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Ultra-Elastic, Durable, Bio-Degradable, and Recyclable Pulp Foam as an Air Dielectric Substitute for Sustainable Capacitive Pressure Sensing
超弹性、耐用、可生物降解和可回收的纸浆泡沫作为可持续电容式压力传感的空气介电替代品

Na Cheng

Na Cheng

Research Division for Sustainable Papermaking & Advanced Materials, Key Laboratory of Biobased Materials Science and Technology of Ministry of Education, Northeast Forestry University, Harbin, 150040 China

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

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Chao Liu

Chao Liu

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

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Yufa Gao

Yufa Gao

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

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Meiyan Wu

Meiyan Wu

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

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Guang Yu

Guang Yu

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

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Chaoji Chen

Corresponding Author

Chaoji Chen

Hubei Biomass-Resource Chemistry and Environmental Biotechnology Key Laboratory, Hubei Provincial Engineering Research Center of Emerging Functional Coating Materials, School of Resource and Environmental Sciences, Wuhan University, Wuhan, 430079 China

E-mail: chenchaojili@whu.edu.cn; jingshen@nefu.edu.cn; libin@qibebt.ac.cn

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Mehdi Rahmaninia

Mehdi Rahmaninia

Wood and Paper Science and Technology Department, Faculty of Natural Resources, Tarbiat Modares University, Noor, 46417-76489 Iran

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Jing Shen

Corresponding Author

Jing Shen

Research Division for Sustainable Papermaking & Advanced Materials, Key Laboratory of Biobased Materials Science and Technology of Ministry of Education, Northeast Forestry University, Harbin, 150040 China

Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, E3B 6C2 Canada

E-mail: chenchaojili@whu.edu.cn; jingshen@nefu.edu.cn; libin@qibebt.ac.cn

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Bin Li

Corresponding Author

Bin Li

CAS Key Laboratory of Biobased Materials, Qingdao New Energy Shandong Laboratory, System Integration Engineering Center, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao, 266101 China

Shandong Energy Institute, Qingdao, 266101 China

E-mail: chenchaojili@whu.edu.cn; jingshen@nefu.edu.cn; libin@qibebt.ac.cn

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First published: 09 February 2025
Citations: 2

Abstract

Green carbon-based cellulosic pulp foams with excellent renewable and biodegradable properties are promising alternatives to traditional petroleum-based lightweight materials, for reducing carbon emission and plastic pollution. However, the fabrication of super-elastic and durable pulp-based foams for high-value utilization remains challenging. Herein, a novel composite bio-foam material is prepared by a simple strategy of wet foaming and ionically cross-linking. The obtained foam assembled by cellulosic pulp fibers and polylactic acid (PLA) fibers at atmospheric pressure shows an oriented lamellar structure with interconnected macropores and super-elastic property. The prepared PLA@Pulp-20 foam shows a high compressive strain of up to 90% with the maximum stress of 150 kPa, while retaining ≈91% of its original height even after 30 000 compressive cycles (far superior to the reported pulp-based foams with compressive cycles <10). Furthermore, the foam exhibits outstanding recyclability and stability in a wide range of temperature and humidity. Remarkably, the potential application of PLA@Pulp foam as a dielectric layer for capacitive sensors is first demonstrated because of its electrical non-conductivity, and low dielectric constant (comparable to air). The corresponding device achieves non-contact touch or contact touch sensing, demonstrating highly attractive performance in sustainable super-elastic pressure sensing, monitoring, and beyond.

