Elsevier

Chemical Engineering Journal
化学工程杂志

Volume 424, 15 November 2021, 130418
第424卷，2021年11月15日，130418
Chemical Engineering Journal

Flexible, transparent, and antibacterial ionogels toward highly sensitive strain and temperature sensors
灵活、透明、抗菌的离子凝胶，适用于高灵敏度的应变和温度传感器

https://doi.org/10.1016/j.cej.2021.130418Get rights and content 获取权限和内容

Highlights 突出

  • •

    Highly flexible and transparent (94.3%) sensor are fabricated through TPU and IL.
    高度柔性和透明 （94.3%） 传感器通过 TPU 和 IL 制造。

  • •

    TPU/IL-based strain sensor exhibits wide strain sensing range (0.1–500%) and fast response (96 ms).
    基于 TPU/IL 的应变传感器具有宽应变感应范围 （0.1–500%） 和快速响应 （96 ms）。

  • •

    TPU/IL-based temperature sensor can work in a wide temperature window (−40-100 °C) with high detecting accuracy (0.1 °C).
    基于TPU/IL的温度传感器可在宽温度窗口（−40-100°C）内工作，检测精度高（0.1°C）。

  • •

    TPU/IL sensor exhibits excellent antibacterial activity and acceptable cytocompatibility.
    TPU/IL 传感器具有出色的抗菌活性和可接受的细胞相容性。

Abstract 抽象

Skin-like sensor has recently emerged for its promising applications in human motion monitoring and healthcare. However, it is challenging to develop flexible skin-like sensors multifunctionally coupled with high transparency and strain sensitivity, accurate thermosensation, and good antibacterial activity. Herein, we fabricate flexible and transparent (94.3%) ionogels by facilely mixing ionic liquid (IL) and thermal-plastic polyurethane (TPU) to be as skin-like sensors. The prepared TPU@IL ionogel-based sensor exhibits wide strain range (0.1–500%) and fast response (96 ms) to strain stimuli. Attributed to the temperature-dependent conductivity and low freezing point of IL, the ionogels can be served as stretchable temperature sensor, containing wide operating temperature window (−40-100 °C) and high detecting accuracy (0.1 °C). Moreover, the ionogels exhibit excellent antibacterial activity. This work may inspire future research to design full-featured skin-like sensors.
类皮肤传感器最近因其在人体运动监测和医疗保健方面的应用而崭露头角。然而，开发多功能的柔性皮肤状传感器具有挑战性，这些传感器具有高透明度和应变灵敏度、准确的热感和良好的抗菌活性。在这里，我们通过将离子液体 （IL） 和热塑性聚氨酯 （TPU） 轻松混合作为类皮肤传感器来制造柔性和透明 （94.3%） 的离子凝胶。制备TPU@IL离子凝胶传感器具有较宽的应变范围（0.1–500%）和对应变刺激的快速响应（96 ms）。由于IL的电导率与温度有关，凝固点低，离子凝胶可用作可拉伸温度传感器，具有较宽的工作温度窗口（−40-100°C）和高检测精度（0.1°C）。此外，离子凝胶表现出优异的抗菌活性。这项工作可能会激发未来的研究，以设计功能齐全的类皮肤传感器。

Introduction 介绍

Skin-like sensors have gained tremendous attentions due to their potentials in human body motion monitoring [1], [2], [3], [4], [5], healthcare management [6], [7], and human–machine interaction [8], [9]. Generally, most of skin-like sensors originate from two types of sensors, namely electronic sensors and ionic sensors. The electronic sensors are normally constructed by integrating rigid electronic conductors (e.g., metal nanoparticles or nanowires, carbonaceous nanoparticles, and conducting polymers) or liquid metal with stretchable or elastic polymers [10], [11], [12], [13], [14]. This type of sensors cannot bear large mechanical strain and their complex geometric structures degrade the optical transparency [12], [14]. It is clear that transparent architecture of the strain sensors not only provides a user acceptance for daily wearing but also allows the possibility to integrate with other devices such as display components and photovoltaic cells. [15], [16], [17]. By contrast, ionic sensors, including ion-conductive hydrogels and ionogels, exhibit high stretchability, sensitivity, and optical transparency [18], [19], [20], [21]. These ionic sensors still maintain high sensitivity even at large deformation because the sensing component is mobile ions [22]. Although hydrogels, consisting of polymer network and plenty of water, can serve as an ideal ionic sensor when incorporating with aqueous inorganic salt [23], [24], [25], these hydrogel-based sensors suffer from poor environmental stability. The reason is that their ionic conductivity can be severely deteriorated due to the dehydration or evaporation of water. Moreover, the hydrogel-based sensors also lose their sensing functions once the temperature increases above the boiling point or decreases below the freezing point of water. Though, organohydrogels constructed by introducing binary solvents (glycerol-water or glycol–water) instead of pure water into polymer networks, exhibit excellent freezing tolerance [26], water loss is still an inevitable problem at high-temperature conditions.
类皮肤传感器因其在人体运动监测[1]、[2]、[3]、[4]、[5]、医疗管理[6]、[7]和人机交互[8]、[9]方面的潜力而受到广泛关注。一般来说，大多数类皮肤传感器起源于两种类型的传感器，即电子传感器和离子传感器。电子传感器通常由刚性电子导体（例如，金属纳米颗粒或纳米线、碳质纳米颗粒和导电聚合物）或液态金属与可拉伸或弹性聚合物集成[10]、[11]、[12]、[13]、[14]来构建。这种类型的传感器不能承受较大的机械应变，其复杂的几何结构会降低光学透明度[12]，[14]。很明显，应变传感器的透明结构不仅为用户提供了日常佩戴的接受度，而且还允许与其他设备（如显示组件和光伏电池）集成。[15], [16], [17].相比之下，离子传感器，包括离子导电水凝胶和离子凝胶，表现出高拉伸性、灵敏度和光学透明度[18]、[19]、[20]、[21]。这些离子传感器即使在大变形下仍能保持高灵敏度，因为传感组件是移动离子[22]。尽管由聚合物网络和大量水组成的水凝胶在与水性无机盐结合时可以作为理想的离子传感器[23]，[24]，[25]，但这些基于水凝胶的传感器的环境稳定性较差。原因是它们的离子电导率会因水的脱水或蒸发而严重恶化。 此外，一旦温度升高到沸点以上或降低到水的冰点以下，基于水凝胶的传感器也会失去其传感功能。尽管通过将二元溶剂（甘油-水或乙二醇-水）而不是纯水引入聚合物网络而构建的有机水凝胶表现出优异的耐冻性[26]，但在高温条件下，水分损失仍然是一个不可避免的问题。

