A Bio-Inspired Model Design for Developing Hierarchically Structured Composite Hydrogels

A hierarchical fabrication strategy is developed by leveraging the fundamental design principles of natural materials to achieve strong and tough ceramic-reinforced organo-hydrogels.
Published in Materials
A Bio-Inspired Model Design for Developing Hierarchically Structured Composite Hydrogels

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1) The Question

Composite hydrogels can be formulated by introducing micro- or nano-fillers, such as ceramics. Indeed, nature presents abundant examples of organic-inorganic composite materials, including nacre, bone, and glass sponge (Euplectella aspergillum).  Nonetheless, these materials are typically hard biological tissues with a predominant mineral content, and their mechanical properties differ from those of soft materials that are elastic and flexible like hydrogels. This raises an interesting question: Can the fundamental design principles of these hard biological tissues still be extracted and applied to soft and elastic composite hydrogels?

2) The Inspiration

In general, the excellent stiffness, strength, and toughness of natural materials result from their intricate hierarchical organic-inorganic composite structure. For example, the hierarchical structure of bone has up to seven levels of organization, constructed by hydroxyapatite nanocrystals and collagen molecules as basic building blocks. This hierarchical organization, from molecular to macro scales, achieves a strong coupling between the organic and inorganic constituents, fostering synergistic interactions across multiple length scales. The hierarchical composite design behind these natural materials can be extracted:

  1. Aligned stiff anisotropic particles or fibers with a thickness in the nm range;
  2. A soft and tough matrix;
  3. Tight interfaces between the stiff elements and the soft matrix;
  4. Assembled hierarchically by the most fundamental building blocks via bottom-up growth. 

3) The Implementation

Leveraging the fundamental design principles of natural materials, a hierarchical fabrication strategy can be formulated to achieve strong and tough ceramic-reinforced organo-hydrogels. Accordingly, the composite organo-hydrogels consist of:

  1. Ceramic platelets of around 200 nm thickness aligned by DIW 3D printing;
  2. A highly crystalline poly(vinyl alcohol) (PVA) organo-hydrogel matrix reinforced by substitution in a glycerol-water solution with ferric chloride;
  3. Enhanced ceramic-polymer interfaces by treating the ceramic platelets with (3-aminopropyl)triethoxysilane (APTES);
  4. Composite organo-hydrogel filaments, which serve as the basic building block, are further 3D printed into a selection of bioinspired macro-architectures. 
Fig. 1 Schematic illustration of the fabrication process of composite organo-hydrogels by shear-induced alignment via DIW 3D printing, freeze-thawing, and solution substitution.

4) The Validation

As indicated by computed tomography (CT) scans, the 35-mm long cylindrical nozzle resulted in effective platelet alignment with more ceramic platelets in the lower angle range (0–10°) than using the short, tapered nozzle. SEM images of a composite organo-hydrogel filament with 5 wt.% ceramic content (CHF-5) show that the platelets aligned compliantly to the circumference of the filament, resulting in a concentric lamellar microstructure that mimicked the spicules of Euplectella aspergillum.

With ceramic platelets, the composite organo-hydrogel filaments revealed significantly higher mechanical properties than those of pure organo-hydrogel (≈3.2 MPa). In particular, CHF-5 had the best combination of Young’s modulus (≈20.3 MPa), tensile strength (≈6.9 MPa), and strain (≈347.3%). It also revealed the highest work of extension (≈17.5 MJ/m3), indicating its excellent strength and toughness among composite hydrogels.

Two other groups of filaments were also prepared with 5 wt% ceramic content but without platelet alignment (W/O Alignment), or APTES treatment (W/O APTES). Both filaments revealed significantly lower mechanical properties when compared to CHF-5, and even to pure PVA organo-hydrogel. This suggested that simply introducing untreated ceramic platelets into the organo-hydrogel compromised mechanical properties, with randomly distributed platelets and poor ceramic-hydrogel interfaces effectively creating weak points in the materials.

Fig. 2 Microstructure and tensile properties of the composite organo-hydrogel filament.

