Delft University of Technology
Pros and Cons
Supramolecular or Macromolecular: What Is Best for Functional Hydrogels with Advanced
Properties?
Eelkema, Rienk; Pich, Andrij
DOI
10.1002/adma.201906012
Publication date
2020
Document Version
Final published version
Published in
Advanced Materials
Citation (APA)
Eelkema, R., & Pich, A. (2020). Pros and Cons: Supramolecular or Macromolecular: What Is Best for
Functional Hydrogels with Advanced Properties? Advanced Materials, 32(20), [1906012].
https://doi.org/10.1002/adma.201906012
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Pros and Cons: Supramolecular or Macromolecular: What
Is Best for Functional Hydrogels with Advanced Properties?
Rienk Eelkema* and Andrij Pich*
DOI: 10.1002/adma.201906012
1. Introduction
Herein, we lay out the pros and cons of constructing (hydro) gels from polymer networks held together by noncovalent inter-actions or covalent bonds. Hydrogel materials currently find many applications in personal care products, biomaterials, coat-ings, and plant fertilizers. They have a large potential for future application in sensing, drug delivery, soft robotics, and biohy-brid or biointerfacing materials. To scientists working in those fields, the choice of material type may depend heavily on the intended application and associated required properties. There, it can be useful to consider a generalized overview and com-parison of hydrogel structure, properties, and performance in various scenarios.
Hydrogels are fascinating soft materials with unique properties. Many biological systems are based on hydrogel-like structures, underlining their versatility and relevance. The properties of hydrogels strongly depend on the structure of the building blocks they are composed of, as well as the nature of interactions between them in the network structure. Herein, gel networks made by supramolecular interactions are compared to covalent macromolecular networks, drawing conclusions about their performance and application as responsive materials.
Dr. R. Eelkema
Department of Chemical Engineering Delft University of Technology
Van der Maasweg 9, 2629 HZ Delft, The Netherlands E-mail: r.eelkema@tudelft.nl
Prof. A. Pich
Institute of Technical and Macromolecular Chemistry RWTH Aachen University
Worringerweg 2, 52074 Aachen, Germany E-mail: pich@dwi.rwth-aachen.de Prof. A. Pich
DWI – Leibniz-Institute for Interactive Materials e.V. Forckenbeckstraße 50, 52056 Aachen, Germany Prof. A. Pich
Aachen Maastricht Institute for Biobased Materials (AMIBM) Maastricht University
Brightlands Chemelot Campus
Urmonderbaan 22, 6167 RD Geleen, The Netherlands
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201906012.
Historically, it has been difficult to pro-vide a comprehensive definition of a gel, but the one by J. D. Ferry suits our pur-pose: “Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.”[1] It
con-tains a network-forming minority compo-nent as well as a majority swelling agent. In the case of a hydrogel, the swelling agent is water. As such, many biological structures and tissues can be considered hydrogels. The network-forming compo-nent can have many origins and structures, including parti-cles, crystallites, polymers, proteins, and small molecules. The important requirement for the formation of hydrogels using above-mentioned building blocks is their ability to form physical or chemical crosslinks to form a network-like structure. In this viewpoint, we will address and compare two widely investigated and applied gel classes, i.e., macromolecular hydrogels (MHGs) and supramolecular hydrogels (SHGs). Both types have been extensively reviewed[2] in recent years. It is not the objective of
this viewpoint to add another exhaustive review, but rather to provide insight into the opportunities and limitations of both hydrogel types in particular scenarios. One of the conclusions will be to arrive at hybrid systems using advantageous proper-ties of each approach to create truly advanced soft materials.
2. Supramolecular or Macromolecular Functional
Hydrogels?
