研究背景
电子废弃物(e-waste)的积累是一个紧迫的全球性问题，它对自然生态系统造成环境威胁对宝贵资源(如贵金属)的损失或回收不足造成经济负担，对有毒物质的使用和不当丢弃造成健康问题。随着可穿戴、可打印和类贴纸系统的使用，电子产品变得无处不在，预计这一挑战只会进一步加深。与传统电子产品一样，柔性薄膜电路可能会加剧日益严重的电子垃圾问题，因为其独特的材料和制造工艺还没有成熟的回收方法。此外，与用于长期使用的传统刚性电子产品不同大多数薄膜软电子产品被设计为一次性设备用于健康监测、物联网和智能包装，有可能在未来几十年内大幅增加电子垃圾的数量。
另一个新出现的环境挑战是在软性可穿戴电子系统的合成和分解过程中使用有毒材料。目前的研究通常依赖在聚合物基底上印刷的导电油墨和复合材料，并经常依赖有毒的有机溶剂来获得印刷和加工所需的足够的流变性。
研究成果
尽管在软性贴纸式电子产品方面取得了进展，但很少有人致力于应对电子垃圾的挑战。为了解决这一问题，卡内基梅隆大学Mahmoud Tavakoli和Carmel Majidi教授推出了一种用于薄膜电路的环保型导电油墨，该油墨由银薄片和水基聚氨酯分散体组成。这种油墨独特地结合了高导电性 (1.6 x 105 S m-1)、高分率数字印刷性、用于微芯片集成的强大粘附性、机械弹性和可回收性。通过生态友好型加工方法将电路分解为组成元素并回收导电油墨，导电率仅降低 2.4%。此外，添加液态金属可实现高达 200%的应变伸展性，尽管这需要更复杂的回收步骤。最后，展示了皮肤电生理监测生物标记以及集成传感器的可回收智能包装，用于监测易腐食品的安全储存。相关研究以“Recyclable Thin-Film Soft Electronics for Smart Packaging and E-Skins”为题发表在Advanced Science期刊上。
图文导读
Figure 1. Green soft electronics with a circular life cycle.
Figure 2. A) Synthesis of the sinter free water-based conductive ink. B) Synthetized Ag-WPU ink with paste-like consistency. C) Soft circuits fabrication process. Conductive lines are printed using (i) stencil printing or (ii) direct ink writing using a digital printer. (iii) Microchips and SMD components are placed on the printed circuit. (iv) Droplets of the Ag-WPU conductive compound are dispensed on the microchip terminals and dried at room temperature, forming a strong and reliable electromechanical bonding between the rigid component and the underlying conductive tracks. D) (i) Detail of an SMD flat cable connector bonded to a set of printed conductive lines (680 um pin separation). This circuit can work as a flexible multisignal conductor which is (ii) ultraflexible, (iii) highly deformable, (iv) and can be molded to nonplanar surfaces to work as a MID. E) (i) Example of digitally printed circuit traces (ii) populated with 0603 size SMD resistors and LEDs. Line width from top to bottom is 1000, 600, and 400 μm. (iii) Detail of the soft bonding points between a 0603 size LED and the underlying conductive tracks. F) (i) The circuit remains functional even when (ii, iii) conformed to nonplanar surfaces or (iv, v) highly deformed, withstanding crumpling, as well as 90° and 180° bends.
Figure 3. A) Ag-WPU conductivity and aging of exposed printed traces for up to 30 days. B) Ag-WPU conductivity for ink vials stored up to 30 days at room temperature. Error bars represent standard deviation C) Strain versus resistance curve for Ag-WPU traces printed over a thermoplastic polyurethane (TPU) substrate (three samples from distinct ink batches). D) Strain (0–30%) vs stress plot for the three printed ink samples. E) Strain at break for the three ink samples. F) Estimated Young modulus for the 3 ink samples. G) Cyclic test of the Ag-WPU ink. H) Printed Ag-WPU track with integrated 0 Ω SMD resistor at 0% strain. I) Printed Ag-WPU track with integrated 0 Ω SMD resistor at 350% strain, before mechanical failure (electrical failure had already occurred). J) Strain versus resistance curve for Ag-WPU traces printed over TPU substrate with integrated 0 Ω SMD resistors (three samples from distinct ink batches). K) Strain (0–30%) versus stress plot for the three printed ink samples with integrated resistors. L) Strain at break for the three ink samples with integrated SMD resistors. M) Mechanical fracture of the samples occurs at the interface between soft printed lines and the rigid SMD component.
Figure 4. A) Circuit degradation and separation process. The circuit is soaked in IPA (i, ii) and stirred in a magnetic stirrer for 30 min (iii). At this point, the clean TPU substrate (iv) and rigid components (v) can be removed from the solution. The suspended Ag flakes and PU residues are left to precipitate (vi) and are decanted, while any remaining IPA traces is then evaporated. B) Separation efficiency. While the rigid electronic components and TPU substrate can be fully recovered with trace amounts of ink residues, the ink (Ag flakes and PU residues) can be separated with ≈90% efficiency due to losses during decanting. C) SEM image depicting no changes in morphology of the Ag flakes before and after the separation process (scale bar 10 μm).
