Engineering Hydrogel Adhesion: A Leap Towards Future Applications

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The pursuit of programmable hydrogel adhesion harbors transformative potential for fields as diverse as engineering, biology, and medicine. This ambition stems from a need to precisely control adhesion properties—energy, spatial distribution, and kinetics—to meet the varied demands of applications ranging from tissue repair to the development of soft robotics. Central to this challenge is the task of mimicking nature’s sophisticated adhesion mechanisms, such as those found in marine animals, which use a combination of adhesive and de-adhesive agents to dynamically control substrate attachment.

Innovations in Hydrogel Adhesion

Recent advancements have showcased the power of nanotechnology and material science in overcoming the limitations of traditional hydrogel adhesives. One notable approach involves the use of nanohesives, which employ a sandwich structure at the nanoscale to bridge hydrogels to substrates through a layer of adhesive nanoparticles (ANPs). This interaction, characterized by a combination of electrostatic interactions, van der Waals forces, and hydrogen bonds, allows for robust and versatile adhesion capabilities across a wide range of materials. The interaction strength, and thereby the adhesion energy, can be finely tuned by adjusting factors such as nanoparticle size and the degree of crosslinking within the hydrogel, offering a level of control previously unattainable with covalent or physical bonding methods alone​​.

Simultaneously, advancements in hydrogel material science have further broadened the biomedical applications of hydrogels. By categorizing hydrogels into natural and synthetic, with each category offering distinct advantages in terms of biocompatibility, mechanical strength, and physiological responsiveness, researchers are able to tailor hydrogel properties to specific applications more effectively than ever before. For instance, collagen and gelatin-based natural hydrogels provide excellent biocompatibility and can mimic the extracellular matrix, promoting cellular attachment and proliferation. On the other hand, synthetic hydrogels boast superior mechanical properties, essential for applications requiring high durability under physiological conditions​​.

Towards a Future of Topologically Engineered Adhesives (TEA)

At the heart of recent innovations is the concept of engineering surface network topology to achieve multifaceted control over hydrogel adhesion. By constructing adhesion units with distinct topological configurations—slip and stitch linkages—researchers have unlocked new dimensions in adhesion programmability. Slip linkages, characterized by a polymer chain entangled with a crosslinked network, allow for variable adhesion energy and can be dynamically adjusted based on loading rate. Stitch linkages, formed by two interwoven crosslinked networks, offer a different set of properties, including less tunability but increased stability. This dual-linkage approach enables the precise spatial patterning of adhesion properties, allowing for the creation of hydrogel adhesives that can strongly attach to healthy tissues while avoiding unwanted adherence to sensitive areas, such as wound beds.

The development of topologically engineered adhesives (TEA) represents a significant stride towards achieving robust, predictable, and highly controllable hydrogel adhesion. This methodology, inspired by the complex biological adhesion mechanisms observed in nature, offers a scalable and versatile solution to the longstanding challenges of hydrogel adhesion programming. With applications ranging from wound healing patches to drug delivery systems and soft robotic actuators, the implications of this research are vast and varied.

Figure : Engineered network topology and linkages for multifaceted programming of hydrogel adhesion. (a) Schematics of the stitch linkage (Top) and slip linkage (Bottom) formed between a bridging polymer and networks without and with surface dangling chains. The thickness of the dangling chain layer and the penetration depth of the bridging polymer are denoted as hdc and hpen, respectively. (b) Hydrophilic and hydrophobic molds are used to form a regular network (Top) and a network carrying surface dangling chains (Bottom), respectively.(c) Rate dependence and magnitude of the adhesion energy depend on the interfacial linkage types: stitch linkages with hpen/hdc → ∞, slip linkages with hpen/hdc « 1, and their hybrid with hpen/hdc ≈ 1. (d) The slip linkage offers programmable adhesion kinetics through tuning hdc (Top), which is also insensitive to processing conditions such as the thickness of bridging polymer solution hsol (Bottom). (e) Spatially controllable adhesion obtained from patterning the topological linkages at the interface.

Universal Applicability and Spatial Programming of Hydrogel Adhesion

Universal Applicability of Topologically Engineered Adhesives (TEA)

The versatility of TEA is underscored by its universal applicability across a diverse array of material systems, including various bridging polymers, targeted substrates, and TEA networks. This adaptability is evident in the creation of slip linkages on the gel-bridging network interface, which can seamlessly integrate with different interactions such as slip, stitch linkages, or even covalent bonds. The use of chitosan, with its triggered crosslinking and abundant amino groups, exemplifies how TEA can interact with various substrates through both covalent and physical interactions. This interaction flexibility is further demonstrated through the achievement of slip-slip, slip-stitch, and slip-bond linkages, highlighting the TEA strategy’s robustness in adhesion programming across different materials and mechanisms.

Gelatin, another bridging polymer, reinforces the principle that the formation of slip linkages, and thereby the overarching adhesion behavior, is more a function of polymer topology than of material chemistry. This insight is crucial for designing adhesives that are less dependent on the specific chemical properties of the substrates and more on the structural and mechanical interplay at the interface.

Double-Network Hydrogels: A New Frontier in TEA

The exploration of double-network (DN) hydrogels as part of the TEA framework opens new avenues for enhancing adhesion strength and toughness. DN hydrogels, characterized by their superior fracture toughness compared to single-network hydrogels, present an attractive option for TEA networks due to their inherent energy dissipation capabilities. This attribute is particularly significant in applications requiring robust adhesion under dynamic or mechanically demanding conditions. The integration of DN hydrogels into the TEA strategy, as demonstrated with PAAm-alginate and PAAm-chitosan gels, showcases the methodology’s applicability across both SN and DN hydrogel systems, provided the network topology can be meticulously engineered.

Spatially Programmable Adhesion: A Paradigm Shift

The ability to program adhesion spatially represents a paradigm shift in material design, enabling the creation of adhesives with predefined weak and strong adhesion zones. This capability allows for the precise control over where and how adhesives interact with surfaces, opening up possibilities for innovative applications such as wound patches, drug depots, fluidic channels, and soft actuators. The use of hydrophilic and hydrophobic patterning to control the distribution of slip and stitch linkages exemplifies how adhesion can be tailored to specific needs, offering a level of customization previously unachievable.

TEA-Based Devices: Bridging Innovation and Application

The practical applications of TEA span a wide range, from medical devices to soft robotics, each benefiting from the unique properties of programmable adhesion. For instance, wound patches designed with TEA can gently adhere to sensitive wound beds while firmly attaching to surrounding healthy tissue, promoting healing without causing additional trauma. Similarly, the creation of drug depots and fluidic channels demonstrates the potential for TEA in controlled drug delivery and biofluid management, respectively. The versatility of TEA extends to the development of soft actuators, which can be dynamically reconfigured for different functions, illustrating the broad impact of this technology.

Conclusion

The universal applicability and spatial programmability of TEA represent significant advancements in the field of hydrogel adhesion. By leveraging the principles of polymer topology and the unique properties of DN hydrogels, researchers have opened new pathways for creating versatile, robust, and precisely controllable adhesive systems. These innovations not only enhance our understanding of material science but also pave the way for groundbreaking applications across medicine, engineering, and beyond. The integration of TEA into various devices showcases the practical impact of this research, highlighting the potential for further exploration and application in the future.


reference link : https://arxiv.org/pdf/2303.16262.pdf

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