2.1. Structure and Basic Properties of Hydrogels
MHGs contain a physically or covalently crosslinked amorphous polymer network. Crosslinks can be introduced during poly-merization using a multifunctional monomer (crosslinker), or through chemical post-modification reactions on existing mers. A typical example of MHG formation is the radical poly-merization of acrylamide with N,N′-methylenebis(acrylamide) crosslinker (0.5 mol%) in water, giving after 10–15 min a trans-parent hydrogel with a swelling ratio of 13.3 and E-modulus of 0.03 MPa (Figure 1). SHGs have a network made up of low-molecular-weight molecules, oligomers, or polymers that self-assemble on timescales of minutes to hours into larger, often fibrous or otherwise somewhat ordered, superstructures through noncovalent interactions.[3] Subsequent crosslinking through
noncovalent interactions, entanglement, or covalent means then leads to gelation. A typical example here is the protonation-induced self-assembly and subsequent gelation of hydrophobic dipeptide derivatives in water, giving stiff transparent gels with low yield stress (Figure 1).[4] A third class of hydrogels is an
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intermediate between SHGs and MHGs, consisting of an amor-phous network of polymer chains crosslinked using noncovalent interactions (Figure 1). In some scenarios, these supramacro-molecular gels (SMHGs) act more as MHGs (e.g., regarding mechanical properties[5]), in others more as SHGs (e.g.,
revers-ibility[6]). In all approaches, crosslink density has a large impact
on many properties of the resulting gel, such as mesh size and stiffness.[7] For MHGs, control over crosslink density is
straight-forward and predictable as it relates to the ratio of introduced crosslinker monomer or the extent (yield) of the crosslinking chemical reaction. For SHGs control over crosslink density is much less predictable as these crosslinks are often somewhat dynamic, their strength can depend on many environmental factors and can also be influenced through processing steps and the kinetics of network formation.[8] A higher level of
con-trol over crosslink density can be achieved by having covalent[9]
crosslinks between fibers, leading to remarkable mechanical behavior (strain stiffening). Alternatively, the use of highly spe-cific and programmable noncovalent crosslinking interactions based on, e.g., that DNA can generate responsive hydrogels with cell interactions that depend on the material superstructure.[10]
Hydrogels are soft materials with an open porous structure and high water content.[11] As such, they have many properties
that align with those of biological tissues, leading to many cur-rent and future applications in biomedical or bioinspired mate-rials.[12] In the development of those applications we also see a
tendency to substitute static, permanent gel materials by more dynamic or reversible, responsive, and interactive gels with tran-sient programmable properties. In these various circumstances, gel performance can depend on static as well as dynamic prop-erties, and choice of material also depends on ease of design, synthesis, and functionalization. Considering these specific requirements, MHGs and SHGs may show stark differences in properties, performance, and applicability. We will discuss sev-eral considerations here illustrated by recent examples.
2.2. Chemical Design of Hydrogels
The design and synthesis of hydrogels are first important steps to target various applications. The synthesis approaches for MHGs and SHGs are hugely different because of the dif-ferent mechanisms of gel formation. In the majority of the cases for MHGs, gelation is based on covalent network forma-tion starting from often readily available soluble monomers or prepolymers and crosslinkers. The formation of MHGs can be realized by the polymerization/crosslinking of monomers[13] or
crosslinking of prepolymers.[14] Using the second approach, one
may benefit from low reaction temperatures, avoid residue of nonreacted monomer, and benefit from shorter gelation times (5–15 min), which is very important for biomedical applications of hydrogels.[15] As such, MHGs formation and certain
mechan-ical properties are largely predictable from the amount and nature of the monomers or prepolymers and the crosslinker content. New functional groups can be introduced in the net-work by adding small amounts of functionalized monomers to such conventional gelation mixtures, as long as these function-alized monomers are compatible with the used polymerization procedure.[16] Alternatively, the new functional groups can be
integrated into the prepolymer structure before or after hydrogel formation.[17] The chemical structure of the crosslinking agent
may influence the network topology (bifunctional vs multifunc-tional crosslinkers) or introduce specific functions to hydrogels like degradability (e.g., redox- or mechano-cleavable disulfide bonds; enzyme-cleavable ester and amide bonds), re-shaping and self-healing (supramolecular bonds like host–guest com-plexes, H-bonds), or toughness and elasticity (ionic bonds).[18]
For SHGs, both gelator structure and crosslink density can be extremely difficult to predict.[19] The ability to self-assemble
into a network has to be built into the monomers (or “gelators”). Gelators are often not commercially available and have to be synthesized. The ability to assemble can be extremely sensitive to the introduction of other functional groups in the gelator. These constraints can put substantial structural limitations on the design of SHGs. Still, tethering a functional group to a small amount of gelator through a long soluble spacer will often allow decoration of gel fibers.[20] For both MHGs and SHGs,
one can include click chemistry functionalities in the network-forming mixture, for post-gelation decoration of the network.[21]
Rienk Eelkema is an associate professor at TU Delft in The Netherlands. He obtained his Ph.D. in chemistry (cum laude) with Prof. Ben Feringa at the University of Groningen. After postdoc-toral work at the University of Oxford with Prof. Harry Anderson, he joined the TU Delft Faculty in 2008 (tenured in 2013). His main research interests include the use of chemical reactivity to control self-assembly processes and soft materials, and the design and synthesis of new materials for applications in physics, biology, and biomedical engineering.
