Molecular Design of Photoresponsive Hydrogels for Biomimetic Actuation
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摘要: 高分子水凝胶是一类具有高含水量的聚合物交联网络，可在外界环境刺激下通过吸水或失水过程产生体积溶胀或收缩，良好的生物相容性和可调的力学性能使其在组织工程、仿生形变以及智能驱动等领域备受关注。光作为一种无接触式、可快速开关和具备高时空分辨率的刺激信号，可通过光热效应、光化学反应或光异构化等途径来调控水凝胶的宏观结构和性能，其中光异构化方法因其光照条件温和、高度可逆等优势有着更加广阔的应用前景。本文从光响应性水凝胶的分子设计、制备以及仿生智能驱动等方面，综述了当前基于分子开关异构化的光响应性水凝胶，并对其发展现状、面临的挑战和应用前景进行了展望。Abstract: Hydrogels are a type of high-water content and crosslinked polymer networks, which can produce a volumetric expansion or contraction through a water absorption or loss process under external environmental stimulation. Due to their good biocompatibility and adjustable mechanical properties, hydrogels have attracted wide attentions and have been extensively applied in the fields of tissue engineering, biomimetic deformation and intelligent actuation. Light is a non-contact, fast switching and high spatiotemporal resolution stimulus, and therefore is used to regulate the macroscopic structure and properties of hydrogels through photothermal effects, photochemical reactions or photoisomerization. Among them, photoisomerization of molecular switches has a broader application potential because of its advantages of mild irradiation conditions and high reversibility. In this review, we summarized the state-of-the-art of photoresponsive hydrogels based on molecular photoisomerization, emphasizing the principle of molecular design as well as applications in biomimetic actuation, and eventually address their perspectives and challenges in the future development.
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Key words:
- molecular switch /
- photoresponsiveness /
- hydrogel /
- soft actuator /
- spiropyran
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图 1 用于构筑光功能水凝胶的代表性分子开关的化学结构式
Figure 1. Chemical structures of the typical molecular photoswitches used in preparation of photofunctional hydrogels
图 2 含偶氮苯凝胶剂的光致凝胶-溶胶转变（a）[22]；含偶氮苯超分子水凝胶的DNA释放（b）[28]；含偶氮苯DNA水凝胶的溶胀-收缩（c）[29]；交联型偶氮苯异构化调节水凝胶力学强度（d）[30]
Figure 2. Sol-gel transition of azobenzene-containing hydrogelators triggered by light (a)[22]; DNA release by azobenzene-containing supramolecular hydrogels (b)[28]; Volume expansion-contraction of azobenzene-containing DNA hydrogels (c)[29]; Tunable mechanical strength of hydrogels induced by azobenzene isomerization (d)[30]
图 3 环糊精主体高分子-偶氮苯客体高分子识别导致的光响应超分子水凝胶（a）[31]；环糊精-偶氮苯接枝的光响应水凝胶的溶胀-收缩（b）[36]；基于环糊精-偶氮苯聚轮烷的人工肌肉（c）[38]
Figure 3. Photoresponsive supramolecular hydrogels formed by azobenzene-containing polymer and α-CD-containing polymer (a)[31]; Photoinduced expansion-contraction of azobenzene- and α-CD-containing hydrogels (b)[36]; Artificial muscles formed by azobenzene-based polyrotaxanes (c)[38]
图 4 基于二芳基乙烯的光致凝胶-溶胶转变（a）[42]；联吡啶-二芳基乙烯介导的水凝胶形状记忆（b）[44]；二芳基乙烯介导的金属-有机笼高分子网络拓扑结构改变（c）[45]
Figure 4. Photoinduced sol-gel transition based on diarylethylene (a)[42]; Shape memory effect of bipyridine-diarylethylene containing hydrogel (b)[44]; Diarylethylene-mediated topology change of metal-organic cage containing polymer network (c)[45]
图 5 紫外光驱动的分子马达微观转动导致凝胶宏观收缩（a）[49]；整合分子马达和调制器的紫外-可见光可逆响应凝胶（b）[50]；分子马达自组装形成光敏超分子水凝胶人工肌肉（c）[51]；分子马达自组装形成光敏水凝胶用于干细胞培养（d）[53]
Figure 5. Macroscopic contraction of the gel induced by ultraviolet light triggered microscopic rotation of the molecular motor (a)[49]; Reversible contraction-expansion of the gel integrating molecular motors and modulators upon irradiation by UV and visible light (b)[50]; Photoresponsive supramolecular hydrogel artificial muscles formed by self-assembly of molecular motors (c)[51]; Self-assembled photoresponsive supramolecular hydrogels for stem cell culture (d)[53]
