Tissue‐Adaptive Materials with Independently Regulated Modulus and Transition Temperature

Abstract

Various types of clinical complications may emerge when a medical device (catheter, microelectrode, or microneedle patch) is deployed to the human body. It has been widely recognized that mechanical mismatch between an implant and a host tissue is one of the leading factors for adverse effects such as irritation and inflammation, which is evident in orthopedic, [1] neural, [2,3] and reconstructive implants. [4] It is generally desired that an implant material precisely matches Young's modulus of the surrounding tissue that may range between E ≈ 10 2 Pa of supersoft brain and adipose tissues to E ≈ 10 5 Pa of muscle and skin. However, forceful implantation of biomedical devices such as piercing, insertion or twisting impose significant mechanical stresses on the device. [5-8] Therefore a certain rigidity is desired to prevent device failure, minimize tissue damage, and simplify storage and handling procedures. [9,10] This presents an oxymoronic challenge for an ideal implant: designing an adaptive material that readily penetrates tissue, but transitions into a tissue-soft implant upon insertion into the body (Figure 1A). Thermoplastics are one of the most commonly used materials possessing a well-defined transition between hard and soft states with Young's moduli of E hard ≈ 1 GPa and E soft ≈ 1 MPa, respectively (Figure 1B, Table S1, Supporting Information). However, conventional thermoplastic materials have two problems: i) E soft ≈ 1 MPa is significantly higher than soft tissue modulus ranging within 10 2-10 5 Pa and ii) softening (melting or glass) transition temperature of high molecular weight polymers cannot be tuned without altering their chemical composition. It is even more challenging to adjust the mechanical and thermal properties independently of each other. The softness issue alone could be resolved by using different types of bioinspired polymeric gels. [14-17] However, not only it is difficult to prepare gels with a sharp GPa-to-kPa transition (Figure 1C, Table S2, Supporting Information), their solvent fraction instigates uncontrolled leaching inside the body, [13] which causes various health risks. The second problem can be addressed via copolymerization of monomers with different melting temperatures. [18-20] However, this involves chemical composition and biocompatibility variation and yet, copolymer modulus is limited by chain entanglements to >10 5 Pa. [21,22] The ability of living species to transition between rigid and flexible shapes represents one of their survival mechanisms, which has been adopted by various human technologies. Such transition is especially desired in medical devices as rigidity facilitates the implantation process, while flexibility and softness favor biocompatibility with surrounding tissue. Traditional thermoplastics cannot match soft tissue mechanics, while gels leach into the body and alter their properties over time. Here, a single-component system with an unprecedented drop of Young's modulus by up to six orders of magnitude from the GPa to kPa level at a control...

Reversible switching of polymeric gel structure and property by solvent exchange

“…The distinctive attributes of the dynamic polymeric gels, such as configurable mechanical behavior [1-6], external stimuliresponsiveness [7-13] and self-healing ability [14-17], underlie their extensive applications in materials science, ranging from tissue-adaptive biomaterials [18][19][20] through high-efficient energy materials [21][22][23] to intelligent information materials [24][25][26]. For instance, stiffness-changing trait (e.g., rigid-to-soft transition) is especially desired in medical devices, since the rigid state allows for the easy insertion, while the soft state favors biocompatibility with surrounding tissues [19,27].…”

“…The distinctive attributes of the dynamic polymeric gels, such as configurable mechanical behavior [1-6], external stimuliresponsiveness [7-13] and self-healing ability [14-17], underlie their extensive applications in materials science, ranging from tissue-adaptive biomaterials [18][19][20] through high-efficient energy materials [21][22][23] to intelligent information materials [24][25][26]. For instance, stiffness-changing trait (e.g., rigid-to-soft transition) is especially desired in medical devices, since the rigid state allows for the easy insertion, while the soft state favors biocompatibility with surrounding tissues [19,27]. Therefore, tremendous efforts have been devoted to imparting dynamic features to polymer networks through designing unique chemical compositions and structural motifs, which are capable of transforming the chemical structure, topological structure or conformational structure of the cross-links [1, [27][28][29].…”

“…Therefore, tremendous efforts have been devoted to imparting dynamic features to polymer networks through designing unique chemical compositions and structural motifs, which are capable of transforming the chemical structure, topological structure or conformational structure of the cross-links [1, [27][28][29]. However, those existing approaches either require dedicated and sophisticated chemical design or suffer from narrow operational state and poor reversibility [1, 19,28,29]. Overall, there remains a significant need for effective strategies to continuously and reversibly regulate the structures and properties of polymeric networks.…”

Recent Strategies for Strengthening and Stiffening Tough Hydrogels

“…Such synthetic hydrogels have recently started emulating soft and tough tissues such as the neural, muscle, and epithelial tissues whose elastic moduli lie in the range of a few pascals to several kilopascals ( Figure 1 ). [ 14–17 ]…”

“…Such biomechanical match can minimize the implanted hydrogel deformation and mechanical/electrical decoupling between the hydrogel and the bioelectronics. [ 14,16,23,24 ] However, the development of highly strong and stiff hydrogels with mechanical properties similar to those of the load‐bearing tissues still remains a challenge.…”