In many animals, the bonding of tendon and cartilage to bone is extremely tough (e.g.,
interfacial toughness ~ 800 Jm−2)
1,2
, yet such tough interfaces have not been achieved between synthetic hydrogels and
nonporous surfaces of engineered solids
3-9
. Here, we report a strategy to design transparent and conductive bonding of synthetic
hydrogels containing 90% water to nonporous surfaces of diverse solids including glass,
silicon, ceramics, titanium and aluminum. The design strategy is to anchor the long-chain
polymer networks of tough hydrogels covalently to nonporous solid surfaces, which
can be achieved by the silanation of such surfaces. Compared with physical interactions,
the chemical anchorage results in a higher intrinsic work of adhesion and in significant
energy dissipation of bulk hydrogel during detachment, which lead to interfacial toughness
over 1000 Jm−2. We also demonstrate applications of robust hydrogel-solid hybrids,
including hydrogel superglues, mechanically protective hydrogel coatings, hydrogel
joints for robotic structures and robust hydrogel-metal conductors.
Hybrid combinations of hydrogels and solid materials including metals, ceramics, glass,
silicon and polymers are used in areas as diverse as biomedicine
10,11
, adaptive and responsive materials
12
, antifouling
13
, actuators for optics
14
and fluidics
15
, and soft electronics
16
and machines
17
. Although hydrogels with extraordinary physical properties have been recently developed
3-9
, the weak and brittle bonding between hydrogels and solid materials often severely
hampers their integrations and functions in devices and systems. Whereas intense efforts
have been devoted to the development of tough hydrogel-solid interfaces, previous
works are generally limited to special cases with porous solid substrates
18
. Robust adhesion of dry elastomers to nonporous solids has been achieved
19-22
, but such adhesion is not applicable to hydrogels that contain significant amounts
of water
23
. The need for general strategies and practical methods for the design and fabrication
of tough hydrogel bonding to diverse solid materials has remained a central challenge
for the field.
Here, we report a design strategy and a set of simple fabrication methods to give
extremely tough and functional bonding between hydrogels and diverse solids, including
glass, silicon, ceramics, titanium and aluminum, to achieve interfacial toughness
over 1000 Jm−2. The new design strategy and fabrication methods do not require porous
or topographically patterned surfaces of the solids and allow the hydrogels to contain
over 90 wt. % of water. The resultant tough bonding is optically transparent and electrically
conductive. In addition, we demonstrate novel functions of hydrogel-solid hybrids
uniquely enabled by the tough bonding, including tough hydrogel superglues, hydrogel
coatings that are mechanically protective, hydrogel joints for robotic structures
and robust hydrogel-metal conductors. The design strategy and simple yet versatile
method open new avenues not only to addressing fundamental questions on hydrogel-solid
interfaces in biology, physics, chemistry and material science but also to practical
applications of robust hydrogel-solid hybrids in diverse areas
10-17,24
.
The proposed strategy to design tough hydrogel-solid bonding is illustrated in
Fig. 1
. Since interfacial cracks can kink and propagate in relatively brittle hydrogel matrices
(see
Video S1
, for example), the design of tough hydrogel-solid bonding first requires high fracture
toughness of the constituent hydrogels
18
. Whereas tough hydrogels generally consist of covalently-crosslinked long-chain polymer
networks that are highly stretchable and other components that dissipate mechanical
energy under deformation
25,26
, it is impractical to chemically bond all components of the hydrogels on solid surfaces.
We propose that it is sufficient to achieve relatively tough hydrogel-solid bonding
by chemically anchoring the long-chain polymer network of a tough hydrogel on solid
surfaces as illustrated in
Fig. 1a
. When such a chemically-anchored tough hydrogel is detached from a solid, the scission
of the anchored layer of polymer chains gives the intrinsic work of adhesion Γ0
27
(
Fig. 1b
). Meanwhile, the tough hydrogel around the interface will be highly deformed and
thus dissipate a significant amount of mechanical energy
20-22,28
, which further contributes to the interfacial toughness by Γ
D
(
Fig. 1b
). Neglecting contributions from mechanical dissipation in the solid and friction
on the interface, we can express the total interfacial toughness of the hydrogel-solid
bonding as
(1)
Γ
=
Γ
0
+
Γ
D
In Eq. (1), Γ0 may be much lower than Γ
D
for tough hydrogel-solid bonding, but it is still critical to chemically anchor long-chain
polymer networks of tough hydrogels on the solids surfaces. This is because the chemical
anchorage gives a relatively high intrinsic work of adhesion Γ0 (compared with physically
attached cases), which maintains cohesion of the hydrogel-solid interface while allowing
large deformation and mechanical dissipation to be developed in the bulk hydrogel
to give high values of Γ
D
(
Fig. 1b
).
