光驱动偶氮聚合物水凝胶执行器

引言

1995年，两个研究小组报道了非晶侧链偶氮聚合物的光致形变，他们观察到在暴露于周期性光图案的偶氮聚合物薄膜上刻写了被称为光栅的物理表面形貌^1,2。几年后，在均匀强度照明下，人们观察到纯偏振图案也能产生类似的薄膜结构化效应，这表明存在方向性、偏振相关的形变效应³。最终，球形偶氮聚合物颗粒⁴和预制微柱⁵沿可见光写入光束偏振方向的定向拉伸得到了直接证实。此后，这些材料在微纳加工领域得到了更广泛的应用^6,7，该基本效应及其薄膜表面结构化能力的新应用仍在不断涌现，例如在光学和光子学领域^8,9,10。与此同时，这些关于偶氮聚合物光致形变方向性内在控制的发现，也启发人们探索其在动态场景中的应用，例如作为具有动态可调形貌的体外细胞培养基底^11,12,13,14。

然而，要将偶氮聚合物设想为可在显微镜下直接、自由且反复控制其形状的动态致动器，仍存在若干挑战。首先，非晶偶氮聚合物的形变是塑性的，先前的形变不易被覆写，因为通常没有办法逆转已经刻写的形变^15,16,17。这限制了致动器的连续控制，因为致动器会累积光致形变，实际上在所有采用的形变方向叠加下变得越来越变形。其次，在水环境中辐照时缺乏机械稳定性，已被认定为生物或微流控应用的一大挑战^18,19。第三，虽然偶氮聚合物的光致软化可以带来诸如受损薄膜自修复等特性²⁰，但它也会使形变微结构具有黏性。因此，形变结构各部分与底层基底之间的黏附，似乎在较大尺寸的超微米级偶氮聚合物结构形变能力低得多这一现象中发挥了作用^21,22。综合起来，这些因素限制了这一独特但仅被部分理解的光力学现象的应用，该现象既涉及偏振光下偶氮染料的统计性取向（Weigert 效应）^23,24，也涉及光致软化效应^25,26。

另一方面，刺激响应水凝胶，即由外部或环境控制的溶胀水聚合物网络，是材料科学中一个蓬勃发展的领域，近年来提出了许多光控光致形变方法^27,28。目前，这些方法中的许多依赖于由例如光热效应^29,30,31或光化学反应^32,33引起的局部但基本各向同性的体积变化。在某些情况下，这些效应可通过机械约束产生定向的致动器响应，例如弯曲^34,35。光力学响应的固有方向性可通过将溶胀/去溶胀及其他致动类型与基底材料各向异性相结合来实现，例如源于凝胶中取向排列的纳米片^36,37或纤维结构³⁸，有时还与成分梯度³⁹或额外的例如磁性刺激⁴⁰相结合。然而，在所有这些方法中，局部光致形变方向在制造过程中即被固定，在材料使用过程中无法改变。

在此，我们介绍偶氮聚合物-水凝胶（azoGel）复合致动器，其将偶氮聚合物纳米颗粒作为光致形变驱动单元嵌入水凝胶基体中。有趣的是，该复合材料不仅继承了颗粒在任意方向上发生物理形变的能力——我们通过照明偏振对其进行控制——还表现出由颗粒-水凝胶物理相互作用所决定的良好形变可覆写性。除了通过对立方微柱进行形变和跟踪来量化这些效应外，我们还提出将这些复合材料用于打印微孔中，作为光控微阀。我们展示了后者如何仅以照明偏振作为信号即可多次开启和关闭。

结果

致动器的偏振导向光致形变

首先，按照图 1a 所示的制造流程制备了立方形（约 10 × 10 × 10 μm³）azoGel 微柱致动器（详见“方法”部分）。简而言之，采用溶剂蒸发法制备了由疏水性光致形变偶氮聚合物聚（分散红1甲基丙烯酸酯-共-甲基丙烯酸甲酯）p(DR1m-co-mma) 构成的尺寸分布较宽（10 nm 至 250 nm，见图 S1）的纳米颗粒，并用低分子量（约 13–23 kDa）聚乙烯醇（PVA）稳定剂进行位阻稳定⁴¹。为促进形成稳固的物理交联水凝胶复合网络⁴²，在将颗粒水溶液浇铸到柔性微结构模板中之前，加入了高分子量（约 360 kDa）聚乙烯吡咯烷酮（PVP），随后干燥，最终进行退火处理并在水中再溶胀。注意，需要在存在足量偶氮聚合物颗粒的情况下进行高温退火（135 °C），以防止纳米复合结构在水再浸入过程中溶解（图 S2）。

