Emerging interactive materials and soft robotics attract growing inspiration from biological functions1,2. Therein, to efficiently store and release mechanical energy to achieve rapid burst-like motions3 is of emerging interest for emerging soft robotics4. The underlying resilient biological proteins, e.g., resilin, and elastin , characteristically combine high stiffness and strength at small reversible strain, allowing efficient storing of mechanical energy, stability and release on demand with little dissipation5.
Hydrogels are synthetic analogues of biological tissues. In synthetic hydrogels, double network hydrogels6,7 have widely been used for toughening, by using combinations of brittle and highly crosslinked polymeric network as sacrificial networks with coiled connecting polymer networks with different chemical compositions8,9. Therein, polymerization of different networks have been incorporated up to quadruple networks10, also exploiting pre-stretching of the lower-level chains. Such mechanisms, while fundamentally irreversible in the stress-strain curves, are of great interest, as they can be applied to a variety of approaches for dissipative toughening. On the other hand, hydrogel reinforcement with minimal mechanical energy dissipation and hysteresis have been demonstrated by entanglement-based mechanisms11,12, nanoparticle reinforcement13, slide ring gels14, tetra-arm PEG gels15, clay gels16, tri-branched gels17, polyprotein crosslinkers18, and crosslinking density adjusted gels19. Therein, several benefits of non-hysteretic hydrogels such as absence of fatigue have been shown14. On the other hand, methods like phase-separation in ionic liquids20, force-triggered chemical reactions21, hydrogel training22 and hybrid gels23 yield significantly stronger gels at the cost of mechanical dissipation. However, synthetic low-hysteretic hydrogels typically show low stiffness and excessively high strain, unlike the biological resilient counterparts24.
In conclusion, new synthetic approaches for hydrogel reinforcement to promote stiffness are needed, still involving high non-dissipative energy storage and release at low strains. They would allow new avenues for emerging bio-inspired soft robotics and facilitate rapid burst-like mechanical actions.
Herein, we show a concept based on hierarchically swollen hydrogels, upon repeated swellings of previously formed hydrogels using fixed monomer, crosslinker, and water mixtures with subsequent photopolymerizations up to 7 hierarchical levels N. This achieves reinforcement towards promoted stiffness at low strains, while still avoiding dissipation, i.e., bioinspired resilience. Importantly, this concept can be applied to different homopolymer hydrogels allowing hierarchically swollen networks. The concept introduces self-reinforcement by hierarchically stretching of lower hierarchical level polymeric chains, i.e., it uses a monomaterial approach to improve mechanical properties based on hierarchical control of chain stretching25.
We first explored poly(acrylamide) (PAAm) hydrogels involving up to 7 hierarchical levels of networks, where each hierarchical level shows characteristic stretching due to sequential swellings for the generation dependent chain stretching. PAAm hydrogels were selected as model materials as their hydrogelation is well known in the state of the art26 and their swelling capacity is easily tunable via the concentration of the crosslinker. Herein, a fixed optimized photoinitiated radical polymerization protocol is sequentially used, using a high acrylamide monomer concentration of 60 wt% in water, additionally incorporating a low concentration (10-4 mol/mol) of N,N-methylenebisacrylamide (BIS) crosslinkers vs. the acrylate monomers (for details, see Supplementary Information). This specific acrylamide/BIS/water composition is taken fixed within the subsequent hierarchical levels of swellings of the previously formed hydrogels and subsequent photo-controlled radical polymerizations.
Concretely, such an initially formed PAAm hydrogel, denoted as hierarchical level N = 1 (see Fig. 1a), is next re-immersed in the above-mentioned fixed acrylamide/BIS/water composition, i.e., compositionally identical as used in the original PAAm hydrogel synthesis. This leads to swelling of the originally formed PAAm hydrogel chains, whereupon the newly added acrylate monomers are next polymerized to lead the next level hydrogel network with hierarchical level N = 2 (Fig. 1a). Such PAAm chains are more coiled than the previously formed swollen and stretched chains. Similarly, the procedure is repeated upon consecutive swellings and polymerizations using the above fixed acrylamide/BIS/water mixture to allow higher level hydrogel hierarchies, presently up to N = 7 (Fig. 1a,b).