To test the proposed design strategy, we use a functional silane, 3-(Trimethoxysilyl)
Propyl Methacrylate (TMSPMA), to modify the surfaces of glass, silicon wafer, titanium,
aluminum and mica ceramic (
Fig. 2a
)
29
. We then covalently crosslink the long-chain polymer network of polyacrylamide (PAAm)
or polyethylene glycol diacrylate (PEGDA) to the silanes on the modified surfaces
of various solids. (See Methods and
Fig. S1a
for details on the modification and anchoring process.) To form tough hydrogels, the
long-chain polymer network is interpenetrated with a reversibly-crosslinked network
of alginate, chitosan or hyaluronan
6,26
, in which the reversible crosslinking and chain scission dissipates mechanical energy
as illustrated in
Fig. 1a
and
Fig. 1b
. (See Methods for details on the formula and procedures to make various hydrogels.)
As control samples, we chemically anchor a pure PAAm or PEGDA hydrogel on silanized
solid surfaces, and physically attach the pure PAAm or PEGDA hydrogel and corresponding
tough hydrogels on untreated solid surfaces as illustrated in
Fig. 1c
. The shear moduli of all hydrogels in as-prepared states are set to be on the same
level, ~ 30 kPa, by controlling the crosslinking densities in the hydrogels.
The samples of tough (e.g., PAAm-alginate) and common (e.g., PAAm) hydrogels chemically
anchored and physically attached on glass substrates all look identical, as they are
transparent with transmittance over 95%. We demonstrate the transparency of a sample
in
Fig. 2b
by placing it above the “MIT MECHE” color logo. We then carry out a standard 90-degree
peeling test with a peeling rate of 50 mm/min to measure the interfacial toughness
between hydrogel sheets with thickness of 3 mm and the glass substrates. A thin (~
25 μm thick) and rigid glass film backing is attached to the other surface of the
hydrogel sheet to prevent its elongation along the peeling direction. Thus, the measured
interfacial toughness is equal to the steady-state peeling force per width of the
hydrogel sheet
30
. (See Methods and
Fig. S2
for details of the peeling test.)
Video S1
and
Figs. 2c-e
demonstrate the peeling process of the common hydrogel chemically anchored on the
glass substrate. It can be seen that a crack initiates at the hydrogel-solid interface,
kinks into the brittle hydrogel, and then propagates forward. The measured interfacial
toughness is 24 Jm−2 (
Figs. 2i
), limited by the hydrogel's fracture toughness, validating that tough hydrogels are
indeed critical in the design of tough hydrogel-solid interfaces.
Video S2 and Figs. S3
demonstrate a typical peeling process of a tough or common hydrogel physically attached
on the glass substrate. Different from the previous process shown in
Video S1
and
Figs. 2c-e
, the crack can easily propagate along the interface without kinking or significantly
deforming the hydrogel, giving very low interfacial toughness of 8 Jm−2 (
Figs. 2i
).
Video S3
and
Figs. 2f-h
demonstrate the peeling process of the tough hydrogel with its long-chain network
chemically anchored on the glass substrate. As the peeling force increases, the hydrogel
around the interfacial crack front highly deforms and subsequently becomes unstable
31,32
, developing a pattern of fingers before the interfacial crack can propagate. When
the peeling force reaches a critical value, the crack begins to propagate along the
hydrogel-solid interface (
Fig. 2g
). During crack propagation, the fingers coarsen with increasing amplitude and wavelength,
and then detach from the substrate (
Fig. 2h
). The measured interfacial toughness is over 1500 Jm−2 (
Figs. 2i
), superior to natural counterparts such as tendons and cartilages on bones. As control
cases, we vary the thickness of the tough hydrogel sheet from 1.5mm to 6mm, and obtain
similar values of interfacial toughness (
Fig. S4
). We further vary the peeling rate of the test from 200 mm/min to 5 mm/min, and find
that the measured interfacial toughness decreases from 3100 Jm−2 to 1500 Jm−2 accordingly
(
Fig. S5
). It is evident that the measured interfacial toughness of chemically anchored PAAm-alginate
hydrogel is rate-dependent, possibly due to viscoelasticity of the hydrogel (
Fig. S5
). Furthermore, the peeling rate used in the current study (50 mm/min) gives an interfacial
toughness around the lower asymptote, which reflects the effects of intrinsic work
of adhesion and rate-independent dissipation such as Mullins effect
33
.