当溶胀的azoGel立方体随后从顶部受到线偏振绿色（波长532 nm）激光束照射90 s时，观察到沿偏振方向平行方向上持续增长的伸长（图1b和补充影片S1）。因此，azoGel立方体继承了单个偶氮聚合物纳米粒子的偏振引导变形行为^4,43，但处于超粒子尺度。这印证了我们最近在热塑性弹性体中在低得多的应变值下的发现，即弹性基体能有效地将嵌入的偶氮聚合物粒子的单个变形传播到整个复合材料，使其整体发生定向变形¹⁷。除了明场显微镜图像（上排）外，变形序列还使用共聚焦显微镜图像以3D方式可视化（下排）。这些图像显示出沿z轴（入射方向）变形递减的轮廓，这是由于基底附着以及激光束在立方体高度方向上的渐进吸收^44,45。实际上，通过降低偶氮粒子含量从而降低复合材料的吸收率，可以减小面外变形梯度（图S3）。注意，当使用圆偏振照明代替线偏振时，嵌入的偶氮聚合物粒子预期会在平面内发生各向同性压扁⁴⁶。因此，我们观察到在这种情况下从azoGel立方体产生了帽沿两个面内方向扩展的蘑菇形状（图S4），令人联想到在纯偶氮聚合物微柱中报道的类似结构⁴⁷。我们还更一般地注意到，复合材料驱动器的上表面在这些共聚焦图像序列中经历了相当均匀的拉伸，这表明偶氮聚合物纳米粒子在水凝胶中分散良好，在驱动器微尺度上形成了均匀响应的纳米复合材料。

上图所示立方体在线偏振光照下的单向变形行为随时间的量化结果见图 1c。这些立方体的干质量由 50% PVA 包覆的偶氮聚合物纳米颗粒和 50% 长链 PVP 组成（Azo NP:(50%)PVP Gel）。图中绘制了不同光强下干燥状态（虚线）和溶胀状态（实线）的行为。我们注意到，由于溶胀本身，溶胀立方体的初始宽度似乎略大，这一点在图 S5 中有更精确的评估。有趣的是，在干燥状态下没有产生明显的立方体变形，我们认为这是由于此时 PVP 基质的刚度高得多，处于吉帕量级⁴⁸。由于偶氮聚合物纳米颗粒在光照下也会发生光致软化，它们的光致变形受到大得多且坚硬的干燥基质的力学约束抑制，这是合理的。在湿润的溶胀状态下——PVP 的刚度通常低几个数量级——变形则相当显著。然而，尽管初始变形速率高度依赖于照射光强，所有曲线都趋于约 200% 的最大相对变形饱和值。这一出人意料的行为表明，水凝胶基质——尤其是溶胀的 PVP 网络——对整体光致变形有明显影响，恢复力将最大可达变形程度限制在一个与光强无关的饱和值。

相比之下，作为一种特殊情况，我们还制备了不添加高分子量 PVP 的样品（Azo NP Gel）。在这种情况下，亲水基质仅由最初添加的低分子量 PVA 稳定剂构成，样品的溶胀程度较小（图 S5）。尽管如此，在水溶胀状态下仍可观察到大得多的光致变形，而且与上述行为不同，这些样品不存在独特的变形极限。相反，这些立方体会变形为细长的线状结构（图 1d）。事实上，尽管这些结构可能变得起伏不定、难以追踪，但在较高强度照射下，Azo NP Gel 立方体上可观察到高达 1600% 的塑性伸长（图 S6 和补充影片 S2）。我们推测，对于 Azo NP Gel，由于缺乏长 PVP 链，仅由包覆颗粒的 PVA 稳定剂形成弱物理网络，因此在低应力下即可发生塑性变形。实验上，这与流变测试中溶胀 Azo NP Gel 表现出的强机械压痕滞后相一致（图 S7），而 PVP 交联的凝胶（如 Azo NP:(50%)PVP Gel）则表现出更偏向弹性的行为。尽管如此，要解释 Azo NP Gel 立方体这种异常的伸长，还需要额外的效应。事实上，这些伸长不仅超过了文献中基于分子取向材料的复杂方法所获得的变形（380%）⁴⁹，也超过了我们下文介绍的纯 Azopolymer Control 立方体（285%），甚至远超文献报道的纯偶氮聚合物纳米粒子在固体基底上的最高伸长率（约 500%）⁴³。不粘附行为可能是部分解释。在纯偶氮聚合物中，光致软化变形的偶氮聚合物与基底之间的粘附常被认为是限制变形的因素，例如会将致动器变形的翼片钉扎在下方表面上²¹。在这里，亲水基质阻止了偶氮聚合物颗粒与基底的直接接触，尽管明显的翼状结构看似接触到底部表面（图 S3a），变形仍不受阻碍地继续进行（补充影片 S2）。此外，还可以设想立方体厚度方向上类似洋葱的逐层变形。由于该样品中偶氮聚合物颗粒含量高，因而顶层非常薄且变形强烈（图 S3a），最初被照射的顶部体积部分可能被充分位移，从而使更深层的立方体层逐渐暴露于光照之下。这些层随后可进一步促进变形结构边缘的位移，导致 1600% 的极端表观伸长。

