A 3D printable and highly stretchable tough hydrogel is developed by combining poly(ethylene glycol) and sodium alginate, which synergize to. Hydrogels are used as scaffolds for tissue engineering, vehicles for drug delivery, actuators for optics and fluidics, and model extracellular matrices for biological. In this investigation, we successfully prepared extremely stretchable, transparent and tough DN hydrogels by using neutral synthetic polymer–poly(vinyl alcohol).
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Hydrogels are used as scaffolds for tissue engineering [ 1 ], vehicles for drug delivery [ 2 ], actuators for optics and fluidics [ 3 ], and model extracellular matrices for biological studies [ 4 ]. The scope of applications, however, is often severely limited by the mechanical behavior of hydrogels [ 5 ]. Most hydrogels do not exhibit high stretchability.
For example, an alginate hydrogel ruptures when stretched to about 1. Some synthetic elastic hydrogels [ 67 ] have achieved stretches in the range of 10—20, but elastic gels are known to reduce achievable stretches markedly when samples contain notches. Despite the exciting achievements, much of the property space of hydrogels remains uncharted.
Here we report hydrogels made of polymers forming networks via ionic and covalent crosslinks. Even for samples containing notches, a stretch of 17 is demonstrated. The toughness is attributed to the synergy of two mechanisms: Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed upon unloading.
The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation.
Highly stretchable and tough hydrogels
Hydrogels with enhanced mechanical properties will expand the scope of their applications. Certain synthetic hydrogels higgly achieved exceptional mechanical behavior. These gels deform elastically. An elastic gel is known to be brittle and notch-sensitive—that is, the high stretchability and strength drop markedly when samples contain notches, or any other features that cause inhomogeneous deformation [ 19 ]. A gel can be made tough and notch-insensitive by introducing energy-dissipating mechanisms.
When the gel is stretched, the short-chain network ruptures and dissipates energy [ 20 ]. The rupture of the short-chain network, however, causes permanent damage. After the first loading the gel does not recover from the damage; hydroggels subsequent loadings the fracture energy is much reduced [ 21 ].
To enable recoverable energy-dissipating mechanisms, several recent works have replaced the sacrificial covalent bonds with noncovalent bonds. In a gel with a copolymer of triblock chains, for example, the end blocks of different chains form glassy domains, and the midblocks of different chains form ionic crosslinks [ 22 ].
When the gel is stretched, the glassy domains remain intact, while the ionic crosslinks break and dissipate energy.
The ionic crosslinks reform during a period of time after the first loading [ 22 ]. Recoverable energy dissipation can also be effected by hydrophobic associations [ 1718 ]. When a gel highoy with hydrophobic bilayers sttretchable a hydrophilic polymer network is stretched, the bilayers dissociate and dissipate energy; upon unloading, the bilayers re-assemble, leading to recovery [ 17 ].
The existing works, however, have demonstrated fracture energy comparable to, or lower than, that of the double-network gels. Here we demonstrate extremely stretchable and tough hydrogels by hivhly two types of crosslinked hydrogrls An alginate chain consists of mannuronic acid M unit and guluronic acid G unitarranged in blocks rich in G units, blocks rich in M units, and blocks of alternating G and M units.
In an aqueous solution, the G blocks on different alginate chains form ionic crosslinks through divalent cations e. By contrast, in a polyacrylamide hydrogel, the polyacrylamide chains form a network by covalent crosslinks. The solution was poured into a glass mold, The gel was then left in a humid box for 1 day to stabilize atretchable reactions.
Hivhly curing, the gel was taken out of the humid box, and water on the surfaces of the gel was removed with N 2 gas for 1 minute. The gel was glued to two clamps made of polystyrene, resulting in specimens of All mechanical tests were performed in air, at room temperature, using a tensile machine Instron model with a N load cell. In both loading and unloading, the rate of stretch was kept constant at 2 per minute.
We stretched an alginate-polyacrylamide hybrid gel over 20 times its original length without rupture Fig. The hybrid gel was also extremely notch-insensitive. When we cut a notch into the gel Fig. At a critical applied stretch, a crack initiated at the front of the notch, and ran rapidly through the entire sample Supplementary Movie 1.
Large, recoverable deformation is demonstrated by dropping a metal ball on a membrane of the gel fixed by circular clamps Supplementary Movie 2. Upon hitting the membrane, the ball hydroggels the membrane greatly and then bounced back. The membrane remained intact, vibrated, and recovered its initial flat configuration after the vibration was damped out. A ball of a higher kinetic energy, however, caused the membrane to rupture after large deformation Supplementary Movie dtretchable.
The alginate-to-acrylamide ratio was 1: The covalent crosslinker, MBAA, was fixed at 0. The ionic crosslinker, CaSO4, was fixed at 0. The extremely stretchable hybrid gels are even more remarkable when compared with their parents: The amounts of alginate and acrylamide in the hybrid gels were kept the same as those in the alginate gel and polyacrylamide gel, respectively.
When the stretch was small, the elastic modulus of the hybrid gel was 29kPa, which was close to the sum of the elastic modulus of the alginate gel 17kPa and that of the polyacrylamide gel 8kPa. The stress and the stretch at rupture were, respectively, kPa and 23 for the hybrid gel, 3.
That is, the properties at rupture of the hybrid gel far exceeded those of either of its parents.
New Process for 3D Printing of Highly Stretchable and Tough Hydrogels
The nominal stress s is defined by the force applied on the deformed gel divided by the cross-sectional area of the undeformed gel. Hybrid gels dissipate energy effectively, as shown by pronounced hysteresis. The area between the loading and unloading curves of a gel gave the energy dissipated per unit volume Fig. The alginate gel exhibited pronounced hysteresis and retained significant permanent deformation after unloading.