To understand the phenomena described above and the interfacial toughening mechanism,
we develop a finite-element model to simulate the peeling process of a hydrogel sheet
from rigid substrate under plane-strain condition. In the model, the intrinsic work
of adhesion Γ0 is characterized by a layer of cohesive elements and the dissipative
property of the PAAm-alginate hydrogel is characterized by Mullins effect
33
. (See Supplementary Information and Fig. S13-19 and Video S8, S9
for details of the model.)
Figure 2j
gives the calculated relation between the intrinsic work of adhesion Γ0 and the interfacial
toughnessΓ. It is evident that the interfacial toughness increases monotonically with
the intrinsic work of adhesion, which is effectively augmented by a factor determined
by the dissipative properties of the hydrogel. We also vary the thickness of the PAAm-alginate
hydrogel in the model from 0.8 mm to 6 mm and find that the calculated interfacial
toughness is approximately the same, consistent with the experimental observation
(
Fig. S4 and Fig. S19
). As a control case, we model the peeling test of a hydrogel with no Mullins effect
(i.e., no dissipation) but otherwise the same mechanical properties as the PAAm-alginate
hydrogel. From
Fig. 2j
, it is evident that the calculated interfacial toughness for the control case is
approximately the same as the prescribed the intrinsic work of adhesion. Although
the current finite-element model does not account for the effects of fingering instability
or viscoelasticity on mechanical dissipation, it clearly demonstrates that high values
of the intrinsic work of adhesion and significant mechanical dissipation of the bulk
hydrogel are key to design tough bonding of hydrogels on solids (
Fig. 2j
).
The proposed design strategy and fabrication methods for tough hydrogel-solid bonding
is applicable to multiple types of nonporous solid materials.
Figure 3a
shows that the measured interfacial toughness is consistently high for the PAAm-alginate
tough hydrogel chemically anchored on glass (1500 Jm−2), silicon (1500 Jm−2), aluminum
(1200 Jm−2), titanium (1250 Jm−2) and ceramics (1300 Jm−2). Replacing the PAAm-alginate
with other types of tough hydrogels including PAAm-hyaluronan, PAAm-chitosan, PEGDA-alginate
and PEGDA-hyaluronan still yields relatively high interfacial toughness, 148 – 820
Jm−2, compared with the interfacial toughness in controlled cases, 4.4 – 16 Jm−2 (
Fig. S6
). (See Methods for details on other hydrogel-solid bonding). To explain the difference
in interfacial toughness of different tough hydrogels with long-chain networks chemically
anchored on substrates, we measure the maximum dissipative capacity and fracture toughness
of these hydrogels (
Fig. S7
). It can be seen that, for tough hydrogels with the same chemically-anchored long-chain
networks (i.e., PAAm-based or PEGDA-based tough hydrogels), both the interfacial toughness
and fracture toughness increase with the maximum dissipative capacity of the hydrogels
(
Fig. S7
). These results validate that significant energy dissipation in bulk hydrogels is
critical to achieving high interfacial toughness.
Since hydrogels are commonly used in wet environments, we further immerse the PAAm-alginate
hydrogels with PAAm networks anchored on various solid substrates in water for 24
hours to allow the hydrogels to swell to equilibrium states. We find that the anchored
hydrogels do not detach from the solid substrates in swollen state. The interfacial
toughness of the swollen samples is measured using the 90-degree-peeling test. From
Video S4
, it can be seen that the detaching process of the swollen hydrogel is similar to
that of the same hydrogel in as-prepared state (i.e.,
Fig. 2f-h
and
Video S3
). As shown in
Fig. S8b
and
Fig. 3a
, the measured interfacial toughness for swollen hydrogels bonded on glass (1123 Jm−2),
silicon (1210 Jm−2), aluminum (1046 Jm−2), titanium (1113 Jm−2) and ceramics (1091
Jm−2) are consistently high, indicating that the design strategy and fabrication methods
can give tough bonding of hydrogels to diverse solids in wet environment. The slight
decrease in interfacial toughness from as-prepared to swollen hydrogels may be due
to the decrease of dissipative capability of hydrogels
34
and/or the residual stress generated in the hydrogels during swelling.