1 Introduction

Pressure sensors that respond to external mechanical stimuli and provide real-time information are increasingly in demand in the fields of wearable electronics, soft robotics, human–machine interface, and more. The sensors can be classified as capacitive,[1, 2] piezoresistive,[3, 4] piezoelectric,[5, 6] and triboelectric[7, 8] sensors based on their working principles. Capacitive flexible pressure sensors have a relatively simple structure, mainly consisting of two conductive electrode plates and an elastic dielectric layer in the middle, similar to a sandwich structure. Capacitance (C) of the capacitive sensing obeys the following equation.
C=ε0εrAd$$\begin{equation} \hspace*{80pt}C = {\varepsilon _0}{\varepsilon _{\mathrm{r}}} \frac{A}{d}\end{equation}$$(1)
where, ε${\varepsilon}$0 and ε${\varepsilon}$r are the dielectric constants of the vacuum and dielectric layers, respectively, A is the overlap area, and d is the distance between the two electrode plates. C variation is dependent upon both the dielectric constants of the dielectric layer and the size of the capacitor. Although constructing microstructure of dielectric layer is a common strategy to increase the contact area of electrodes and improve sensitivity, the variability of the dielectric layer is the most critical aspect in capacitive sensing.[9, 10] This variability can be greatly altered by modulating the dielectric constant and compressibility/elasticity of the dielectric layer. Therefore, the elastic dielectric layer, which serves as the primary material for these devices, has a significant impact on the performance of capacitive pressure sensors.

Polymer foams have been used as dielectric layers in capacitive sensors due to their low density and high surface area, and the used materials mainly include polyolefins,[11, 12] polyurethanes,[1, 13] polyamides,[14, 15] and polydimethylsiloxane (PDMS).[16-18] To achieve high compressibility and sensitivity in capacitive pressure sensing devices, control of the porosity of the foam is of critical importance for the working performance of capacitor.[19, 20] The above-mentioned polymer foams offer advantages in terms of flexibility and elasticity. However, these foams are non-renewable, and non-biodegradable, which should be replaced by biodegradable materials for more sustainable development. Cellulose, as the most abundant renewable biopolymer in nature, has been considered as an attractive candidate for the development of green and sustainable materials.[21-23] Cotton fabric with 94% cellulose content and undulating porous structure was loaded with highly conductive substances to make electrodes for parallel plate capacitors.[24] In addition, cellulose-based aerogels/foams with more homogeneous porous structure have been widely used in recent years for flexible sensing.[4, 25-28] As far as the preparation methods of cellulose porous materials are concerned, cellulose-based aerogels are mostly prepared from wet-gels by freeze-drying[29] or supercritical drying,[30] while cellulose-based foams are typically prepared by wet foaming with surfactant at room temperature and atmospheric pressure, followed by molding and air or oven drying.[31-36] Cellulose-based aerogels mostly use nano-sized cellulose (i.e., nanocellulose), such as cellulose nanocrystal,[37] cellulose nanofibril (CNF),[38] and bacterial cellulose.[39] Pure CNF aerogels exhibit 90% compressibility, and the application of freezing technology enables the aerogels to be endowed with an anisotropic honeycomb-like structure, which confers upon cellulose aerogel's excellent mechanical strength and elasticity.[40] Furthermore, the introduction of strong covalent bonds can endow the aerogels with an oriented structure, which represents an effective strategy for preparing elastic aerogels in recent years.[25, 26, 29, 38, 41] Therefore, due to their excellent elasticity and tunable conductivity,[3, 25, 26] cellulose-based aerogels have been successfully applied to piezoresistive pressure sensors. However, the high energy consumption of freeze drying and supercritical drying technologies limits their practical application.
聚合物泡沫由于其低密度和高表面积而被用作电容式传感器的介电层,使用的材料主要包括聚烯烃、 11, 12 聚氨酯、 1, 13 聚酰胺和 14, 15 聚二甲基硅氧烷 (PDMS)。 16-18 为了在电容式压力传感设备中实现高可压缩性和灵敏度,控制泡沫的孔隙率对于电容器的工作性能至关重要。 19, 20 上述聚合物泡沫在柔韧性和弹性方面具有优势。然而,这些泡沫是不可再生和不可生物降解的,应该被可生物降解的材料所取代,以实现更可持续的发展。纤维素作为自然界中含量最丰富的可再生生物聚合物,一直被认为是开发绿色和可持续材料的有吸引力的候选者。 21-23 纤维素含量为 94% 的棉织物和起伏的多孔结构加载了高导电性物质,用于制造平行板电容器的电极。 24 此外,近年来,具有更均匀多孔结构的纤维素基气凝胶/泡沫已被广泛用于柔性传感。 4, 25-28 就纤维素多孔材料的制备方法而言,纤维素基气凝胶大多由湿凝胶通过冷冻干燥 29 或超临界干燥制备, 30 而纤维素基泡沫通常是通过在室温和常压下用表面活性剂湿法发泡,然后成型和空气或烘箱干燥来制备的。 31-36 基于纤维素的气凝胶主要使用纳米级纤维素(即纳米纤维素),例如纤维素纳米晶体、 37 纤维素纳米纤维 (CNF) 38 和细菌纤维素。 39 纯 CNF 气凝胶具有 90% 的可压缩性,冷冻技术的应用使气凝胶具有各向异性蜂窝状结构,这赋予了纤维素气凝胶优异的机械强度和弹性。 40 此外,强共价键的引入可以赋予气凝胶取向结构,这是近年来制备弹性气凝胶的有效策略。 25, 26, 29, 38, 41 因此,由于其优异的弹性和可调的导电性, 3, 25, 26 纤维素基气凝胶已成功应用于压阻式压力传感器。然而,冷冻干燥和超临界干燥技术的高能耗限制了它们的实际应用。