Andrij Pich studied chemistry at the National University “Lvivska Polytechnika” in Ukraine and obtained his Ph.D. in macromolecular chem-istry with Prof. Hans-Jürgen Adler at TU Dresden in 2001. Afterward, he was a postdoc at the University of Toronto with Prof. M. Winnik and completed his habilitation at the Dresden University of Technology in 2008. In 2009, he was appointed as Professor for Functional and Interactive Polymers at RWTH Aachen University and, since 2019, he has been Professor for Biobased Polymers at Maastricht University. His main research interests are the synthesis of functional polymers and responsive hydrogels for the design of interactive soft materials.
2.3. Network Structure of Hydrogels
The sequence structure of the polymers making up the MHG network is commonly random when starting from a mixture of monomers. The use of sequence controlled macromonomers (e.g., block copolymers) is the preferred method to introduce internal structure in the gel network. After network formation, the chemical composition of polymer chains and distribution of crosslinks in MHGs are fixed. This can be quite the oppo-site in SHGs, where monomer (gelator) exchange dynamics (on the second to hour timescale[22]) within the fiber are mainly
dictated by interaction strength. Several cases now exist where, either before or after initial network formation from a mixture of gelators, the gelators selectively form interpenetrating[23] or
phase-separated networks[24] of homopolymer fibers through
a self-sorting process. Similarly, driven by the combination of dynamics, bond exchange, and interaction strength, potent gelators can self-select from a mixture of interchanging gelator precursors.[25]
Spatial differentiation in hydrogel materials is classically achieved through patterning, most commonly using light and masks. For MHGs, post-gelation functionalization is preferred as the diffusion is stopped in the gel state.[21b,c] Analogous
approaches exist for SHGs,[26] but there are also examples of
pre-gelation spatial differentiation in SHGs, through localized gelator formation using, e.g., light, electrochemical potential, seeding surfaces, or local catalysts as a spatially resolved stim-ulus.[27] An entirely different method for spatial differentiation
is by reaction diffusion, and this is an example where the worlds of MHGs and SHGs meet. Controlled reaction and diffusion of SHG gelator precursors can lead to SHG object and pattern formation,[26d,28] as well as the stabilization of emulsions,[29]
typically on timescales of hours to days depending on diffusion lengths. Diffusion is often carried out inside an MHG, leading to the formation of MHG–SHG interpenetrated network hybrid gel materials. Inversely, a recent example shows damage repair in bulk polymer materials by first forming a supramolecular gel network in the crack, that then acts as a medium for the diffu-sion and reaction of polymerization monomers, making up the final network.[30] Although not a hydrogel, it does demonstrate
the power of combining supramolecular and macromolecular network formation.