The above results prove that chemically anchoring the long-chain networks of tough
hydrogels on solid substrates can lead to tough hydrogel-solid bonding. Since the
tough hydrogels used in the current study are composed of covalently-crosslinked long-chain
networks and reversibly-crosslinked dissipative networks, it is also important to
know the effects of chemically anchoring dissipative networks on the resultant interfacial
toughness. We chemically anchor the dissipative networks (i.e., alginate or hyaluronan)
in PAAm-alginate, PEGDA-alginate and PEGDA-hyaluronan hydrogels on glass substrates
using EDC-Sulfo NHS chemistry, and then measure the interfacial toughness of resultant
samples (see
Fig. S1b-c
and Methods for details on anchoring alginate and hyaluronan). As shown in
Fig. S9a-b
, the measured interfacial toughness for PEGDA-alginate and PEGDA-hyaluronan hydrogels
with dissipative network anchored on substrates is 13 Jm−2 and 16 Jm−2 respectively
– much lower than the values of the same hydrogels with long-chain networks anchored
on substrates (365 Jm−2 and 148 Jm−2 respectively). On the other hand, the interfacial
toughness for PAAm-alginate hydrogel with alginate anchored on substrate is 1450 Jm−2
(
Fig. S9c
), similar to the value of PAAm-alginate hydrogel with PAAm anchored on substrate
(1500 Jm−2). It is evident that anchoring either long-chain or dissipative networks
gives similar interfacial toughness in PAAm-alginate hydrogel but very different values
in PEGDA-alginate (or PEGDA-hyaluronan) hydrogel (
Fig. S9
). The different results obtained in PAAm-alginate and PEGDA-alginate (or PEGDA-hyaluronan)
hydrogels may be due to much stronger interactions between long-chain and dissipative
networks in PAAm-alginate hydrogel than in PEGDA-alginate and PEGDA-hyaluronan hydrogels.
6,35
To compare our results with existing works in the field, we summarize the interfacial
toughness of various hydrogel-solid bonding commonly used in engineering applications
vs. water concentrations in those hydrogels in
Fig. 3b
. (See supplementary materials and Fig. S10
for detailed references). Whereas our approach allows the PAAm-alginate tough hydrogels
to contain 90 wt. % of water and does not require porous or topographically patterned
surfaces of the solids, it can achieve extremely high interfacial toughness up to
1500 Jm−2. In comparison, most of synthetic hydrogel bonding has relatively low interfacial
toughness, below 100 Jm−2. Although previous work on hydrogels and animal skin tissues
impregnated in porous substrates gave interfacial toughness up to 1000 Jm−2, the hydrogels
and tissues contains 60 to 80 wt. % water and the requirement of porous solids significantly
limits their applications
18
. Further notably, our fabrication methods of tough hydrogel bonding are relatively
simple compared with previous methods and generally applicable to a wide range of
hydrogels and solid materials.
Owning to its simplicity and versatility, the design strategy and fabrication methods
for tough hydrogel-solid bonding can potentially enable a set of unprecedented functions
of hydrogel-solid hybrids. For example, the tough hydrogels may be used as soft (e.g.,
30 kPa), wet (e.g., with 90% water) and biocompatible
36
superglues for glass, ceramics and Ti, which have been used in biomedical applications.
(See Methods and
Fig. S12
for details on biocompatibility of tough hydrogels bonded on solid surfaces.)
Figure 4a
demonstrates that two glass plates bonded by the tough hydrogel superglue (dimension,
5 cm × 5 cm × 1.5 mm) are transparent, and can readily sustain a weight of 25 kg.
(See Methods for details on fabrication of hydrogel superglue.) As another example,
the tough hydrogel-solid bonding can re-define the functions and capabilities of commonly-used
hydrogel coatings, which are usually mechanically fragile and susceptible to debonding
failure.
Video S5 and Fig. 4b
demonstrates the process of shattering and consequently deforming a silicon wafer
coated with a layer of chemically-anchored tough hydrogel. Thanks to the high toughness
of the hydrogel and interface, the new coating prevents detachment of the shattered
pieces of silicon wafer and maintains integrity of the hydrogel-solid hybrid even
under high stretch of 3 times, demonstrating hydrogel coating's new capability of
mechanical protection and support. (See Methods for details on fabrication of mechanically
protective hydrogel coating.) The tough hydrogel bonding can also be used as compliant
joints in mechanical and robotic structures.
Video S6 and Fig. 4c
demonstrate an example of four ceramic bars bonded with the chemically-anchored tough
hydrogels. The compliance of the hydrogel combined with high toughness of the bonding
enables versatile modes of deformation of the structure. (See Methods for details
on fabrication of hydrogel joints.) In addition, the tough hydrogel bonding is electrically
conductive and thus can provide a robust interface between hydrogel ionic conductors
and metal electrodes
16
. Existing hydrogel-metal interfaces usually rely on conductive copper tapes whose
robustness is uncertain.
