Highly Elastic and Self-Healing Composite Colloidal Gels

Highly Elastic and Self-Healing Composite Colloidal Gels
高弹性和自愈合复合胶体凝胶

Colloidal gels are composed of a continuous network of assembled particles dispersed in a liquid. They are frequently used in applications such as ceramic and glass processing, paint and coating production, food products, cosmetics, and biomedical engineering. Unlike monolithic polymeric gels, colloidal gels exhibit a heterogeneous structure characterized by assembly of colloidal particles into strands which form a mechanically stable particulate network due to attractive interparticle forces. Colloidal gels are particularly appealing in view of their fascinating viscoelastic properties. By exploiting the reversibility of noncovalent interparticle interactions, colloidal gels can be rendered self-healing. However, current self-healing colloidal gels consist of polymeric particles and are mechanically weak, which restrict their applicability. The mechanical properties of colloidal gels can be improved by reinforcement with inorganic nanoparticles, but such composite colloidal gels that resist substantial compressive and/or tensile loads have not yet been reported.
胶体凝胶是由分散在液体中的聚集粒子组成的连续网络。它们经常用于陶瓷和玻璃加工、涂料和涂料生产、食品、化妆品和生物医学工程等领域。与单聚合物凝胶不同的是，胶体凝胶呈现出一种非均质结构，其特征是将胶体颗粒组装成束，在吸引粒子间力的作用下形成机械稳定的颗粒网络。胶体凝胶由于其迷人的粘弹性特性而特别吸引人。通过利用非共价粒子间相互作用的可逆性，胶体凝胶可以自愈。然而，目前的自愈胶体凝胶是由聚合物颗粒组成的，机械强度弱，限制了其适用性。无机纳米粒子的增强可以改善胶体凝胶的力学性能，但这种复合胶体凝胶能够抵抗大量的压缩和/或拉伸载荷，目前还没有报道。由于胶体凝胶的力学性能较弱，迄今为止只有流变试验证明其自愈能力。此外，用无机填料对胶体凝胶进行机械强化会降低合成材料的自愈能力。由于机械稳健性和自愈能力都是实际应用自愈凝胶所必需的，因此开发一种结合机械稳健性和自愈能力的胶体凝胶仍然是一项重大挑战。

Here, we developed a novel nanocomposite colloidal gel with an unprecedented combination of mechanical properties and self-healing capacity by: i) careful control of particle assembly, ii) fundamental understanding of the mechanism of gel network formation, and iii) precise tuning of composition and structure of the resulting gel networks. We used amphoteric soft gelatin nanoparticles and negatively charged hard silica nanoparticles as the organic and inorganic colloidal building blocks, respectively. To prevent uncontrollable aggregation resulting from direct mixing of attractive binary colloids, electrostatic assembly of the nanoparticles into homogeneous binary networks was facilitated by controlling the pH of the colloidal system. These homogeneous gels exhibit a remarkable self-healing capacity, as evidenced by immediate recovery of gel elasticity upon destructive shearing to values which even exceed initial gel elasticity. In addition, we show—for the first time—that colloidal gels can be prepared which not only resist substantial compressive and tensile loads, but also are able of complete self-healing upon shear- or cutting-induced failure. Our study provides new, critical insight into the structural and mechanical properties of composite colloidal gels and, more importantly, opens up a new horizon for applications of colloidal gel systems.
在这里，我们开发了一种新型的纳米复合胶体凝胶，它将机械性能和自愈能力前所未有地结合在一起:i)仔细控制颗粒组装，ii)对凝胶网络形成机制的基本理解，iii)对产生的凝胶网络的组成和结构的精确调整。我们分别使用两性软明胶纳米颗粒和负电荷硬二氧化硅纳米颗粒作为有机和无机胶体构建块。为了防止由于直接混合有吸引力的二元胶体而导致的不可控制的聚集，通过控制胶体系统的pH值，使纳米颗粒静电组装成均匀的二元网络。这些均匀凝胶表现出显着的自愈能力，如通过破坏性剪切立即恢复凝胶弹性所证明的甚至超过初始凝胶弹性的值。此外，我们首次展示了可以制备的胶体凝胶不仅能够抵抗大量的压缩和拉伸载荷，而且能够在剪切或切割引起的失效时完全自我修复。我们的研究为复合胶体凝胶的结构和机械性能提供了新的，重要的见解，更重要的是，为胶体凝胶系统的应用开辟了新的视野。

Colloidal gels formed by direct mixing of attractive colloids are often characterized by uncontrollable, nonuniform aggregation and phase separation which compromise their structural integrity and mechanical strength. Furthermore, pre-shear resulting from sample mixing and loading substantially alters the structural and mechanical properties of colloidal gels, thereby hampering analysis of the relationship between their structure and mechanical properties. Therefore, we mixed basic aqueous suspensions (20 × 10−3 m NaOH; pH ≈ 11) of gelatin nanoparticles (diameter ≈ 400 nm) and silica (diameter ≈ 80 nm) nanoparticles to ensure that all particles were negatively charged and homogeneously mixed. We then introduced glucono delta-lactone (GDL) as an acidifier which upon decomposition decreased the pH gradually from ≈11 to below the isoelectric point (IEP) of gelatin nanoparticles (pH < 8). As a consequence, the net charge of gelatin nanoparticles changed from negative to positive, thereby triggering self-assembly between oppositely charged silica and gelatin nanoparticles. Scheme 1 shows a general overview of the gel formation method used in our study.
通过直接混合有吸引力的胶体形成的胶体凝胶通常以不可控制的，不均匀的聚集和相分离为特征，这会损害它们的结构完整性和机械强度。此外，由样品混合和加载产生的预剪切基本上改变了胶体凝胶的结构和机械性质，从而妨碍了它们的结构和机械性能之间关系的分析。因此，我们将明胶纳米粒子（直径≈400nm）和二氧化硅（直径≈80nm）纳米粒子的碱性水性悬浮液（20×10 -3 m NaOH;pH≈11）混合，以确保所有粒子带负电并均匀混合。然后我们引入葡萄糖酸δ-内酯（GDL）作为酸化剂，其在分解时将pH从≈11逐渐降低至明胶纳米颗粒的等电点（IEP）以下（pH <8）。结果，明胶纳米粒子的净电荷从负变为正，从而引发带相反电荷的二氧化硅和明胶纳米粒子之间的自组装。方案1显示了我们研究中使用的凝胶形成方法的一般概述。