2.4. Functional and Responsive Hydrogels
Since hydrogels are very attractive materials for numerous applications, their structure and morphology need to be tailored to enable specific properties and functions. The mechanical properties of hydrogels are extremely important for their use in medical materials and devices.[31] One such property is shear
thinning, and recovery after removal of the applied stress. This behavior can allow application as tissue engineering scaffolds or injectable hydrogels, for subcutaneous drug delivery. There, after shear-induced transition to the sol state, the gel network rearranges and re-forms. In SHGs, this effect is observed in a limited number of cases as fiber and monomer exchange dynamics are often too slow to obtain the effect.[32] True
self-healing (for instance after cutting a gel into pieces) is very rare for SMGs.[33] For MHGs, the permanent nature of MHG
net-works generally prevents self-healing entirely. An important exception is the use of reversible crosslinks, either in SMHGs using noncovalent interactions,[34] or in MHGs using dynamic
covalent bonds. There, covalent crosslinks can exchange contin-uously[35] or after a stimulus, rearranging the network. SMHGs
and dynamic covalent MHGs are often capable of self-healing after damage, recovery after shear-thinning, or remodeling their network to release an applied stress.
Mechanical properties such as tensile strength, elasticity, extensibility, and toughness are important for numerous appli-cations. It is important to note that larger is not necessarily better: for instance, when used as a tissue engineering scaffold, a property such as the gel storage modulus should be somewhat matched to that of the tissue and certainly not be as high as pos-sible. Where for SHGs, these properties are difficult to predict or tune, for MHGs they can be tuned through the amount and distribution of crosslinks and chemical structure of functional groups integrated into polymer chains. Pushing the boundaries of mechanical properties of hydrogel materials, Gong and co-workers recently introduced a new family of SMHGs, which exhibit high mechanical strength and extreme elasticity. Their hydrogels consist of amphiphilic polymer chains crosslinked by ionic or a combination of ionic and covalent crosslinks (Figure 2).[36] These so-called polyampholytes, bearing randomly
distributed cationic and anionic groups along the polymer chain, form extremely tough and viscoelastic hydrogels. The random
Figure 1. Schematic representation of the network topology for MHGs (left), SHGs (right), and the intermediate case: SMHGs where the crosslinks
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distribution of charges provides ionic bonds with a wide distri-bution of strength, where the strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds revers-ibly break and re-form, dissipating energy. The Young’s mod-ulus and swelling degree of these SMHGs with supramolecular crosslinks can be tuned over a wide range by using combina-tions of different ionic monomers and tuning their concentra-tion (Cm) during the polymerization process (Figure 2).
Degradation of hydrogels is extremely important aspect if they are used as delivery systems, implant coatings, or scaf-folds. Hydrogel degradation is a complex process where both the hydrogel structure and environment are changing with time. It includes changes of the network structure and simultaneous release of degradation products (small molecules, oligomers etc.). Very few systematic studies of hydrogel degradation processes have been reported.[37] MHGs and SHGs are different in their
degradation behavior and nature of degradation products. Basi-cally, if MHGs contain degradable or supramolecular crosslinks, they degrade to the (mostly) linear soluble oligomer chains.[18b,38]
If the hydrogels consist of polymer chains with cleavable bonds in the main chain, then degradation may result in the formation of a mixture of small molecules and branched oligomers.
Contrary, the largest molecular unit in an SHG is the gelator molecule itself. Fiber-bound gelators are in dynamic equilib-rium with gelators that are free in solution. Degradation of SHGs can proceed via three pathways, often simultaneously. Depletion of gelator molecules in solution will lead to fiber
breakdown. Mechanical forces can cause breaking of fibers, and fiber fragments can then leave the gel. Sometimes it is possible to either breakdown a gelator molecule (commonly through hydrolysis, analogous to biodegradable MHGs) or change its gelation ability through a chemical reaction (e.g., (de-)protonation, redox), both leading to fiber destabilization. The latter is an interesting example where SHGs are stimuli-responsive, leading to advanced functions in, for instance, drug delivery.