Video S7 and Fig. 4d
demonstrate that the hybrid combination of a tough hydrogel chemically anchored on
two titanium electrodes is conductive enough to power a LED light, even when the hydrogel
is under high stretch of 4.5 times. In addition, the conductivity of the hydrogel-metal
hybrid maintains almost the same even after 1000 cycles of high stretch up to 4 times.
(See Methods and
Fig. S11
for details on fabrication of robust hydrogel-metal conductors and measurement on
its electrical conductivity.)
In summary, we demonstrate that the chemical anchorage of long-chain polymer networks
of tough hydrogels on solid surfaces represents a general strategy to design tough
and functional bonding between hydrogels and diverse solids. Following the design
strategy, we use simple methods such as silane modification and EDC chemistry to achieve
tough, transparent and conductive bonding of hydrogels to glass, ceramic, silicon
wafer, aluminum and titanium with interfacial toughness over 1000 Jm−2 — superior
to the toughness of tendon-bone and cartilage-bone interfaces. The ability to fabricate
extremely robust hydrogel-solid hybrids makes a number of future research directions
and applications possible. For example, electronic devices robustly embedded in (or
attached on) tough hydrogels may lead to a new class of stretchable hydrogel electronics,
which are softer, wetter and more biocompatible than existing ones based on dry elastomers
matrices. New microfluidic systems based on tough hydrogels bonded on nonporous substrates
may be able to sustain high flow rate, high pressure and large deformation to better
approximate physiological environments than existing microfluidics based on weak or
brittle hydrogels. Neural probes coated with tough and bio-compatible hydrogels with
reduced rigidity
34
may be used to better match the mechanical and physiological properties of brains,
spinal cords and peripheral nervous systems.
Methods
Materials
Unless otherwise specified, the chemicals used in the current work were purchased
from Sigma-Aldrich and used without further purification. For the long-chain polymer
networks in the hydrogels, acrylamide (AAm; Sigma-Aldrich A8887) was the monomer used
for the polyacrylamide (PAAm) networks, and 20 kDa polyethylene glycol diacrylate
(PEGDA) was the macromonomer used for the PEGDA networks. The PEGDA macromonomers
were synthesized based on a previously reported protocol
37
using polyethylene glycol (PEG; Sigma-Aldrich 81300), acryloyl chloride (Sigma-Aldrich
549797), triethylamine (TEA; Sigma-Aldrich 471283), dichloromethane (Sigma-Aldrich
270997), sodium bicarbonate (Sigma-Aldrich S6014), magnesium sulfate (Sigma-Aldrich
M7506) and diethyl ether (Sigma-Aldrich 346136). For the polyacrylamide (PAAm) hydrogel,
N,N-methylenebisacrylamide (MBAA; Sigma-Aldrich 146072) was used as crosslinker, ammonium
persulfate (APS; Sigma-Aldrich A3678) as thermal initiator and N,N,N’,N’-tetramethylethylenediamine
(TEMED; Sigma-Aldrich T9281) as crosslinking accelerator. For the PEGDA hydrogel,
2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959; Sigma-Aldrich
410896) was used as photo initiator. For the dissipative polymer networks in tough
hydrogels, a number of ionically crosslinkable biopolymers were used including sodium
alginate (Sigma-Aldrich A2033) ionically crosslinked with calcium sulfate (Sigma-Alginate
C3771), chitosan (Sigma-Aldrich 740500) ionically crosslinked with sodium tripolyphosphate
(TPP; Sigma-Aldrich 238503), and sodium hyaluronan (HA; Sigma-Aldrich H5542) ionically
crosslinked with iron chloride (Sigma-Aldrich 157740). For chemical modification of
various solid materials, functional silane 3-(Trimethoxysilyl) propyl methacrylate
(TMSPMA; Sigma-Aldrich 440159) and acetic acid (Sigma-Aldrich 27225) were used. For
anchoring alginate and hyaluronan on solid substrates, (3-Aminopropyl) Triethoxysilane
(APTES, Sigma-Aldrich 440140), N-Hydroxysulfosuccinimide (Sulfo-NHS, Sigma-Aldrich
56485), N-(3-Dimethylaminopropyl)-N′ ethylcarbodiimide (EDC, Sigma-Aldrich 39391),
2-(N-Morpholino)ethanesulfonic acid (MES, Sigma-Aldrich M3671) and Sodium Chloride
(Sigma-Aldrich 746398) were used.
