers and thickness of the coating was done with an optical microscope and can be subjective since the researcher is the one who places the location of the cursors. In the future we would like to measure thickness of the fibers and the coating more accurately by looking a cross-section of the fiber in an SEM or by using other methods that may be less subjective. Having more accurate measurements for the dimensions of this system can help us tune the ratio of coating thickness to fiber diameter, in order to see how that affects the overall mechanical properties of the coated fibers.
Another open question is the fact that at high strains, in the first cycle, the uncoated PDMS showed extra strain stiffening as compared with later cycles. We would like to explore the reason for this further and then help isolate that effect to more accurately understand the full mechanical effect of the coating in the first strain cycle.
Lastly, we would like to measure the tensile behavior of the dried gels in the bulk.
Previously, the properties of the gels have only been measured in the hydrated state, but as we measured the fibers while the coating was dry, it would be interesting to see how the Young’s Modulus of the dried gel compares with that of the coated fibers.
6. Conclusions
This study sought to develop a synthetic analog of a mussel byssal thread in order to isolate the origins of its unique mechanical properties. We coated PDMS fibers with a thin layer of a metal-coordinated PEG gel. We then measured their mechanical properties
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using a uniaxial tension tester. We successfully showed that a stiff coating on a soft elastomeric fiber does in fact have the same behavior as that of a mussel thread, proving that the cuticle of the mussel thread could contribute dramatically to the overall mechanical properties. By changing the pH of the coating it is possible to tune the stiffness of the fiber as shown by the change from the NaOH dipped and Bis-Tris dipped fiber. After the coating has failed catastrophically, placing it in a humid or hydrated environment allows the coating to self-heal and regain its stiffness.
Possible applications for these fibers include any instance where a fiber needs to be stiff but may undergo high strain intermittently. This would allow the fiber to be stiff and in case the fiber coating breaks under a sudden strain it can heal and regain stiffness.
As evidenced by the big area under the stress-strain curve in the first cycle, coated fibers can dissipate a lot of energy and then heal and recover over time to once again dissipate a lot of energy. These fibers, on a much larger scale, could be used to tie up boats or any other device that, like a mussel, must stay attached to a surface even under a heavy barrage of waves. Another application could be medical sutures. If a patient moves or strains in such a way that would normally stretch out or tear their sutures, these fibers would dissipate the energy and then over a short amount of time regain their stiffness and mechanical integrity. Lastly, the fiber stiffness can change dramatically based on pH so the fibers could be used as a way of mechanically detecting a change in pH in certain systems. Future work includes refining the coating process to be more consistent and yield more even coatings as well as exploring a variety of metal-coordination bond crosslinked coatings that could be used to detect different properties (like color, pH etc.) as a function of strain.
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