最后，作为最后一组对照实验，我们还考察了由纯疏水性偶氮聚合物（Azopolymer Control）制成的立方体。作为唯一的样品，这些立方体在干燥和湿润条件下的形变相似，因为它们不会溶胀。此外，我们观察到，湿润的偶氮聚合物残余层在高光照强度下以气泡状方式分解，正如其他人所报道的⁵⁰，这与凝胶材料的完整性形成对比。此外，与 Azo NP Gel（不含 PVP 的样品）类似，Azopolymer Control 立方体在长时间光照后的形变程度取决于强度，且不存在饱和极限，这再次表明其具有粘塑性行为。事实上，在无恢复力的情况下，纯偶氮聚合物在辐照下的纳米压痕塑性形变行为是众所周知的²⁵。Azopolymer Control 立方体以及所有其他测试样品（还包括含 85% PVP 干质量的（Azo NP:(85%)PVP Gel）立方体）的定向形变视频和连续形变曲线，见补充影片 S3 和图 S8。

覆盖先前的形变

接下来，我们关注水凝胶驱动器沿不同方向依次形变的可能性。这种形变的可覆盖性在软体水凝胶驱动器需要抓取或固定物体的应用中似乎至关重要，例如脑类器官⁵¹，且可能需要经过多次后续的操控和释放步骤。有趣的是，形变的可覆盖性此前已在弹性体基质中的单个偶氮聚合物颗粒中被报道⁵²。在他们的开创性工作中，Ryabchun & Bobrovsky 用基质中储存的弹性恢复力——连同光照下嵌入的偶氮聚合物颗粒的光致软化——来解释为什么当对颗粒写入第二个不同取向的光致形变时，颗粒会沿初始形变方向完全回缩。后来，我们的研究组在由致密偶氮聚合物颗粒和热塑性弹性体基质组成的复合材料中，也发现了类似的小应变宏观形变（<1.5%）效应¹⁷。值得指出的是，这种行为对偶氮聚合物而言并非不言而喻。相反，在多种情形下，偶氮聚合物的形变——在没有恢复力的情况下——已被证明是叠加过程。例如，在薄膜上施加不同取向的顺序干涉图案可以产生叠加的方形光栅¹⁸。类似地，对纯偶氮聚合物微柱施加两个正交偏振（且剂量相同）的顺序光照步骤，会得到扁平形状，沿两个形变轴的伸长量相近^15,17。而覆盖则要求先前的形变在写入新形变的同时被主动擦除。

在此，为了评估大量 azoGel 形变的可覆写性，我们设计了一个实验：用一系列等间隔曝光（每次 25 s）照射微立方体，交替采用正交的照明偏振，如图 2a 所示。将该序列施加于 Azo NP:(50%)PVP Gel 立方体的结果见补充影片 S4，快照显示于图 2b。图 2c 显示了 32 个立方体在 x 和 y 方向上的跟踪宽度以及样品 xy 平面内的致动器面积。从图和图像中可以看出，尽管拉伸幅度沿照明序列略有下降，但仍可获得受控且可重复的致动器横向尺寸周期性调制。有趣的是，正如影片中也可见的那样，先前照明后的弹性恢复似乎比下一个正交应变的建立更快，这一点反映为每次照明开始时面积的轻微下降。这一观察结果在图 S9 的附加快照中得到说明，显示致动器在覆写过程中向新方向扩展之前，先恢复了其大部分先前形变。最后，我们发现这一特定观察结果以及总体的可覆写性均未受到所用光强度的强烈影响。因此，可根据应用调整该参数，并可获得相似的覆写幅度，例如采用仅 5 s 的短时高强度照明（补充影片 S5，图 S10）。

另一方面，正如前文所述单向变形行为的差异所预期的那样，弹性可覆写行为强烈依赖于样品类型。补充视频 S6 展示了对前述四种样品进行正交偏振、10 s 照明的序列，相关曲线绘制于图 S11。性能最差的是 Azopolymer Control 样品（纯偶氮聚合物）。在这种情况下，变形幅度迅速消失，致动器的边缘在多次曝光后似乎出现劣化，且每次照明步骤中相对面积增加量相对于变形幅度较大。最后这一观察证实了纯偶氮聚合物众所周知的塑性变形行为，即后续变形相互叠加而非相互覆写^16,53，从而导致材料的持久性压平¹⁷。Azo NP Gel 样品在每个照明步骤中也表现出可观的相对面积增加，这同样可归因于其强烈的粘塑性特征（图 S7），有利于塑性光致变形。然而对于该样品，变形幅度并未下降，且尽管残余层容易脱层和屈曲，致动器本身在实验过程中保持完好。最后，含 PVP 的常规 azoGel 样品在所测试的 15 个照明步骤中均表现出良好的稳定性和可覆写性，我们认为这与其在流变测试中主要表现出的弹性性质有关，因而在覆写过程中对光软化、受辐照的偶氮聚合物纳米粒子施加了强大的弹性恢复力⁵²。