In the 90-degree peeling experiments, borosilicate glass (McMaster Carr), silicon
wafers with a thermal oxidized layer (UniversityWafer), nonporous glass mica ceramic
(McMaster Carr), anodized aluminum (Inventables) and titanium (McMaster Carr) plates
were used as the solid substrates. As a stiff backing for the hydrogel sheet, ultrathin
glass films (25 μm; Schott Advanced Optics) were used together with transparent Scotch
tape (3M). In the conductive hydrogel-metal bonding experiments, sodium chloride solution
was used as an electrolyte.
Synthesis of various tough hydrogels
The PAAm-alginate tough hydrogel was synthesized by mixing 10 mL of a carefully degassed
aqueous precursor solution (12.05 wt. % AAm, 1.95 wt. % sodium alginate, 0.017 wt.
% MBAA and 0.043 wt. % APS) with calcium sulfate slurry (0.1328 times the weight of
sodium alginate) and TEMED (0.0025 times the weight of AAm)
6
. The mixture was mixed quickly and poured into a laser-cut Plexiglass acrylic mold.
The lid of the mold included an opening for the functionalized substrates to be in
contact with hydrogel precursor solution. The gel was crosslinked by UV light irradiation
for an hour (254 nm UV with 6.0 mWcm−2 average intensity; Spectrolinker XL-1500).
The PAAm-hyaluronan tough hydrogel was synthesized by mixing 10 mL of degassed precursor
solution (18 wt. % AAm, 2 wt. % HA, 0.026 wt. % MBAA and 0.06 wt. % APS) with 60 μL
of iron chloride solution (0.05 M) and TEMED (0.0025 times the weight of AAm). The
PAAm-chitosan tough hydrogel was synthesized by mixing 10 mL of degassed precursor
solution (24 wt. % AAm, 2 wt. % chitosan, 0.034 wt. % MBAA and 0.084 wt. % APS) with
60 μL of TPP solution (0.05 M) and TEMED (0.0025 times the weight of AAm). The PEGDA-alginate
tough hydrogel was synthesized by mixing 10 mL of a degassed precursor solution (20
wt. % PEGDA and 2.5 wt. % sodium alginate) with calcium sulfate slurry (0.068 times
the weight of sodium alginate) and Irgacure 2959 (0.0018 the weight of PEGDA). The
PEGDA-hyaluronan tough hydrogel was synthesized by mixing 10 mL of a degassed precursor
solution (20 wt. % PEGDA and 2 wt. % HA) with 60 μL of iron chloride solution (0.05
M) and Irgacure 2959 (0.0018 the weight of PEGDA). The curing procedure was identical
to the PAAm-alginate tough hydrogel.
Common PAAm hydrogel was synthesized by mixing 10 mL of degassed precursor solution
(23 wt. % AAm, 0.051 wt. % MBAA and 0.043 wt. % APS) and TEMED (0.0025 times the weight
of AAm). The curing procedure was identical to the PAAm-alginate tough hydrogel. Note
that the modulus of the common PAAm hydrogel was tuned to match the PAAm-alginate
tough hydrogel's modulus (30 kPa) based on the previously reported data
6
.
Chemically anchoring PAAm and PEGDA on various solid surfaces
The surface of various solids was functionalized by grafting functional silane TMSPMA.
Solid substrates were thoroughly cleaned with acetone, ethanol and deionized water
in that order, and completely dried before the next step. Cleaned substrates were
treated by oxygen plasma (30 W with 200 mTorr pressure; Harrick Plasma PDC-001) for
5 min. Right after the plasma treatment, the substrate surface was covered with 5
mL of the silane solution (100 mL deionized water, 10 μL of acetic acid with pH 3.5
and 2 wt. % of TMSPMA) and incubated for 2 hours at room temperature. Substrates were
washed with ethanol and completely dried. Functionalized substrates were stored in
low humidity conditions before being used for experiments.
During oxygen plasma treatment of the solids, oxide layers on the surfaces of the
solids (silicon oxide on glass and silicon wafer, aluminum oxide on aluminum, titanium
oxide on titanium, and metal oxides on ceramics) react to hydrophilic hydroxyl groups
by oxygen radicals produced by oxygen plasma. These hydroxyl groups on the oxide layer
readily form hydrogen bonds with silanes in the functionalization solution generating
a self-assembled layer of silanes on the oxide layers
38
. Notably, the methoxy groups in TMSPMA are readily hydroxylated in acidic aqueous
environment and formed silanes. These hydrogen bonds between surface oxides and silanes
become chemically stable siloxane bonds with removal of water, forming strongly grafted
TMSPMA onto oxide layers on various solids
39
.