Figure 5. A) Inclusion of liquid metal (EGaIn) in the Ag-WPU ink leads to a biphasic stretchable conductive compound. B) Strain versus resistance curve for Ag-EGaIn-WPU traces printed over TPU substrate (three samples from distinct ink batches). C) Strain versus stress plot for the three printed ink samples. D) Strain at break for the three ink samples. E) Estimated Young modulus for the 3 ink samples. F) Cyclic test of the Ag-WPU ink. G) Strains versus resistance curves for both Ag-WPU and Ag-EGaIn-WPU.
Figure 6. Fully recyclable smart packages.
Figure 7. A) Design of multilayer printed patch which includes a microcontroller, LEDs, voltage regulator and analog signal conditioning circuitry for thermal sensing through a skin-contact thermistor. B) Detail of the printed circuit where the interface between rigid SMD components and the soft printed circuit can be observed. C) Thermal sensing patch adhered to the skin, with the thermistor in the armpit region. D) Axillary temperature measured in the armpit for 1 h using the patch (in red, data acquired every 5 s, filtered with a moving average filter with a 60 s window) and a ground truth obtained using a commercial digital thermometer (in blue, data acquired every 5 s).
Figure 8. A) Printed multielectrode patch with integrated flat-cable connector for skin-surface electrophysiology. B) Electrophysiology patch adhered to the right forearm over the flexor carpi radialis muscle. C) EMG signals and corresponding RMS amplitudes produced by the flexion/extension of different fingers. D) Hand dynamometer used to measure hand grip strength. E) EMG signal and corresponding RMS amplitude envelope acquired during a task consisting of squeezing the dynamometer with increasing gripping forces, with rests in between. F) Multielectrode patch adhered on the face near the temple. G) Bioelectronic signals produced by various facial expressions and subtle movements, including smiling, jaw clenching, eye blinking, and eyebrow flashing. Bottom plot shows the corresponding RMS amplitude envelope for each expression. H) Electrophysiology patch adhered to the user’s chest, over the sternal portion of the left pectoralis major muscle, connected to a biopotential recording system and battery. I) ECG signal acquired with the printed patch, where ECG features (P and T waves, QRS complex) can be observed. RR interval and heart rate (HR) can also be calculated.
总结与展望
在本研究中，作者介绍了一种新型的环保型软导电油墨，该油墨由水性聚氨醋、银和液态金属组合而成，具有高导电性 (1.1-1.6 x 105 S m-1)，适用于DW 等数字印刷方法，并可印刷在薄膜基底上，以制造高柔性、高分辨率(线宽可达 200 μm)的贴纸式电子产品。与之前报道的可印刷导电混合物不同，Ag-WPU 油在室温下可保持稳定长达 4 周，而不会影响导电性或可印刷性。利用该导电聚合物的强粘合性，提出了一种简单的“软焊接“工艺可将集成电路芯片和小型化 SMD 元件直接集成到印刷电路中，而无需其他导电粘合剂或复杂的粘合工艺。此外，整个工艺可在室温下进行，无需烧结，从而实现了低成本、快速的功能性芯片集成电路制造。电失效发生在应变30%时，但印刷Ag-WPU 迹线和SMD 元之间的强机械粘合允许电路在机械失效前拉伸高达 380%。
另一个主要特点是 Ag-WPU 油墨完全可回收，符合环境可持续发展的要求: 油分散在水中，不含有机溶剂，芯片集成软电路可通过在异丙醇中浸泡和搅拌进行拆卸，从而分离出刚性元件、TPU 基材薄膜和油墨残留物。在这一阶段，它们可以被适当丢弃、再加工(对于油墨)或直接再利用(对于SMD 元件和基板)。为了完成循环生命周期，分离出的油墨聚集体在IPA 中进行多次洗涤，以分离银薄片和聚氨酷残留物。然后用回收的银合成新油墨，其导电率为初始油墨的 97.6%。这种直接的回收方法无需复杂的设备或有害的化学物质即可实现，这与针对软电子产品提出的少数降解和回收工艺形成鲜明对比，这些工艺复杂、劳动密集，通常会产生对环境有害的化学副产品。为了使油墨具有更强的机械强度，添加了EGaln 以形成双相复合材料。这样制成的导电油墨可拉伸至200%以上的应变而不会出现电气故障。然而，EGaln 增加的可拉伸性是以更复杂的分解和回收方法为代价的，而这是完全分离金属所必需的。通过制作皮肤生物电子贴片展示了这一材料系统的潜力，该贴片可获取多种数字生物标记，包括体温以及运动任务期间与心脏活动和肌肉活动相关的电生理信号。最后，介绍了一种智能包装，其中的 AgWPA 电路用于创建一个电子印刷标签，该标签可记录易腐物品的储存温度，并警告消费者不安全的储存条件可能导致产品污染。通过采用传统方法(食品包装)和建议的软电子温度监测器分离和回收方法，整个智能包装可回收利用。这些实施方案共同证明了 Ag-WPA油墨在减少材料浪费的同时支持电子贴纸功能的潜力。
总之，作者首次展示了一些简单的材料和方法，这些材料和方法可实现具有完全循环生命周期的复杂环保型软电子电路。这些材料和方法包括开发一种与数字印刷方法兼容的环保型导电聚合物一种使用相同导电聚合物实现室温、无烧结、可靠的SMD芯片集成的简便方法以及一种直接、低成本地分离和降解所制造电路的方法，该方法可回收和再利用旧电路的成分，使其用于各个领域的新电路。
文献链接
Recyclable Thin-Film Soft Electronics for Smart Packaging and E-Skins
https://doi.org/10.1002/advs.202301673
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