为了通过直接观察进一步确认这种恢复效应，我们制备了另一个样品，其中将较大的偶氮聚合物颗粒（~1–2 μm，见方法）嵌入由含 PVP 的 Azo NP:(50%)PVP 配方制成的块体凝胶中。在正交偏振光照下，这些较大颗粒的光致形变可以在显微镜下直接观察到（补充视频 S7，图 S12）。因此，它们提供了关于此类复合材料中偶氮聚合物颗粒行为的指示，而对于本工作其他实验中使用的纳米颗粒，无法直接获得这种信息。我们发现，这些单个微粒的光致形变与复合材料本身的行为高度相似。形状不仅被高效覆写，而且在此我们观察到在覆写过程中形状会短暂恢复为圆形、膨胀较小的状态，随后才出现下一次形变。综合这些观察，我们发现有两个过程支配着这些材料中的写入和覆写，嵌入的偶氮聚合物颗粒和水凝胶基质均具有双重作用（示意图总结见图 2d）。具体而言，当原始复合材料被线偏振光照射时，光致软化的偶氮聚合物颗粒发生定向形变，水凝胶基质则传递其形变。光照停止后，形变的偶氮聚合物颗粒恢复其吉帕级刚性状态²⁵，复合材料的形变被固定。在正交偏振光照下，偶氮聚合物颗粒再次光致软化，但这一次除了定向形变过程外，还会在水凝胶基质中储存的弹性力作用下发生被动重塑。我们注意到，这种恢复分量与形状记忆聚合物共混物中遇到的概念有些类似。在其中一些材料中，嵌入的颗粒可以在拉伸的复合材料状态下被冷却至 T_g 以下，从而固定整体形状——直到加热至 T_g 以上使颗粒软化，并允许由周围基质介导的弹性恢复⁵⁴。

光控微孔

作为最后一项实验，为展望azoGel复合材料更广阔的应用前景，我们引入了由反立方体模板制备的微孔阵列。在PVP交联凝胶薄膜中形成的10 μm宽开口阵列，令人联想到植物叶片中调节气体交换的天然气孔⁵⁵，如图3a所示。利用正交偏振的照明序列，这些微孔可以反复闭合与重新开启，如图3b和补充影片S8所示。值得强调的是，这种行为——尤其是微孔的重新开启——之所以可能，仅源于该复合材料两种特性的结合，即良好的变形可覆写性，以及与纯偶氮聚合物自愈合特性形成对比的非黏附行为。纯偶氮聚合物的自愈合特性源于相邻聚合物体积可在光致再流动作用下合并，熔合为单一材料整体。例如，这一效应已被用于演示通过适当照明愈合偶氮聚合物裂纹，甚至在偶氮聚合物最初掺杂导电填料时，可恢复愈合裂纹两侧的导电性²⁰。然而在本文微孔的框架下，这种效应反而会带来不利影响，因为闭合中的微孔表面可能相互熔合，导致后续照明无法将其分离。相反，包覆偶氮聚合物颗粒的非黏附水凝胶基质可防止它们直接接触，即使不同致动部件相互接触也是如此，从而使完全闭合的微孔能够再次开启。

此外，利用共聚焦显微镜可以在 3D 中可视化孔关闭和打开的不同阶段（图 3c）。除了确认孔的关闭外，这些从底部视角展示的图像还可视化了变形顶层下方的封闭空间。具体而言，初始的方形孔形状在基底侧仍然可见，而孔的顶部已关闭。这是由于前面讨论的沿 z 轴的吸收效应，该效应确保只有顶部（面向入射光）部分发生显著变形。然而，由于 AzoGel 复合材料的变形深度可以通过偶氮聚合物浓度方便地调节（图 S3），因此闭合层的厚度也可以根据应用前景进行调节。此外，只有正确的偏振（与用于关闭的偏振正交）才能解锁并打开孔。因此，照明偏振就像一把钥匙，即使对整个样品进行（偏振的）全面曝光，也可以设想针对特定孔的开启。最后，我们指出，鉴于触发复合材料光致变形所需的典型中等强度，图 3c 中孔的打开是直接使用共聚焦显微镜内置的 10 mW、555 nm 激光器完成的。因此，在这种情况下，驱动和 3D 孔表面成像（补充视频 S9）直接在商用仪器上结合进行。

讨论

总之，我们引入了偶氮聚合物纳米粒子作为水凝胶基质中的变形驱动单元。我们成功证明，先前研究⁴³中描述的单个偶氮聚合物纳米粒子的大规模定向变形可以转移到微米级复合凝胶致动器上。与纯偶氮聚合物相比，azoGel 被发现更适合在水环境中长时间照明，并且在所用的较低照明强度（11 Wcm⁻²）下始终产生更大的变形。这两个方面都有利于生物技术器件的开发。尽管这里仅引入了简单的物理交联凝胶基质配方，但结果也表明，变形深度、可重写性和幅度等性质可以很容易地通过成分变化进行调节，在某些情况下还可以相互权衡。在表 1 中，总结了本文所研究的四种不同样品类型的一些关键光机械性能。简而言之，我们在力学纳米压痕研究中观察到，添加高分子量 PVP 使样品的响应比不含 PVP 的样品更具弹性，我们将其归因于更强的物理交联。相应地，含 PVP 样品的光致变形被限制在一个最大值——大概是由于恢复力阻止了进一步变形——而不含 PVP 的样品则发生了极端变形，这是我们在偶氮聚合物领域前所未见的。反过来，不含 PVP 的样品在重写尝试中表现出塑性变形行为，变形不断累积，而含 PVP 的样品由于弹性恢复效应表现出高可重写性。