Wet foaming with surfactant and atmospheric drying, as low energy consuming technologies for preparing newly developed pulp foam materials, have received intensive attention in recent years.[42] More importantly, cellulose-based foams use fibers in a wider range of sizes, including microfibers and nanofibers, and resources, such as waste paper fibers,[43] pulp fibers,[31-33, 44-47] bamboo fibers,[27, 48, 49] and sugarcane bagasse.[50] Solvent replacement[51, 52] and microwave-assisted drying[53, 54] can effectively improve foam wrinkles or deformation after foaming, which is caused by water evaporation during drying.[55] Strategies such as building cross-linked network,[31, 44, 56-58] making fibers microfibrillated,[59, 60] introducing functional coating,[48, 61] and co-assembling at the gas–liquid interface[62] can improve the mechanical strength of foams and provide them with antimicrobial, thermal insulation, flame retardancy and sound insulating properties. Nevertheless, they are not resilient in compression and usually limited in shape recovery, limiting their potential application in capacitive sensors as dielectric layer. It remains highly challenging to develop highly compressible dielectric cellulose foams for new-generation of sustainable and high-performing capacitive sensors.
表面活性剂湿法发泡和常压干燥作为制备新开发的纸浆泡沫材料的低能耗技术,近年来受到了广泛关注。 42 更重要的是,纤维素基泡沫使用范围更广的纤维,包括超细纤维和纳米纤维,以及废纸纤维、 43 纸浆纤维、 31-33, 44-47 竹纤维 27, 48, 49 和甘蔗渣等资源。 50 溶剂置换 51, 52 和微波辅助干燥 53, 54 可有效改善发泡后泡沫的皱纹或变形,这是由于干燥过程中水分蒸发引起的。 55 构建交联网络、 31, 44, 56-58 使纤维微纤化、 59, 60 引入功能性涂层 48, 61 以及在气液界面 62 共组装等策略可以提高泡沫的机械强度,并为其提供抗菌、隔热、阻燃和隔音性能。然而,它们在压缩时没有弹性,并且通常形状恢复受到限制,这限制了它们在电容传感器中作为介电层的潜在应用。为新一代可持续和高性能电容式传感器开发高度可压缩的介电纤维素泡沫仍然极具挑战性。