Grafted TMSPMA has a methacrylate terminal group which can copolymerize with the acrylate
groups in either AAm or PEGDA under free radical polymerization process, generating
chemically anchored long-chain polymer network onto various solid surfaces
40
. Since the long-chain polymer networks in hydrogels are chemically anchored onto
solid surfaces via strong and stable covalent bonds, the interfaces can achieve higher
intrinsic work of adhesion than physically attached hydrogels. The silane functionalization
chemistry is summarized in
Fig. S1a
.
Chemically anchoring alginate and hyaluronan on various solid surfaces
We anchored alginate and hyaluronan via EDC-Sulfo NHS chemistry following previously
reported protocols
41,42
(
Fig. S1b-c
). Glass substrates were cleaned and oxygen plasma treated following the abovementioned
procedures and covered with 5 mL of the amino-silane solution (100 mL deionized water,
2 wt. % of APTES) and incubated for 2 hours at room temperature. Substrates were washed
with ethanol and completely dried. The amino-silane functionalized glass substrates
were further incubated in either alginate anchoring solution or hyaluronan anchoring
solution (100 mL of aqueous MES buffer (0.1 M MES and 50 mM sodium chloride), 1 wt.
% alginate or hyaluronan, Sulfo-NHS (molar ratio of 30:1 to either alginate or hyaluronan)
and EDC (molar ratio of 25:1 to either alginate or hyaluronan)) for 24 hours. Incubated
glass substrates were finally washed with deionized water and completely dried before
use.
Interfacial toughness measurement
All tests were conducted in ambient air at room temperature. The hydrogels and hydrogel-solid
interfaces maintain consistent properties over the time of the tests (i.e., ~ a few
minutes), during which the effect of dehydration is not significant. Whereas long-term
dehydration will significantly vary the properties of hydrogels, adding highly hydratable
salts into the hydrogels can enhance their water retention capacity
43
. The interfacial toughness of various hydrogel-solid bonding was measured using the
standard 90-degree peeling test (ASTM D 2861) with mechanical testing machine (2 kN
load cell; Zwick / Roell Z2.5) and 90-degree peeling fixture (TestResources, G50).
All rigid substrates were prepared with 7.62 cm in width, 12.7 cm in length and 0.32
cm in thickness. Hydrogels were cured on the solid substrates within Plexiglass acrylic
mold with size of 110 mm × 30 mm × 6 mm. As a stiff backing for the hydrogel, TMSPMA
grafted ultrathin glass film was used with an additional protective layer of transparent
Scotch tape (3M) on top of the glass film. Prepared samples were tested with the standard
90-degree peeling test setup (
Fig. S2
). All 90-degree peeling tests were performed with constant peeling speed of 50 mm/min.
The measured force reached a plateau as the peeling process entered steady state,
and this plateau force was calculated by averaging the measured force values in the
steady state region with common data processing software (
Fig. S8a
). The interfacial toughness Γ was determined by dividing the plateau force F by the
width of the hydrogel sheet W. To test the dependence of interfacial toughness on
hydrogel thickness, we carried out a set of 90-degree peeling tests on PAAm-alginate
hydrogels with different thicknesses (1.5 mm ~ 6 mm) chemically anchored on glass
substrates (
Fig. S4a
). For interfacial toughness measurement of fully swollen samples, each peeling test
samples was immersed in deionized water for 24 hours and tested by the standard 90-degree
peeling test (
Fig. S8b
).
To demonstrate the peeling rate dependency of the measured interfacial toughness,
we performed a set of 90-degree peeling tests on PAAm-alginate hydrogles chemically
anchored on glass substrates with varying peeling rates from 5 mm/min (lowest) to
200 mm/min (highest) (
Fig. S5
).
To demonstrate that the proposed strategy and method is generally applicable to multiple
types of hydrogels, we also performed standard 90-degree peeling tests on various
types of tough hydrogels including PAAm-hyaluronan, PAAm-chitosan, PEGDA-alginate
and PEGDA-hyaluronan hydrogels chemically anchored on glass substrates (
Fig. S6a
). The measured interfacial toughness for these tough hydrogels (148 – 820 Jm−2,
Fig. S6b
) was consistently much higher than the interfacial toughness of the control cases
(4.4 – 16 Jm−2,
Fig. S6b
).