在这方面，与此前报道的各向异性光致形变水凝胶相比，一个重要的根本区别如下：迄今为止，这类驱动器通常是在刺激响应型水凝胶中掺入取向排列的纳米材料来构建的^36,37,39。纳米材料的制备取向决定了最终器件的形变方向，而驱动本身则依赖于合适水凝胶基体的光响应，通常是聚（N-异丙基丙烯酰胺）（PNIPAM）。而在本研究中，形变方向不仅可以在复合材料使用过程中决定和改变，而且驱动形变的正是纳米材料（偶氮聚合物纳米颗粒）自身的各向异性形变。因此，基体只是一个被动元件，用于传递形变、提供弹性回复力，并使偶氮聚合物与周围水环境相接触。因此，我们不局限于某一特定类型的基体材料，而是预见可以探索范围广泛的、可能面向特定应用的不同水凝胶。例如，共价交联的丙烯酸类水凝胶可能是有前景的基体候选材料。在这种情况下，可以通过调节凝胶的交联剂比例来调控弹性性能，而不会间接改变例如偶氮聚合物颗粒含量。这对基础研究和应用研究都是一项有意义的改进。此外，过去已开发出含偶氮苯嵌段的嵌段共聚物^56,57,58，可以设想设计两亲性嵌段共聚物，使其直接相分离形成类似的光致形变偶氮聚合物-水凝胶复合材料。更一般地说，可以利用将偶氮聚合物颗粒掺杂到不同类型水凝胶中的方法、材料特异性的制备工艺以及生物功能化策略⁵⁹，并根据应用需求选择外部基体。

从应用角度来看，采用本工作所示的光致形变水凝胶柱和水凝胶孔，可能在生物技术和微流控的诸多挑战中具有应用价值。例如，光致形变偶氮聚合物柱阵列此前已被用作生物学中按需提供细胞指导的基底^12,13，而为这类结构赋予可重写性，应能实现完整的时空控制。在微流控领域，柱阵列可用于引导流动，也可用于控制待从流中分离颗粒的确定性侧向位移，其中各向异性的柱形状与取向已被提出用于精确裁剪器件^60,61。在此，动态柱元件也可能开辟更多可能性。关于生物技术中的水凝胶孔，我们注意到，越来越多的研究利用这类孔在精确定义的微环境中培养单细胞⁶²、细胞聚集体⁶³或类器官⁶⁴。这些工作所增加的功能包括：通过手动破坏覆盖孔的油封来提取细胞外囊泡；制造非平凡的封闭三维孔形以确定类器官发育；以及通过孔的刺激诱导溶胀响应释放细胞聚集体。借助本工作所示的局部化且厚度可调的定向形变，孔可以动态且非接触地执行这些功能。最后，在微流控领域已有若干光响应水凝胶阀的实例^65,66，其中一些基于与我们的孔相似的微孔⁶⁷，它们通常依赖光诱导的溶胀/去溶胀响应。在此背景下，完全关闭孔并在黑暗中自动保持所产生的任意形状这一可能性可能变得很有意义。尽管如此，我们预计，对各种水凝胶基质配方的探索也将使制造比孔和柱更复杂的几何结构成为可能，例如三维细胞支架⁶⁸或复杂的三维打印水凝胶执行器⁶⁹。

With this prospect of obtaining even more complex pristine azoGel actuator structures in 3D, it is worth briefly discussing the obtainable degrees of freedom in their deformation. Traditionally, works on azopolymer surface structuring have focused heavily on aspects such as optimizing the azopolymer molecular weight and film thicknesses^70,71, on the polymer chemistry^72,73, on refined illumination setups^8,74, and sometimes on surface reconfigurability^75,76. This has permitted strong advances in the obtained surface relief heights, the diffraction efficiencies, and in accurate surface profile shaping. Still, the local degree of freedom is essentially equal to one and corresponds to the local height of the surface profile, even if the directional deformations associated with polarization patterns were involved in the formation of said profile⁷⁷. By starting from overwritable pillars, the degrees of freedom theoretically increase to three, corresponding to the extent of directional pillar deformation, its angular orientation, as well as simultaneous deformation of the minor axis, which can in principle be obtained through the use of elliptical polarization^46,47. More so, if the illumination is not limited to normal incidence, but can hit a protruding structure from any incidence angle, the number of available deformation degrees of freedom increases to five⁷⁸. To this, one may add the possibility of providing illumination only at specific points of the entire actuator. On the other hand, such freedom of incidence direction and target locations would also require advanced illumination set-ups.