Leveraging the design freedom of DIW 3D printing, the composite organo-hydrogel filaments can be easily assembled as basic building blocks to construct free-form bioinspired architectures. Unidirectionally aligned, Bouligand, and crossed lamellar structures are selected by drawing inspiration from natural strong and tough materials. The resulting composite organo-hydrogels exhibited excellent strength and toughness (work of extension). Notably, this was achieved with a relatively small fraction of ceramic reinforcement.

With different bioinspired macro-architectures, these 3D printed composite organo-hydrogels revealed distinct mechanical behaviors. The in-plane mechanical isotropy of composite organo-hydrogels with bioinspired macro-architectures closely resembled their natural prototypes. Meanwhile, three architectures also showed wide-range differences in Young’s modulus (~4–10 MPa). 

Fig. 3 DIW 3D printing and tensile properties of composite organo-hydrogels with different macro-architectures.

5) The Explanation

The fracture toughness of composite organo-hydrogels was investigated by pure shear tests. 3D printed pure PVA organo-hydrogels had a fracture energy of 7.3 kJ/m2, which increased about three-fold (20.3 kJ/m2) in the unidirectionally aligned composite organo-hydrogels. This improvement was attributed to the stiffening and strengthening effects of aligned filaments and platelets, coupled with the crack pinning and deflection effects of the concentric lamellar microstructure. The Bouligand architecture saw a further increased fracture energy of 26.1 kJ/m2, where the rotating filaments led to effective crack pinning and deflection mechanisms that delayed and deflected crack propagation. Remarkably, the crossed lamellar samples revealed the highest fracture energy of 31.1 kJ/m2 owing to a more prominent crack pinning effect with significantly delayed and slowed crack propagation. These results demonstrate that by 3D printing bioinspired macro-architectures, our strategy can translate the toughening mechanisms of natural materials into composite organo-hydrogels to effectively enhance their fracture toughness.

Fig. 4 Fracture behavior and toughness of 3D printed composite organo-hydrogels with different bioinspired macro-architectures.

On top of the crack pinning and deflection mechanisms through 3D printed bioinspired architectures at the macro scale, our strong and tough composite organo-hydrogels exhibited mechanical energy dissipation across multiple length scales, including coupled energy dissipation in both the process and bridging zones at the micro scale, PVA chain deformation at the nano scale, and the breakage of hydrogen and coordination bonds in the PVA matrix, as well as interfacial hydrogen bonds during platelet pull-out at the molecular scale. 

Fig. 5 Multi-scale energy dissipation mechanisms of the composite organo-hydrogels.

6) The Application

In addition to robust mechanical properties, our composite organo-hydrogels were also endowed with multi-functionality, including electrical conductivity (7.1 S/m), strain sensing capability, and wide operation tolerance. To demonstrate their versatile application in flexible electronics, composite organo-hydrogels with different bioinspired macro-architectures were integrated into a multi-functional smart sensing glove for multiple modes of sensing and detection, which include conductive fingertips to allow interaction with touch screens, a pressure-sensing touch pad on the back to control robotic vehicle movement, and strain sensors on finger joints to control the gripper action. Their integration in the smart sensing glove enabled multiple modes of operation, including robotic vehicle and gripper control, and touch screen interaction.

Fig. 6 Demonstration of a multi-functional smart sensing glove that integrates conductive fingertips, touch pad, and strain sensors using the composite organo-hydrogels for touch screen interaction and robotics control.

7) The Conclusion and Outlook

By implementing the hierarchical composite design principles of natural materials, this study proposed a hierarchical fabrication design strategy for preparing strong and tough composite organo-hydrogels intricately engineered with (1) 5 wt.% aligned conductive ceramic platelets, (2) a highly crystalline PVA matrix, and (3) silane-treated platelet-matrix interfaces. The composite organo-hydrogels exhibited bioinspired mechanical mechanisms and tunable mechanical and electrical sensing responses. The groundwork laid by this model strategy opens exciting opportunities for developing advanced composite hydrogels. The versatility of DIW 3D printing in both material and structural design can be further exploited. Therein lies the potential to fabricate composite hydrogels with novel bioinspired architectures, further exploring the hierarchical organization of natural materials for enhanced performance.

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Soft Materials
Physical Sciences > Materials Science > Soft Materials
Structural Materials
Physical Sciences > Materials Science > Structural Materials
Bioinspired Materials
Physical Sciences > Materials Science > Soft Materials > Bioinspired Materials

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