During the sequential swellings and polymerizations, the previously formed polymer chain networks are increasingly swollen and stretched whereas the newly added chains are more coiled see Fig. 1a. The stretched polymer chains serve as scaffolds for hierarchical entanglements of the newly created more coiled chains. This hierarchical process increases the strength and modulus due to formation of new dynamic and hierarchical entanglements while maintaining elasticity and not causing embrittlement which is normally the case for increasing concentration of fixed cross-links.11 In short, the present hierarchical gel consists of sequentially stretched chains due to excessively swollen/less swollen networks (Supplementary Fig. 1&2). The low ratio of cross-linkers to monomers (10-4 mol/mol) guarantees formation of long chains between the crosslinks allowing stretching of the previously formed chains and coiling of the later formed chains to increase the number of entanglements.11 According to ‘slip-link theory’, the entanglements can slip to the optimal configuration and therefore transmit stress across the whole network granting the hydrogel resilience27. The approach leads to increase of the strength and stiffness upon increasing the hierarchical level N.
Upon increasing the final N, the hydrogels first become stronger and stiffer, with an order of magnitude increase from N = 1 to N = 5, still with small strain and hysteresis (Fig. 1b & Supplementary Table 1). At each step, the polymer weight fraction is constant (Supplementary Fig. 3a) and the fraction of swollen polymer to non-swollen polymer increases (Supplementary Fig. 3b). Therefore, the drastic changes in the mechanical properties are not due to changes in gel chemical composition but solely due to network topology, stretching, and entanglements within a single type of monomers within the hierarchies. Thus, the increased mechanical properties can be coined as “self-reinforced” hydrogels, where reinforcement occurs with the chemically same materials using different levels of sequential chain stretching. The energy storage density increases at each level of hierarchy up to a final level of N = 5, beyond which they increase steeply due to high stretching of the first formed networks and the amount of energy stored plateaus (Fig. 1b). In biological spring-driven systems, the extension ratio is limited to be small, i.e., 1.05-1.503. Therefore, to mimic biological systems and still to achieve a sufficient energy storage capacity for snapping movements, a high Young’s modulus is needed (Fig. 1d & Supplementary Table 1 & 2) to achieve efficient resilience at low strains. State of the art hydrogels are unable to achieve the high energy storage at such low strain values, to mimic resilient biological materials such as resilin and elastin28. Compared to the current state-of-the-art literature of hydrogels and biological resilient proteins, the present hierarchically swollen and entangled hydrogels show superior tensile strength and Young’s modulus at small strains (Fig 1d). From hierarchical level N = 1 to N = 7 the Young’s modulus increases from 0.596 +- 0.065 MPa to 1.836 +- 0.253 MPa, the tensile strength in cyclic testing from 0.387+- 0.044 MPa to 3.100 +- 0.792 MPa and the stored energy increases from 0.487 +- 0.049 MJ/m3 to 1.625 +- 0.239 MJ/m3.
However, the stress-strain behavior changes fundamentally when the final hierarchical level N grows beyond a limit. Fig. 1b shows that for N = 7, the stress-strain curves upon loading and reloading deviate considerably from each other, showing emergence of hysteresis, i.e., increase of dissipation and loss of resilience. In other words, the resilience is conserved in PAAm hydrogels up the final hierarchical level of approximately N = 5 beyond which dissipation emerges. The PAAm hydrogels of final hierarchical level N = 5 show high modulus, still low hysteresis, therefore it is close to optimal for hysteresis-free mechanical energy storage for PAAm hydrogels. The resilience sustains even in cyclic tests of 100 cycles of tensile strain of 1 mm/mm without fatigue, i.e., the absence of the Mullins effect (Fig. 2a). The swelling decreases as the final hierarchical level N increases (Fig. 2b). The Young´s modulus increases steeply as a function of N up to N = 7 (Fig. 2c). Relevant for the bio-inspired resilience, the present approach allows efficient reinforcing upon exploiting the hierarchically stretched chains, as compared with the state of the art (Fig. 2d). Upon increased hierarchical level, the hysteresis in the stress-strain curves, indicating dissipation, remain small until N = 7 is reached. Using PAAm hierarchical hydrogels, the limiting hierarchical level between low dissipation and higher dissipation is close to N = 6 and 7 (Fig. 2f).