To overcome these defects, ionogels, composed of polymer networks and ionic liquids (ILs), are constructed to replace hydrogel-based ionic sensors by means of unique characteristics of ILs, such as high ionic conductivity, nonvolatility, and thermal and chemical stability [27], [28], [29], [30]. Therefore, utilizing ionogels is a bright way to construct highly conductive and environmentally stable skin-like sensors. However, most existing ionogels require sophisticated syntheses of conductive polymer or the precise design of complicated internal structure, limiting their further development [29], [30]. In addition, for ionogel-based sensors, the capability of temperature sensing, especially a subtle thermosensation, is an essential function of e-skins. However, most of ionogel-based sensors neglect to investigate the temperature detecting function. Furthermore, when the ionogel-based sensors are harnessed on the human body, human sweat or humid environment are prone to breed harmful bacteria, which is a potential threat to people’s health [31], [32]. Therefore, it is desired to introduce the thermosensitivity and antibacterial functions into the ionogel-based skin-like sensors.
为了克服这些缺陷，构建了由聚合物网络和离子液体（ILs）组成的离子凝胶，以通过ILs的独特特性（如高离子电导率、非挥发性、热稳定性和化学稳定性）取代基于水凝胶的离子传感器[27]、[28]、[29]、[30]。因此，利用离子凝胶是构建高导电性和环境稳定的类皮肤传感器的明智之举。然而，大多数现有的离子凝胶需要复杂的导电聚合物合成或复杂内部结构的精确设计，这限制了它们的进一步发展[29]，[30]。此外，对于基于离子凝胶的传感器，温度传感能力，尤其是微妙的热感，是电子皮肤的一项基本功能。然而，大多数基于离子凝胶的传感器都忽略了对温度检测功能的研究。此外，当基于离子凝胶的传感器被利用在人体上时，人体汗液或潮湿环境容易滋生有害细菌，对人们的健康构成潜在威胁[31]，[32]。因此，需要将热敏性和抗菌功能引入基于离子凝胶的皮肤状传感器中。

Herein, we report a flexible, transparent, and antibacterial ionogel with both strain- and temperature-sensitive features by integrating a mechanically robust thermal-plastic poly(urethane) (TPU) networks with ILs. The resultant TPU@IL ionogel can serve as not only stretchable and transparent strain sensor with large strain sensing range (0.1–500%) and high transparency (up to 94.3%), but also temperature sensor possessing good sensitivity in a wide temperature range (−40-100 °C) and high detecting accuracy (0.1 °C). Meanwhile, this ionogel-based sensor has apparent antibacterial capacity and acceptable cytocompatibility. This work paves the way for the applications of ionogels in flexible strain and temperature sensors, which can inspire future work on the development of skin-like electronics.
在此，我们通过将机械坚固的热塑性聚氨酯（聚氨酯）（TPU）网络与IL集成，报告了一种柔性、透明和抗菌的离子胶，具有应变和温度敏感特性。由此产生的TPU@IL离子胶不仅可以作为可拉伸的透明应变传感器，具有较大的应变感应范围（0.1-500%）和高透明度（高达94.3%），还可以作为温度传感器，在宽温度范围内（−40-100 °C）具有良好的灵敏度和高检测精度（0.1 °C）。同时，这种基于离子凝胶的传感器具有明显的抗菌能力和可接受的细胞相容性。这项工作为离子凝胶在柔性应变和温度传感器中的应用铺平了道路，这可以激发未来开发类皮肤电子学的工作。

Section snippets

Results and discussion

The ionogels are fabricated by mixing TPU and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) (Details see Supporting information). [EMIM][NTf2], a kind of ILs, is selected as the sensing component to fabricate ionogels with TPU due to its good miscibility with the TPU, low viscosity, high ionic conductivity, and good electrochemical stability (Fig S1, Supporting information) [33], [34]. The obtained ionogels are designated as TPU@ILx, in which x represents the mass

Conclusions

In summary, we have successfully developed a flexible and transparent ionogel-based sensor with excellent stretchability, large strain sensing range, wide working temperature range, and superior antibacterial activity. The existence of IL endows the TPU@IL ionogel with the stable electrical property and high sensing performance. Consequently, this ionogel sensor exhibits good electromechanical behaviors, including ultralow detection limit (0.1%), wide strain range (500%), and excellent

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the financial supports of Zhejiang Provincial Natural Science Foundation of China (LR20E030003), National Natural Science Foundation of China (52073078, 21774126, 52003070), and the start-up fund from Hangzhou Normal University.

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