近幾年來有關穿戴式感測器的研究正蓬勃發展。由於這些裝置在受力變形時會產生電阻值差異，因此可藉由電阻值的變化來偵測目標部位的動作。目前多數的穿戴式裝置是由水凝膠製備而成。然而，由於水分的易揮發性，導致這些裝置容易失去原有的性質。此外，受限於傳統模具成形技術，難以製造出具有複雜結構的穿戴式裝置。因此，同時找到一個可以取代水凝膠且在空氣中有高穩定性的材料，以及一個能夠製造複雜結構的方法，是本論文的主要研究方向。
在本研究中，我們結合深共熔溶劑和纖維素奈米晶體以製備可三維列印和紫外光固化的墨水。深共熔溶劑在此作為替代水的溶劑，具有低揮發性以及有效分散奈米纖維素晶體的能力，更富含大量的離子以提供離子導電性。在深共熔溶劑中的纖維素奈米晶體可形成強韌的物理網狀結構，並且可作為流體流變行為的修飾劑以及膠體機械強度的增強劑。在纖維素奈米晶體網狀結構和聚丙烯酸-鋁離子網狀結構共同作用下，此奈米複合凝膠展現出高拉伸性、強韌性和消散能量的能力。在經過許多流變和機械性質上的測試後，我們選擇了含有21 wt% 纖維素奈米晶體和1 mol% 鋁離子的有機凝膠，以三維列印製備具有拉脹行為的穿戴式感測器。結果顯示，此穿戴式感測器不僅具有高敏感度，並且能夠透過拉脹行為表現精準地偵測人類關節彎曲產生的電阻值差異。有了相對應的電阻訊號圖形，我們能夠有效辨識人類的動作。另外，值得注意的是，比較此有機凝膠和相同配方的水凝膠，我們所製備出的凝膠在空氣中具有高穩定性且能夠避免水凝膠會遇到的縮水問題。
總結來說，本研究藉由結合纖維素奈米晶體和深共熔溶劑展示了可再生的奈米複合凝膠，不只如此，因為此種凝膠具有理想的流變性質和機械性質，我們可以利用新興的三維列印程序去製備具有複雜結構的穿戴式裝置，這項研究提供了一個新穎奈米複合材料開發的策略。

Recently, wearable strain sensors were intensively studied due to their ability to detect human motions by the change of their resistance. Most of these sensors were prepared from hydrogels by traditional molding processes. However, the hydrogel sensors may lose their properties due to the water evaporation. It is also difficult to fabricate functional and complex structures by molding. Thus, it is necessary to find new materials as well as new methods to produce stable and structurally complicated sensors with high mechanical strength, conductivity, and stretch-ability.
In this study, we prepared a 3D printable ionogel from deep eutectic solvents (DESs), cellulose nanocrystals (CNCs), and ionically cross-linked polyacrylic acid (PAA). The DES composed of choline chloride (ChCl) and ethylene glycol (EG) served as a non-volatile medium with high ionic conductivity. With the shear-thinning behavior and high yield stress by the CNC physical network, the ink can be extruded and printed into a customized object with high shape fidelity. The ink was further cured into the ionogel by the photo-polymerization of acrylic acid in the presence of Al3+ ions to form another ionically cross-linked network. We found the ink with 21 wt% of CNC and 1 mol% of Al3+ was most suitable for the direct ink writing (DIW) process. By combining the ionically cross-linked PAA network and the CNC physical network, the ionogel exhibited excellent mechanical properties, such as high stretch-ability, toughness, and self-recovery. From the loading-unloading tests, we found the first CNC network indeed played an important role in dissipating energy during mechanical deformation.
When compared to hydrogels, we found the DES/CNC nanocomposite ionogel was more stable in the air due to the low volatility of the DES. In addition, the ionogel was fabricated into an auxetic sensor by the DIW process. The auxetic sensor possessed a high gauge factor (GF) and exhibited high performances in detecting different human activities by the change of resistance during the deformation. These results demonstrate a new strategy to fabricate stable, complex, and sensitive strain sensors from cheap and renewable feedstock by the DIW process.

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