In contrast, the polyacrylamide gel showed negligible hysteresis, and the sample fully recovered its original length after unloading. The hybrid gel also showed pronounced hysteresis, but the permanent deformation after unloading was significantly smaller than that of the alginate gel.
The pronounced hysteresis and relatively small permanent deformation of the hybrid gel were further demonstrated by loading several samples to large values of stretch before unloading Fig.
After the first loading and unloading, the hybrid gel was much weaker if the second loading was applied immediately, and recovered somewhat if the second loading was applied 1 day later Fig.
We loaded a sample of the hybrid gel to a stretch of 7, and then unloaded the gel to zero force. The sample was then sealed in a polyethylene bag and submerged in mineral oil to prevent water from evaporation, and stored in a bath of a fixed temperature for a certain period of time. The sample was taken out of the storage and its stress-stretch curve was measured again at room temperature. The internal damage was much better healed by storing the gel at an elevated temperature for some time before reloading Fig.
Gels of various proportions of alginate and acrylamide were prepared to study why the hybrids were much more stretchable and stronger than either of their parents. When the proportion of acrylamide was increased, the elastic modulus of the hybrid gel was reduced Fig. However, the critical stretch at rupture reached the maximum when acrylamide was 89 wt.
New Process for 3D Printing of Highly Stretchable and Tough Hydrogels
A similar trend was stregchable for samples with notches Fig. The toug of ionic hydrogrls covalent crosslinks also strongly affect the mechanical behavior of the hybrid gels Supplementary Figs. Each test was conducted by pulling an unnotched sample to rupture. On the basis of our experimental findings, we discuss mechanisms of deformation and energy dissipation. When an unnotched hybrid gel is subject to a small stretch, the elastic modulus of the hybrid gel is nearly the sum of that of the alginate gel and that of the polyacrylamide gel.
This behavior is further ascertained by viscoelastic moduli determined for the hybrid and pure gels Supplementary Fig. Thus, in the hybrid gel the alginate and the polyacrylamide chains both bear loads.
Moreover, alginate is finely dispersed in the hybrid gel homogeneously, as demonstrated by using fluorescent alginate and by measuring local elastic modulus with atomic force microscopy Supplementary Fig.
The load sharing of the two networks may be achieved by entanglements of the polymers, and by possible covalent crosslinks formed between the amine groups on polyacrylamide chains and the carboxyl groups on alginate chains Fig.
As the stretch increases, the alginate network unzips progressively [ 23 ], while the polyacrylamide network remains intact, so that the hybrid gel exhibits pronounced hysteresis and little permanent deformation. Since only the ionic crosslinks are broken, and the alginate chains themselves remain intact, the ionic crosslinks can reform, leading to the healing of the internal damage.
The relatively low fracture energy of a hydrogel of a single network with covalent crosslinks is understood in terms of the Lake-Thomas model [ 8 ].
Highly stretchable and tough hydrogels
When the gel contains a notch and is stretched, the deformation is inhomogeneous: For the notch to turn into a running crack, only the chains directly ahead the notch needs to break. Once a chain breaks, the energy stored in the entire chain is dissipated. In the ionically crosslinked alginate, fracture proceeds by unzipping ionic crosslinks and pulling out chains [ 24 ].
After one pair of G blocks unzip, the high stress shifts to the neighboring pair of G blocks and causes them to unzip also Supplementary Fig. For the notch in the alginate gel to turn into a running crack, only the alginate chains crossing the crack plane need to unzip, leaving the network elsewhere intact. In both polyacrylamide gel and alginate gel, rupture results from localized damage, leading to small fracture energies.
That a tough material can be made of brittle constituents is reminiscent of transformation-toughening ceramics, as well as composites made of ceramic fibers and ceramic matrices. The toughness of the hybrid gel can be understood by adapting a model well studied for toughened ceramics [ 25 ] and for gels of double networks of covalent crosslinks [ 2627 ]. When a notched hybrid gel is stretched, the polyacrylamide network bridges the crack and stabilizes deformation, enabling the alginate network to unzip over a large region of the gel Supplementary Fig.
The unzipping of the alginate network, in its turn, reduces the stress concentration of the polyacrylamide network ahead the notch. The model highlights the synergy of the two toughening mechanisms: The idea that gels can be toughened by mixing weak and strong bonds has been exploited in several ways, including hydrophobic associations [ 18 ], particle filled gels [ 715 ] and supramolecular chemistry [ 1722 ].
When the hybrid gel is stretched, the polyacrylamide network remains intact and stabilizes the deformation, while the alginate network unzips progressively, with closely spaced ionic crosslinks unzipping at a small stretch, followed by more and more widely spaced ionic crosslinks unzipping as the stretch increases.
Because of the large magnitude of the fracture energy and the pronounced blunting of the notches, we ran a large number of experiments to determine the fracture energy, using three types of specimens, as well as changing the size of the specimens Supplementary Figs. The experiments showed that the measured fracture energy is independent of the shape and size of the specimens.
Our data suggest that the fracture energy of hydrogels can be dramatically enhanced by combining weak and strong crosslinks. The combination of relatively high stiffness, high toughness and hyrdogels of stiffness and toughness, along with an easy method of synthesis, make these materials an ideal candidate for further investigation.
Further development is needed to relate macroscopically observed mechanical behavior to microscopic parameters.