图 6 光致收缩螺吡喃水凝胶的化学结构式（左）和收缩-溶胀示意图（右）（a）[57]；螺吡喃分子接枝量对水凝胶光致收缩性能的影响（b）[58]；不同取代基螺吡喃对水凝胶暗恢复速率的影响（c）[60]；螺吡喃水凝胶基底拓扑结构的变化（d）[58]；螺吡喃水凝胶用于电路微阀门（e）[63]；基于螺吡喃水凝胶的非线性光学器件（f）[65]
Figure 6. Chemical structure of photoshrinkable spiropyran hydrogels (left) and schematic diagram of the contraction-expansion process (right) (a)[57]; Effect of grafting ratio of spiropyran on hydrogel photoactuation (b)[58]; Effect of different substituents of spiropyran on hydrogel recovery rate in the dark (c)[60]; Changes in substrate topology of spiropyran hydrogel (d)[58]; Spiropyran hydrogel based for circuit microvalves (e)[63]; Spiropyran hydrogel-based nonlinear optical devices (f)[65]
图 7 含磺酸根修饰的光致溶胀螺吡喃水凝胶结构示意图[66]（a）；光致溶胀螺吡喃水凝胶光照后净电荷和体积变化（b）；光信号和pH信号的依次和同时施加对光扩展行为的影响（c）；高分子相转变温度与光致溶胀率之间的关系（d）
Figure 7. Schematic representation of the photoexpanding spiropyran hydrogel[66] containing sulfonate groups (a); Changes in net charge and volume of photoexpanding spiropyran hydrogels upon illumination (b); The effect of sequential and simultaneous application of light and pH signal on the phoptoexpanding behaviors (c); Relationship between polymer phase transition temperature and photoexpanding ratio (d)
图 8 棒状螺吡喃水凝胶的向光运动（左）[59]和背光运动（右）（a）[66]；花瓣型螺吡喃水凝胶的模仿植物花朵向光开放过程（b）[67]；光致溶胀-光致收缩协同效应导致双层水凝胶快速致动（c）[68]；由光活性双层和光惰性双层水凝胶组成的嵌段平整结构在照光后呈现出3D光致折纸变化（d）[68]
Figure 8. Positive phototropic motion (left)[59] and negative phototropic motion (right)[66] of rod-shaped spiropyran hydrogels (a); Flower-shaped spiropyran hydrogel mimicking the blooming process of plant flowers under light (b)[67]; The synergistic effect of photoexpansion-photocontraction leads to a rapid actuation of the bilayer spiropyran hydrogel (c)[68]; The block flat structures composed of photoactive bilayer and photoinactive bilayer hydrogel exhibit 3D origami shape changes after illumination (d)[68]
图 9 弧形螺吡喃水凝胶在倒刺基底上受光单向行走（a）[69]；分枝形超分子-共价杂化螺吡喃水凝胶在倒刺基底上受光单向爬行和转向运动（b）[67]
Figure 9. Arch-shaped spiropyran hydrogel walks unidirectionally under light on a ratcheted substrate (a)[69]; Branched supramolecular-covalent hybrid spiropyran hydrogel crawls and rotates on a ratcheted substrate subjected under light (b)[67]
图 10 两亲性多肽超分子纤维取向导致分枝形螺吡喃水凝胶爬行步幅的各向异性（a）[67]；具有倒刺结构的线形（b）和十字形（c）螺吡喃水凝胶在玻璃基底上受光单向行走[68]
Figure 10. The orientation of peptide amphiphile assembled supramolecular fibers leads to anisotropy of the crawling amplitude of the branched spiropyran hydrogel (a)[67]; Linear (b)[68] and cross-shaped (c)[68] spiropyran hydrogels containing ratcheted structures inside their bottom surfaces of feet walks unidirectionally under light on a glass substrate
图 11 光和磁双重响应螺吡喃水凝胶[70]结构示意图（a）；十字形螺吡喃水凝胶在光照下变形、在磁场中行走的过程（b）；光和磁双重响应螺吡喃水凝胶在磁场驱动下可在水平面（左）或倾斜面（右）行走（c）
Figure 11. Schematic representation of the spiropyran hydrogels with dual response of light and magnetic field[70] (a); The process of cross-shaped spiropyran hydrogels deforming under light and walking in a magnetic field (b); Light and magnetic dual responsive spiropyran hydrogels can walk in a plane (left) or inclined plane (right) driven by a magnetic field (c)
图 12 水凝胶机器人行走速率受光致形变调控（a）；水凝胶机器人在光和磁场作用下的精确转向运动（b）；水凝胶机器人用于货物的负载、运输和释放（c）；水凝胶机器人用于黏性货物的运输和释放（d）[70]
Figure 12. The walking rate of the hydrogel robot is regulated by photodeformation of the hydrogels (a); Precise steering motion of hydrogel robots under the synergy of light and magnetic fields (b); Hydrogel robots for capturing, transporting and releasing non-sticky cargos (c); Hydrogel robots for the transport and release of sticky cargos (d)[70]
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