Breakdowns of polymer chains due to excessive polymer stretching upon tension is suggested29, leading to hysteresis and dissipation (Fig 3a). Such a hierarchical self-entanglement resilient swollen/polymerized gel approach appears to be general as suggested by the use of different polymers (Fig. 3b-e): Four different hydrogels, i.e., poly(N-isopropylacrylamide) (PNIPAm), poly(oligo(ethylene glycol) methyl ether methacrylate), poly(di-methyleneacrylamide) (PDMAA), and poly(acrylic acid) (PAAc), all allow sequential radical polymerization. This shows the wide range of the proposed self-reinforcement and resilience approach by the hierarchical swellings and polymerizations (Supplementary Table 3 and Supplementary Fig. 10). Thus, the mechanism allows for significant increase of mechanical strength and stiffness and low-hysteretic stress-strain behavior for hydrogels with a variety of different properties, such as thermoresponsive hydrogels (POEGMA, PNIPAm), biocompatible hydrogels (POEGMA), and pH sensitive hydrogels (PAAc).
Mechanical energy storage and release in biological systems use resilient proteins extensively, e.g., elastin and resilin30. Locusts and other insects use an intricate, spring-like interplay of muscles, elastic tissue and chitin, i.e., Latch-Mediated Spring Actuation Mechanism (LAMSA), to store mechanical energy and non-dissipatively release it with a snapping motion at low strains (Fig. 4a)3. Inspired by these systems, we employed the resilient hierarchically swollen and entangled PAAm gel to store elastic energy for a demonstration of a locust inspired jumping soft robot, fabricated via 3D printing (Fig. 4b). Firstly, we load the soft robot manually with mechanical energy. Then, we lock the structure mechanically with a frozen hydrogel and release the latch via heat to initiate the snapping motion. With the LAMSA mechanism we show a proof of concept for resilient, hierarchical entanglement hydrogels to be used in actuating systems for soft robotics. Actuation strength and speed have been commonly acknowledged as a major bottleneck in soft robotics31,32. Latch mediated spring actuation with resilient, high stiffness, and high strength hydrogels guarantees efficient energy storage and conversion to power ultra-fast actuation which is 30 % faster than that achieved with a comparable classic hydrogel, resulting in a higher and faster jump (Fig. 4c&d).
In summary, we describe a principle to store and release non-dissipatively mechanical energy of hydrogels using single monomer types to allow sequential chain swelling, stretching and entanglements hierarchically. This allows bioinspired non-dissipative high mechanical energy storage involving high stiffness and low strain. We show polyacrylamide hydrogels of hierarchical level up to N = 7, suggesting an increase of an order of magnitude in Young’s Modulus and nearly 300% higher mechanical energy storage efficiency while retaining small hysteresis up to hierarchical level at N = 5 where the energy increases from 0.5 MJ m-2 to 1.6 MJ m-2 This principle is general, as shown using different polymers. Importantly, the concept to avoid stress-strain hysteresis for resiliency conceptually starkly contrasts the classic double network or other hydrogel concepts for toughening where dissipation is pursued. The massively enhanced mechanical energy storage properties of these hierarchical entangled gels offer great promise for applications where mechanical energy storage is crucial, such as soft robotics, specifically for jumper robots4. We show a proof of concept of a jumper robot powered by the mechanical energy storage of hierarchically entangled hydrogels. Our jumper prototype highlights the increase in jumping power of hierarchically entangled gels relative to a non-entangled control. Further systematic studies to study the mechanism of self-entangled gels and the effect of various synthesis conditions on their mechanical properties will be the scope of future work.