Stimuli-responsiveness is one of the attractive properties of hydrogels, which is useful for the fabrication of hydrogel actua-tors, dosing devices, and sensors. The classical way to imple-ment stimuli-responsiveness into MHGs[39] is the
incorpora-tion of temperature-responsive (leading to lower critical solu-tion temperature (LCST) or upper critical solusolu-tion temperature (UCST) behavior) or pH-responsive monomers in the polymer chain.[40] Recently, the utilization of light as stimulus emerged,
by the incorporation of photo-switchable molecules (azoben-zenes, spiropyrans, donor–acceptor Stenhouse adduct (DASAs), etc.) as side groups into polymer chains.[41] In the case of
iso-tropic MHGs, stimuli such as temperature, pH, or light lead to the reversible change of the volume and swelling degree. Ani-sotropic MHGs can perform complex shape changes, which is useful for the fabrication of swimmers or robotic devices.[42]
In SHGs, changes in the gelator structure often induce a response in the properties of the gel. As there are many ways to change the gelator structure (chemical bond formation
Figure 2. Extremely tough and elastic SMHGs exhibiting weak and strong ionic bonds formed by crosslinked polyampholyte chains.[36] Total molar
concentration of ionic monomers (Cm), dependence of the swelling volume ratio Qv, and Young’s modulus, E, of the polyampholyte hydrogels (bottom
or destruction, complexation, protonation, photochemical isomerization, etc.), gels responding to a very broad range of stimuli have been reported.[2d,43] However, the range in
resulting responses is substantially smaller than for MHGs, with the majority leading to a sol–gel or gel–sol transition.[44] A
few exceptions include thermo-induced gel-to-gel transitions,[45]
chemical-reaction-induced viscosity changes,[25a] and light or
charge-transfer-induced changes in optical properties.[46] The
reason for this typical phase change response is that such a change in gelator structure often has a drastic positive or nega-tive impact on its ability to form a gel. Also, because of limited elasticity of typical SHG fibers, mechanical stress, changes, or perturbations often lead to gel collapse instead of expansion, deformation, or actuation as one may see for MHGs. Still, these stimulus-induced phase transitions find many applications, for instance in controlled drug release,[47] catalysis,[48] switchable
membranes,[49] in interactions with biological structures,[2c] or
as sensors.[50]
Although stimulus response can be programmed into hydro-gels, more advanced functions such as autonomous multi-stimuli response (logic gates, computing), or self-regulation of gel structure and properties, are still out of reach. These are all typical features seen in some advanced biological hydrogels. An interesting example of such an advanced biological hydrogel is fibrin. Fibrin is responsible for the clot formation and fast termination of bleeding caused by injury, which is an essential function for mammalian organisms. Fibrin is formed when needed by hierarchical self-assembly and eventually covalent crosslinking of peptide fibrinogen triggered by the enzyme thrombin (Figure 3).[51,52] The final fibrin network consists of
long fibrillar structures which are crosslinked by covalent and supramolecular bonds. Remarkably, this fiber-like structure of fibrin is responsible for the strain-stiffening behavior of the hydrogel, which is required to resist and efficiently terminate blood flow.
3. Conclusion
The example of fibrin hydrogels demonstrates that the hier-archical assembly of covalent and supramolecular bonds in combination with the use of molecular (amino acids, peptides) and macromolecular (proteins) building blocks is required to ensure advanced properties and specific functions. Biological hydrogels are frequently composed of macromolecular and low-molecular components, containing covalent and noncova-lent crosslinks, and are assembled in a modular way exhibiting different hierarchy levels. Such materials often exhibit hys-teresis effects and nonlinearity of their properties, which are essential to functions like adaptability and time-programming. Finally, many biological hydrogels can regenerate through con-tinuous growth, solving a problem related to destruction or damage caused by bacteria or inflammation processes. At pre-sent, the performance of synthetic hydrogel materials is not on par with these advanced biological hydrogels. Synthetic MHGs and SHGs each have areas where they outperform the other, and individually start to show some of the base principles and properties needed to reach the level of biological hydrogels. So, merging of the two worlds (MHGs and SHGs) seems to be an optimal way to design life-like adaptive soft hydrogel materials.
Acknowledgements
A.P. thanks Deutsche Forschungsgemeinschaft (DFG) and Collaborative Research Center SFB 985 “Functional Microgels and Microgel Systems” for financial support. R.E. thanks the European Research Council (ERC, consolidator grant 726381) for financial support.
Conflict of Interest
The authors declare no conflict of interest.
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Keywords
hydrogels, polymers, responsive materials, soft materials, supramolecular interactions
Received: September 13, 2019 Revised: October 28, 2019 Published online:
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