The incorporation of flexible materials into foams is a viable route for improving the elasticity of pulp foam. Polylactic acid (PLA) fibers with sound resilience[63] is an environmentally friendly material produced by melt spinning process. Co-wet foaming of flexible PLA fibers with pulp fibers may be a viable way to achieve foam elasticity and strength. Thus, in this study, we employed wet foaming and ionic cross-linking technique to construct a super-elastic PLA@Pulp foam, in which PLA fibers were assembled as the elastic skeleton and pulp fibers were integrated to reinforce the elastic skeleton. The flexible structure of PLA fibers prevented foam shrinkage during oven drying. Results showed that the fabricated PLA@Pulp foams with lightweight and high porosity had excellent mechanical toughness, high compressibility (up to 90%), super-elasticity (height retained 91% even after 30 000 compressive cycles at 50%), and good durability. Most importantly, the prepared foam exhibited a very low dielectric constant almost comparable to air due to its extremely high porosity (>90%) and the insulating nature of PLA and pulp fibers. Thus, it was expected that this PLA@Pulp foam could be used as a dielectric layer for capacitive pressure sensors. Impressively, based on the results obtained in this work, it was found that the fabricated sensor device had both contact and non-contact sensing capabilities, which provided new ideas for the use of fully bio-based foams in high resilience and pressure sensing applications. Therefore, this PLA@Pulp foam is first considered an excellent substitute for petroleum-based resilient materials to prepare capacitive pressure sensors due to its unique and impressive properties, including resilience, mechanical strength, non-conductivity, recyclability, and biodegradability.
将柔性材料掺入泡沫中是提高纸浆泡沫弹性的可行途径。聚乳酸 (PLA) 纤维具有良好的回弹性, 63 是一种通过熔融纺丝工艺生产的环保材料。软质 PLA 纤维与纸浆纤维的共湿发泡可能是实现泡沫弹性和强度的可行方法。因此,在本研究中,我们采用湿法发泡和离子交联技术构建了超弹性 PLA@Pulp 泡沫,其中 PLA 纤维组装成弹性骨架,纸浆纤维整合在一起以增强弹性骨架。PLA 纤维的柔性结构可防止烘箱干燥过程中泡沫收缩。结果表明,轻质、高孔隙率的 PLA@Pulp 泡沫具有优异的机械韧性、高压缩性(高达 90%)、超弹性(即使在 50 000 次压缩循环后仍保持 91% 的高度)和良好的耐久性。最重要的是,由于其极高的孔隙率 (>90%) 以及 PLA 和纸浆纤维的绝缘性,制备的泡沫表现出非常低的介电常数,几乎与空气相当。因此,人们期望这种 PLA@Pulp 泡沫可以用作电容式压力传感器的介电层。令人印象深刻的是,根据这项工作获得的结果,发现制造的传感器设备同时具有接触和非接触式传感功能,这为在高回弹和压力传感应用中使用全生物基泡沫提供了新思路。 因此,这种 PLA@Pulp 泡沫由于其独特而令人印象深刻的特性,包括弹性、机械强度、非导电性、可回收性和生物降解性,首先被认为是石油基弹性材料制备电容式压力传感器的绝佳替代品。

2 Results and Discussion
2 结果与讨论

2.1 Design Concept and Preparation of PLA@Pulp Foam
2.1 PLA@Pulp 泡沫的设计概念和准备

Figure 1a displays the schematic of the fabrication process of bio-based PLA@Pulp foam. Herein, PLA@pulp foams are fabricated by wet-foaming technology,[31] including foaming, draining, and oven-drying. In this typical preparation process, fiber slurry with a solid content of 1.5 wt % was first prepared by stirring of the slurry containing PLA fiber and pulp fiber. Sodium dodecyl sulfate (SDS) was added to the slurry to create sufficient air bubbles under stirring to uniformly disperse PLA and pulp fibers, leading to the formation of 3D porous wet foam. After the wet foam was stable, sodium alginate and calcium ions were added successively under stirring for cross-linking. The stable networks among PLA fiber and pulp fiber could resist capillary force and maintain the porous structure during ambient draining. Finally, a stabilized PLA@pulp foam was obtained by oven drying at 60 °C, while the foam and process filtrate could be directly reused after recycling.
图 1a 显示了生物基 PLA@Pulp 泡沫塑料的制造工艺示意图。在本文中,PLA@pulp 泡沫是通过湿发泡技术制造的, 31 包括发泡、排水和烘箱干燥。在这个典型的制备过程中,首先通过搅拌含有 PLA 纤维和纸浆纤维的浆料来制备固体含量为 1.5 wt % 的纤维浆料。将十二烷基硫酸钠 (SDS) 添加到浆料中,在搅拌下产生足够的气泡,使 PLA 和纸浆纤维均匀分散,从而形成 3D 多孔湿泡沫。湿泡沫稳定后,在搅拌下依次加入海藻酸钠和钙离子进行交联。PLA 纤维和纸浆纤维之间的稳定网络可以抵抗毛细管力,并在环境排水过程中保持多孔结构。最后,通过在 60 °C 下烘箱干燥获得稳定的 PLA@pulp 泡沫,而泡沫和工艺滤液在回收后可以直接再利用。