Preparation of hydrogel superglue, coating and joints
For the hydrogel superglue, two TMSPMA grafted glass plates (5 cm × 12 cm × 2 cm)
were connected by thin tough hydrogel (5 cm × 5 cm × 1.5 mm) and subjected to weight
up to 25 kg. Weight was applied by hanging metal pieces of known weights with metal
wires. Hydrogel joints were fabricated by curing tough hydrogel using Plexiglass acrylic
mold between four TMSPMA grafted nonporous glass mica ceramic rods (75 mm length with
10 mm diameter) forming an interconnected square structure. To test the robustness
of these hydrogel joints, each joint was twisted and rotated to large angles. Hydrogel
coating was fabricated by curing a thin (1 mm) tough hydrogel layer onto the TMSPMA
grafted thermal oxide silicon wafer (100 μm thickness with 50.8 mm diameter). To test
the hydrogel coating's protective capability, we shattered the wafer with metal hammer
and stretched the hydrogel-coated wafer by hand up to 3 times of its original diameter.
In preparation of samples, we used the PAAm-alginate tough hydrogel. The grafting
of TMSPMA on various solids was conducted as discussed in the previous section.
Electrically conductive hydrogel interface
Ionic tough hydrogel was prepared by curing tough PAAm-alginate hydrogel on two TMSPMA
grafted titanium slabs and then soaking in sodium chloride solution (3 M) for 6 hours.
The electric resistance of the ionic hydrogel-titanium hybrid was measured using the
four-point method
44
. The ionic hydrogel-titanium hybrid was connected in series with a function generator
and galvanometer, and the voltage between titanium slabs was measured with a voltmeter
connected in parallel (
Fig. S11a
). The ratio of the measured voltage to the measured current gave the electric resistance
of ionic hydrogel-titanium hybrid. The resistivity was then calculated using the relation
R = ρL/A for a given geometry of the ionic hydrogel in test where ρ is resistivity,
L length of the gel and A cross-sectional area. The rate of stretch was kept constant
at 100 mm/min using a mechanical testing machine. All electric connections other than
the ionic tough hydrogel-titanium interface were established using conductive aluminum
tapes. Cyclic extension of the ionic tough hydrogel was done by mechanical testing
machine based on predetermined numbers of cycles. The ionic tough hydrogel's ability
to transmit power was tested by lighting up LEDs using AC power source (1 kHz 5V peak-to-peak
sinusoidal).
Figure S11b
illustrates the test setup.
Biocompatibility of tough hydrogel bonding
The biocompatibility of tough hydrogels including PAAm-alginate and PEGDA-alginate
hydrogels has been validated in previous studies
35,36
. In the current study, the biocompatibility of PAAm-alginate hydrogel bonded on silane-grafted
glass was tested in vitro with a live/dead viability assay of hTERT-immortalized human
Mesenchymal Stem Cells (MSCs) (
Fig. S12
). A hydrogel disk was chemically anchored on a glass slide following the abovementioned
procedure using TMSPMA and then swelled in PBS for two days. To focus on the biocompatibility
of the hydrogel-solid interface, the hydrogel was peeled off from the glass slide
to expose the previously bonded interface. Thereafter, both the hydrogel and the glass
slide were placed in 24-well plates with the exposed interfaces facing up (
Fig. S12a). MSCs were seeded at a density of 25,000 cells/well on the exposed interfaces
of hydrogel and glass, and incubated for seven days at 37 °C and 5% CO2 in complete
cell culture media (high-glucose DMEM with 10% FBS, 1mM sodium pyruvate, 1X MEM (non-essential
amino acids), 2mM glutamax, and 100U/mL penicillin-streptomycin) from Life Technologies.
A life/dead staining was performed using the LIVE/DEAD kit for mammalian cells (Life
Technologies) per manufacturer's instructions, and fluorescent images were obtained
using a Leica DMI 6000 microscope with Oasis Surveyor software. As seen in
Fig. S12c
, the MSCs proliferated and survived on the exposed interface of the glass slide.
On the exposed interface of the hydrogel, there was a lower number of cells as the
MSCs did not attach well on the hydrogel, but most cells that attached were alive,
consistent with previous report
36
(
Fig. S12b
). In both cases, the percentage of viable MSCs on the exposed interfaces is over
95 % after seven days of incubation. (It should be noted that although the formed
tough hydrogel-glass interface is biocompatible, the bonding process is not since
AAm monomers used in the process are toxic.)
Supplementary Material
1