In the context of illumination strategies and how to practically deploy the potential of the azoGel actuators introduced here, we point out that the illumination is not limited to Gaussian laser beams. In fact, no coherent illumination is needed to deform azopolymers⁴⁴, and compact devices with tailored incidence directions and spatial control may even rely on micro-LED arrays or integrated waveguides to address and steer the actuators⁷⁹. Conversely, polarized blanket LED exposure, e.g., to selectively address pre-closed wells, or the use of the built-in components of conventional confocal laser microscopes¹³ may constitute cheaper and very convenient ways to control the photo-deformations in biological application scenarios. Finally, we note that in-situ patterning approaches for large 3D cell scaffolds usually rely on multiphoton illumination for localized functionalization⁸⁰. In this regard, it is worth noting that the directional deformation of the azopolymer type used here has also been demonstrated using two-photon inscription⁸¹.

Methods

Particle fabrication

An azopolymer solution of 10 mg/mL poly(Disperse Red 1 methacrylate-co-methyl methacrylate) p(DR1m-co-mma) (purchased from Sigma Aldrich, ~15 mol% dye monomer) in chloroform is prepared. Subsequently, the azopolymer solution is combined with an aqueous solution containing 0.5 wt% polyvinyl alcohol (PVA, purchased from Sigma Aldrich, MW: 13–23 kD, 87–89%, hydrolyzed) at a volume ratio of 1:12.5. The immiscible two-phase system is emulsified in a closed container immersed in an ultrasonic bath (Elmasonic P30H, 37 kHz, 100%) for several hours, with temperature constantly kept between 45° and 55 °C, and under occasional manual shaking, until appearing fully homogeneous by eye. Particles are obtained from the emulsion by transferring the system to an open beaker and slowly raising the temperature of the water bath until the chloroform is evaporated (final temperature 80 °C). Excess PVA is removed from the particle suspension by centrifuging and re-suspending the particles in pure DI water three times. The supernatant of all centrifugation steps is kept apart and measured against a 0.0025 mg/mL reference concentration in acetone using a UV-VIS spectrometer, to estimate the total loss of azopolymer during purification (~18% loss after three rounds, see Fig. S13). Then, the water amount during the last resuspension is adjusted to obtain a small volume of concentrated ~30 wt% azopolymer particle solution, which can be used as a precursor for the AzoGel fabrication below. In the case of the larger azopolymer particles used for the individual particle deformation studies in Fig. S12, a slightly modified protocol with a polyvinylpyrrolidone (PVP) precursor solution and different proportions was utilized as reported previously⁴⁶.

AzoGel fabrication

The particle precursor solution obtained above, consisting of 30 wt% azopolymer particles coated with PVA, is either used as such (for Azo NP Gel samples) or mixed with an aqueous solution containing 10 wt% polyvinylpyrrolidone (PVP, purchased from Sigma Aldrich, MW: 360 kD) at the proportion required to obtain the desired ratio of solid content between nanoparticles and PVP (for Azo NP:(50%)PVP and Azo NP:(85%)PVP samples). The precursor solutions are degassed in a vacuum desiccator and then dispersed between a clean (acetone and O₂ plasma treated) microscopy glass slide and polydimethylsiloxane (PDMS, Sylgard 184 purchased from Dow Corning) master molds. The master molds contain the inverse microstructures to be printed, i.e., negative replicas of 10 × 10 × 10 μm³ cubes/well arrays previously fabricated by conventional lithography with SU-8 photoresist. After drying, the PDMS mold is peeled off, and the microstructures are annealed on a hotplate in the dry state for 10 min at 135 °C, which exceeds the glass transition temperature of p(DR1m-co-mma) (Tg~105 °C, according to the provider). Before utilization as composite gels, the structures are routinely immersed in DI water for >1 h. Finally, control azopolymer samples are fabricated directly from a 30 wt% solution of p(DR1m-co-mma) in anisole, using the same PDMS master mold imprinting procedure as for the composite gels, without annealing.

Particle characterization

To obtain an estimate of the particle size distribution, diluted droplets of aqueous particle solution are dried on a piece of silicon wafer. The substrate is then sputter-coated with 10 nm of platinum-palladium. Images are obtained using a scanning electron microscope (SEM Apreo, Thermo Fisher) using the T1 detector (back-scattered electrons), 5 kV acceleration voltage, and a 50 pA electron current (magnification 30000×, image size 4096 × 3536 pixels, 1.1 nm/pixel). Particle outlines and sizes are detected using ImageJ by the following procedure: Convert to 8 bit ◄ Adjust Contrast (160–255) ◄ Auto Local Threshold v1.11 (“mean”, 15 pixels, P1 = 0) ◄ Convert to Mask ◄ Filter (“median”, 5 pixels) ◄ Fill Holes ◄ Watershed ◄ Analyze Particles (Circularity: 0.6–1.0). Finally, an average diameter is computed based on the detected area for each particle, and 20 nm are subtracted to account for the metal coating. The procedure is applied to 8 images from distinct regions of the sample (example images shown in Fig. S1), leading to the detection of 3491 particles. Finally, to obtain an estimate of the volume distribution the absolute particle counts were reweighted using the following equation (which leaves the total amount of renormalized particle counts unaltered):