Details are in the caption following the image
a) Schematic illustration of the fabrication process of PLA@Pulp foam by wet foaming and ionic cross-linking. b) Schematic illustration of the multiscale structure of PLA@Pulp foam. c) The PLA@Pulp foam is customized into various shapes, demonstrating excellent large-size producibility. d) Photographs of X-Z cross-sectional views of the PLA@Pulp foam in cyclic compression at 50% strain. e) Schematic illustration of signal conversion in capacitive pressure sensor with PLA@Pulp foam as dielectric layer. f) Radar plots comparing the overall performance of PLA@Pulp foam, cellulose aerogel, polyurethane (PU) foam, and polydimethylsiloxane (PDMS) foam.
a) 湿法发泡和离子交联制造 PLA@Pulp 泡沫的示意图。b) PLA@Pulp 泡沫多尺度结构的示意图。c) PLA@Pulp 泡沫定制成各种形状,表现出优异的大尺寸生产能力。d) PLA@Pulp 泡沫在 50% 应变下循环压缩的 X-Z 横截面图照片。e) 以泡沫为介电层的电容式压力传感器 PLA@Pulp 信号转换示意图。f) 比较 PLA@Pulp 泡沫、纤维素气凝胶、聚氨酯 (PU) 泡沫和聚二甲基硅氧烷 (PDMS) 泡沫整体性能的雷达图。

The prepared PLA@Pulp foam features low density (0.05 g·cm−3) that can stay on the top of a leaf without any noticeable deformation (Figure 1b). As the main components of foam, PLA fiber and pulp fiber have different sizes and microstructures. According to the fiber quality analyzer measurement, PLA fiber with weight-average length of 4.66 mm and width of 18.98 µm and pulp fibers with weight-average length of 2.47 mm and width of 25.09 µm (Figure S1, Supporting Information). PLA fiber is stiff with smooth surface (Figure S2a, Supporting Information), while pulp fiber has “hairy” fibrillated surface (Figure S2b, Supporting Information). Two kinds of fibers are mixed and distributed alternately (Figure S2c, Supporting Information), with PLA fibers playing a supporting role in the network of foam, while fibrillated pulp fibers can enhance the degree of entanglements in the fiber network, leading to a porous network with increased mechanical strength. In addition, Ca2+ crosslinking, hydrogen bonding, and electrostatic interactions at the molecular level, generate a more stable network structure. Fourier-transform infrared spectrometer (FTIR) spectra were used to investigate the interaction among PLA fiber, pulp fiber, alginate, and Ca2+ ions (Figure S3, Supporting Information). The peak at 3330 cm−1 in the pristine pulp fiber spectra is attributed to the ─OH groups of cellulose and hemicellulose. The peak at ≈2889 cm−1 is assigned to the stretching vibrations of ─CH groups. In the spectra of PLA fiber, the peak at 1750 cm−1 is due to the C═O stretching vibrations of ester bonds, which is higher due to the hyperconjugation effect.[64] The peak at 1450 cm−1 is due to vibrational absorption of C═O in the carbonyl group. In the spectra of PLA@Pulp foam, the OH peaks are enhanced and the C═O peaks are weakened, but no new peaks are generated, suggesting that the components in the foam interact by physical cross-linking.
制备的 PLA@Pulp 泡沫具有低密度 (0.05 g·cm −3 ),可以保持在叶子的顶部而不会产生任何明显的变形(图 1b)。PLA 纤维和纸浆纤维作为泡沫的主要成分,具有不同的粒径和微观结构。根据纤维质量分析仪的测量,PLA 纤维的平均重量长度为 4.66 mm,宽度为 18.98 μm,纸浆纤维的平均重量长度为 2.47 mm,宽度为 25.09 μm(图 S1,支持信息)。PLA 纤维坚硬且表面光滑(图 S2a,支持信息),而纸浆纤维具有“毛状”原纤化表面(图 S2b,支持信息)。两种纤维混合交替分布(图 S2c,支持信息),PLA 纤维在泡沫网络中起支撑作用,而原纤化浆纤维可以增强纤维网络中的缠结程度,导致多孔网络具有更高的机械强度。此外,Ca 2+ 交联、氢键和分子水平的静电相互作用会产生更稳定的网络结构。傅里叶变换红外光谱仪 (FTIR) 光谱用于研究 PLA 纤维、纸浆纤维、藻酸盐和 Ca 2+ 离子之间的相互作用(图 S3,支持信息)。原始纸浆纤维光谱中 3330 cm −1 处的峰归因于纤维素和半纤维素的 ─OH 基团。≈2889 cm −1 处的峰值被分配给 ─CH 组的拉伸振动。在 PLA 纤维的光谱中,1750 cm −1 处的峰值是由于酯键的 C═O 拉伸振动造成的,由于超共轭效应而更高。 64 1450 cm −1 处的峰值是由于羰基中 C═O 的振动吸收。在泡沫 PLA@Pulp 光谱中,OH 峰增强,C═O 峰减弱,但没有产生新的峰,表明泡沫中的组分通过物理交联相互作用。

PLA@Pulp foams are conveniently prepared by wet-foaming and atmospheric oven drying, which allows the production of multi-sized foams more economic feasible on a large scale (Figure 1c). As illustrated in Figure 1d, the achieved PLA@Pulp foam almost recovered to its original height after repeated compressive strain of 50% for 30 000 cycles. To the best of our knowledge, this is the first realization of ultra-elasticity and durability in pulp fiber-based pulp foam materials by wet-foaming and atmospheric oven drying. Thanks to the super-elasticity and high porous structure of the PLA@Pulp foam, we assembled the foam as a dielectric layer in a capacitive pressure sensor (Figure 1e). This sensor can generate transmission signals for different pressures quickly and sensitively, that expands the application of foams in the field of pressure sensing. As in Figure 1f, the comparison was conducted among PLA@Pulp foam, cellulose aerogels, and petroleum-based foams, which were employed to assess the biodegradability, reusability, resilience, and cost of each material (see Tables S1 and S2, Supporting Information, for details). Due to its multitude of advantageous characteristics, PLA@Pulp foam has emerged as a promising alternative to petroleum-based materials.

The following section provided a detailed elaboration of the design strategy employed in the fabrication of the elastic PLA@Pulp foam. Contribution of fibers entangled network and ionic cross-linking in the formation of PLA@Pulp foam can be revealed from the dimensional changes of both wet foam and dry foam (Figure 2a). Pure PLA foam without ionic cross-linking can be shaped in wet state but fails to shape and volume expands after drying due to the lack of bonding between smooth PLA fibers. PLA foam with ionic cross-linking (PLA/SA/Ca2+) can be formed after drying, but volume still expanded slightly with weak bonding force that collapsed after one compression (Figure S4, Supporting Information). Un-crosslinked pure pulp foam shrank in volume, while crosslinked pulp foam (Pulp/SA/Ca2+) shrank more severely. This phenomenon was attributed to the hydrogen bonding between the pulp fibers and the electrostatic effect of cross-linking, which resulted in an increase in capillary forces during drying process. Accordingly, PLA@Pulp foam (PLA/Pulp/SA/Ca2+) was prepared by compounding two distinct fiber types, namely PLA and pulp fibers, and employing an ionic cross-linking strategy. The volume of the PLA@Pulp foam remained essentially unchanged after drying, with radial and axial shrinkage below 5% (Figure S4a, Supporting Information). Moreover, the PLA@Pulp foam exhibited a low density of 0.05 g·cm−3 and a high porosity of 96.8% (Figure S4b, Supporting Information). This PLA@Pulp foam was constructed using stiff PLA fibers as elastic skeleton and microfibrillated pulp fibers for strengthening and bonding. Meanwhile, the ionic cross-linking networks enhanced the structural stability. Therefore, it is feasible to attain a foam that exhibits both robust strength and high resilience simultaneously by regulating the ratio of PLA and pulp fibers.