$${h}_{i}=\frac{{n}_{i}\cdot {V}_{i}}{\sum _{j}{n}_{j}\cdot {V}_{j}}\cdot \begin{array}{cc}{\sum }_{j}{n}_{j}, & {V}_{i}=\frac{4}{3}\pi \cdot {r}_{i}^{3}\end{array}$$

where n_i designs the net particle count in the ith histogram interval, h_i designs its reweighted value used for the histograms in Fig. S1f, and r_i designs the radius of the particles at the center of the ith interval, which is used to calculate the characteristic Volume V_i. The distributions were plotted on a logarithmic scale.

Image acquisition procedures

For brightfield actuation videos, the sample glass slides are turned upside down and glued to the bottom of a microscopy dish using 200 μm of spacer tape, with the actuator structures facing downwards (towards the dish surface). The space between the dish and the sample glass slides is then filled with DI water using capillary forces. A 532 nm continuous wave laser diode with tunable output power is used to illuminate the samples from below (i.e., hitting the top surface of the bottom-facing micro-actuators). Optical elements placed in the laser path include lenses to adjust the spot size and polarization elements (polarizer, quarter- and half-wave plates) to control the polarization of the incident laser illumination. The videos are acquired in transmission configuration, with a broadband Tungsten-Halogen lamp providing brightfield illumination and a microscope setup consisting of an infinity-corrected 20×/0.4 NA air objective, a 550 nm long-pass filter, a tube lens, and a color CCD camera (Thorlabs) mounted on the opposite side of the laser and brightfield illumination sources. Note that although the long-pass filter removes direct laser illumination, high intensity creates fluorescence from the azopolymer, which is seen as a brightness change coinciding with the laser spot. For some single image acquisitions of final deformed structures, such as for the snapshots in Fig. 1b, the long-pass filter is removed to enhance contrast in the absence of laser illumination.

For 3D profiles of the actuators, a confocal microscope (Zeiss Airyscan 700) is used. The samples are placed in a dish with the actuator structures facing upwards and imaged using a 40×/1.0 NA water-dipping objective. In general, the 639 nm laser is used for 3D scans to avoid strong absorption of laser light by the azopolymer structures, thus relying on the scattering from the particle-gel structure for signal. The voxel size is 0.16 × 0.16 × 0.2 μm³. In Supplementary Movie S9, the 555 nm laser is used instead for continuous 3D scans, to both induce deformation (well opening) and perform 3D imaging of the top surface (here, the signal depth is absorption-limited). In Fig. S5a, the dry cube is imaged using a fluorescence signal at 580 nm (excitation wavelength 546 nm), due to low scattering in the dry case. When confocal image slices of cubes are segmented, as in Fig. S5 to determine swelling volumes, or in the context of Fig. S3 to determine the deformation depth, the following procedure in ImageJ is applied in general: First, the z-extent of the actuator is estimated by eye, determining an approximate first and last slice. The intensities are then normalized to the maximum found on an intermediate slice at half the height of the actuator. The precise height of the actuator is then set between an onset slice with clear signal detection and a final slice with a substrate background signal of equal intensity to the first slice. All slices are threshold-segmented at 20% of the maximal intensity previously obtained, leading to binary images of each actuator slice. Then, we apply the sequence Maximum 3D (2px, 2px, 4px) ◄ Fill holes ◄ Median (2px) ◄ Erode to detect smooth and continuous areas pertaining to the actuator on each slice. In the context of the Azo NP Gel cubes deformation depth (Fig. S3d, left), due to strong signal and proximity of the substrate background, the procedure is simplified to Erode -> Median (2px) -> Dilate.

Automated image analysis

A deep learning-based segmentation tool, Cellpose 3.1.1.1, is used for the automated image analysis. Firstly, the brightfield actuation videos are cropped to only the region of interest, normalized for brightness, and converted to images at 1 Hz. Next, one of the images is loaded into the Cellpose graphical interface and manually segmented with respect to azoGel cube regions for retraining models (learning rate: 0.1, weight decay: 0.0001 or 0.001, number of epochs: 100 or 200). The image loading and manual segmentation are repeated for approximately 50 images until the model is sufficiently accurate to segment regions. Retrained models are prepared for each material composition and illumination pattern. All converted images are then subjected to automatic segmentation of azoGel cube regions in Cellpose using the corresponding retrained model for each movie. Segmented areas are extracted directly, while widths and heights are calculated from bounding rectangles drawn around the segmented regions. For orthogonal polarization sequences (Fig. 2 and Fig. S11), a low-pass filter with a cutoff frequency of 0.2 Hz is applied to the acquired data of areas, widths, and heights. All procedures, except for model re-training, are performed in Python.

Rheological measurements

The Young’s modulus of the different azoGel composites was measured by a Bioscope (Bruker) atomic force microscope equipped with a Nanoscope IIIa controller. Silicon nitride pyramidal AFM tips (Bruker DNP) with a half angle of about 20° and a tip radius of curvature of about 10–20 nm were used. Different cantilevers were exploited according to the different rigidity of the sample (nominal spring constants: k = 0.06 N/m and k = 0.12 N/m). The effective spring constants of the cantilevers were measured by exploiting the thermal noise method⁸². To obtain a statistical analysis, for each sample, 6 sample points, respectively separated by at least 100 μm, were analyzed, and for each sampling point, a number of force curves ranging from 20 to 40 were acquired. Small indentation values of about 100 nm (corresponding to an applied force of less than 1 nN) were applied in order to limit the contributions coming from the underlying glass support. Vertical scan speeds of 2 μm/s were applied. The Nanoscope Analysis 1.5 software package was used to fit the Hertz model for pyramidal tips to force curves in order to obtain Young’s modulus for each curve. A Poisson ratio of 0.49 was assumed for all fits.

Data availability

The data and analysis codes that support the findings of this study can be made fully available.

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Acknowledgements

Prof. Alberto Puliafito (University of Turin) is acknowledged for technical assistance at the confocal microscope. We thank Dr. Beatrice Masante and Dr. Stefano Gabetti (Politecnico di Torino) for sample preparation during revisions. Prof. Bjørn Torger Stokke (NTNU) is acknowledged for fruitful discussions about hydrogel formulations and their mechanical properties.

Funding

National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 1409 published on 14.9.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– “Metastatic potential of cancer cell aggregates in deformable matrices controlled by light” – CUP E53D23015220001 – E.D. RSO - NTNU strategy and restructuring funds, by the Norwegian University of Science and Technology, funding a PhD fellowship under project number 989454111, and Norges tekniske høgskoles, funding a personal “Reisestipend” travel grant to the recipient – D.U. Research Council of Norway, Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 295864 – E.D. and D.U. Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – SFB 1459/2 2025 – 433682494 – M.R.

Author information

Authors and Affiliations

1. Department of Electronic Systems, Norwegian University of Science and Technology, Trondheim, 7491, Norway

   David Urban & Dag Roar Hjelme

2. Department of Applied Science and Technology, Politecnico di Torino, Torino, 10129, Italy

   David Urban & Emiliano Descrovi

3. Microsystems and Nanotechnology (MiNaLab), SINTEF Digital, Oslo, 0314, Norway

   David Urban

4. Faculty of Engineering, Hokkaido University, Sapporo, 060-8628, Japan

   Ryota Toyohara & Toshiro Ohashi

5. Institut für Physikalische Chemie, Universität Münster, Münster, 48149, Germany

   Marcel Rey

6. Dipartimento di Chimica “Ugo Schiff”, Università di Firenze, Sesto Fiorentino, 50019, Italy

   Daniele Martella

7. European Laboratory for Non-Linear Spectroscopy (LENS), Sesto Fiorentino, 50019, Italy

   Daniele Martella

8. Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, Modena, 41125, Italy

   Andrea Alessandrini

9. CNR-Nanoscience Institute-S3, Modena, 41125, Italy

   Andrea Alessandrini

Authors

1. David Urban
2. Ryota Toyohara
3. Marcel Rey
4. Daniele Martella
5. Dag Roar Hjelme
6. Andrea Alessandrini
7. Toshiro Ohashi
8. Emiliano Descrovi

Contributions

The authors contributed to the manuscript as follows: Conceptualization: D.U., E.D. Methodology: E.D., D.U., A.A., M.R., D.M. Investigation: D.U., E.D., A.A. Data analysis: R.T., T.O., D.U., A.A., E.D. Visualization: D.U. Funding acquisition: E.D., D.U. Project administration: E.D., D.U. Supervision: E.D., D.R.H. Writing – original draft: D.U. Writing – review & editing: D.U., E.D., M.R., D.M., R.T., T.O., A.A., D.R.H.

Corresponding authors

Correspondence to David Urban or Emiliano Descrovi.

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Conflict of interest

D.U. and E.D. are inventors on a patent application (Italian Patent, Application No: 102025000006681) filed by Politecnico di Torino and NTNU Technology Transfer AS on March 31, 2025. The authors declare that they have no other competing interests.

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Urban, D., Toyohara, R., Rey, M. et al. Photo-guided azopolymer hydrogel actuators. Light Sci Appl 15, 383 (2026). https://doi.org/10.1038/s41377-026-02411-5

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• Received: 26 October 2025

• Revised: 25 June 2026

• Accepted: 26 June 2026

• Published: 17 September 2026

• Version of record: 17 September 2026

• DOI: https://doi.org/10.1038/s41377-026-02411-5