Details are in the caption following the image
a) Optical images of PLA foam, Pulp foam and PLA@Pulp foam with/without ionic- crosslinking before and after drying. b) 3D reconstruction of the PLA@pulp foam characterized by micro-computed tomography (micro-CT), and the 2D micro-CT tomograph images of internal structures in the radial section (X-Y view/top view) and axial section (Z-X view/side view). c) SEM images of the X-Y views of the PLA@Pulp foam with different magnifications.

The porous and hierarchical structure structures of PLA@Pulp foam were presented by SEM and Computed Tomography (CT) analysis. The reconstructed 3D structure of the PLA@Pulp Foam and the 2D micro-CT tomograph images of internal structures in the radial section and axial section are shown in Figure 2b and Movie S1 (Supporting Information). There were numerous pores, indicated by the “black” part (yellow circles) in micro-CT tomographs and the 3D “green” one represented the composite structure consisting of PLA fibers and pulp fibers to build the cell walls of the pores. Visible holes can be seen in the radial section, while laminated fibers can be observed with a consistent orientation in the axial section. Furthermore, scanning electron microscopy (SEM) provided a more detailed visual representation of the internal porous structure of the foam (Figure 2c). The pulp fibers were intersected and entangled with PLA fibers to form a firmly interconnected porous network structure. There were micron-sized pores of different shapes and sizes in this network, conforming with the radial cross-section structure in micro-CT observations.

2.2 Elasticity and Mechanical Properties of PLA@Pulp Foam

The mechanical properties of the foam are of crucial importance for a good working performance in sensor applications. Herein, the compressibility and cyclic stability of PLA@Pulp foam were evaluated by compression tests. Figure 3a displays the compressive stress (σ)–strain () curves for the PLA@Pulp foam with different pulp fiber contents at a strain amplitude of 80% (the inset is a local enlargement at 50% strain). It can be observed that the compressive strength increased with the increase of pulp fiber content, and the PLA@Pulp-50 exhibited 3.14 times the compressive strength of the PLA/SA/Ca2+ foam at 50% strain. In addition, PLA foam collapses and loses binding power after one compression (Figure S5, Supporting Information). The foams with different pulp contents were denoted as PLA@Pulp-x, where x was the content of the pulp fiber of the total fiber in wt%. One point that conforms to the general rule is that maximum stress increases with the increasing of foam density (taking PLA@Pulp-10 as an example shown in Figure S6a, Supporting Information). The super-elastic properties of the foam with the lowest density (0.05 g.cm−3) were further characterized to determine the extent of its mechanical resilience to compression. Figure 3b shows the σ- curve for the first compression cycle at a strain amplitude of 50%. Figure 3c,d presents the σ- curves for the fifth and tenth compression cycles, respectively. From the loading curves in each cycle, PLA@Pulp-50 (blue line) exhibited no stress at strains below 5%. This phenomenon was due to the slight shrinkage of pulp fibers during drying, causing a small extent of foam shrinkage. The compressive strength of the foams showed an increasing trend with the increasing of pulp content in ten cycles, which was consistent with the initial loading curve, but the maximum stress for each foam samples slightly decreased with the increase of compression cycles (Figures S6 and S7, Supporting Information). Figure 3e shows the height retention of foam samples extracted from the ten compression cycles. The height retention of all foams decreased with the increasing of compression but remained above 90% after ten compressions. In particular, PLA@Pulp-10 and PLA@Pulp-20 exhibited a height retention of up to 95%, indicating excellent resilience. Figure 3f presents the maximum stress in ten consecutive compressions. The maximum stress diminished with the increasing cycles for all foams, but the extents of the decrease was relatively greater for the foams with higher pulp contents. Remarkably, the maximum stress of PLA@Pulp-10 and PLA@Pulp-20 in the tenth cycle were 12.1 and 15.0 kPa, respectively. This strategy realized the adjustment of both the compression strength and elasticity properties by tuning the pulp fiber content. To explore the intrinsic mechanism of elasticity of PLA@Pulp, we observed the microscopic morphology of the foam under different compression states using in situ-SEM (Figure 3g). In the original state, the fibers in the PLA@Pulp foam were interwoven and entangled, exhibiting a porous structure. We conducted in situ compression observations on the “triangular” shaped hole at the center. Under 10% compressive strain, the fiber walls of the pore became in closer proximity, resulting in a reduction in pore size while maintaining the complete triangular structure. When the compressive strain increased to 30%, the fibers lapped together, and the triangular holes disappeared. Continuing to increase the strain up to 50%, the fibers were more tightly bound to each other, forming strong anchor points where the fibers overlapped together, which explained the gradual increase in strength of the foam during compression. After being released to the initial state, the PLA@Pulp-20 foam almost recovered to its original state, which is consistent with the result that the height retention rate of foam in Figure 3e is >95%. The slight decrease in height may be caused by friction between fibers during compression and release.

Details are in the caption following the image
a) 80% stress-strain curves of PLA@Pulp Foam with different pulp fiber contents. The inset shows a magnified plot of (a) up to 50% strain. 50% compression cyclic curves of PLA@Pulp Foams at b) cycle 1, c) cycle 5, d) cycle 10, respectively. Summary plots of the height retention e) and maximum compressive stress f) of foam samples extracted from the ten cyclic tests. g) In situ SEM images of the X-Z views of PLA@Pulp-20 in the original state, in the compressed state of 30%, 50% strain, and after releasing completely.

The good shape recovery of the pores after compression gives the foam excellent elasticity, and the fatigue resistance of foam during compression is further investigated. Figure 4 shows the compressibility and fatigue resistance of PLA@Pulp foam. The strain–stress curves at different strains (20–90%) exhibited crescent shape that steepened dramatically with the increase of strain (Figure 4a). PLA@Pulp-10 and PLA@Pulp-20 also exhibited excellent fatigue resistance, maintaining a similar stress–strain curve after 30 000 compression cycles without noticeable deformation (Figure 4b,c). The height loss of PLA@Pulp-30 after 500 cycles was >10% (Figure S8a, Supporting Information). Besides, PLA@Pulp-40 (Figure S8b, Supporting Information) and PLA@Pulp-50 (Figure S8c, Supporting Information) experienced permanent deformation >10% after 50 cycles of compression. This results further verified that the balance between mechanical strength and elasticity can be adjusted by varying the pulp content in the foam. Figure 4d shows that the height retention of the foam only slightly decreased as compression cycles increased and the height retention of the foam was still over 90% after 20 000 cycles, demonstrating its remarkable compressibility and elasticity. During the first cycle, owing to the destruction of unstable fiber networks, a high energy loss coefficient of ≈0.5 can be observed. The energy loss coefficient eventually <0.40 after 10 cycles, suggesting good structural durability of the PLA@Pulp foam (Figure 4e). PLA@Pulp-20 showed excellent elasticity at both 20% (Figure S9a, Supporting Information) and 30% strain (Figure S9b, Supporting Information). At 30% compression stain, the strain–stress curve of PLA@Pulp-20 remained similar after 10 000 cycles, with a high retention of 94.5% (Figure S9c, Supporting Information). In Figure 4f, we compared the fabricated PLA@Pulp foam with other cellulose-based 3D materials reported in literature. The elasticity of the PLA@Pulp foam is far superior to other cellulose-based foams and can match most freeze-dried cellulose-based aerogels. The specific information is shown in Table S1 (Supporting Information). In addition, as shown in Figure 4g and Movie S2 (Supporting Information), PLA@Pulp-20 can almost recover to original shape after 3×104 compressions, indicating that the foams with higher stress could have excellent fatigue resistance. Notably, bleached hardwood kraft pulp, bleached bamboo kraft pulp, and unbleached softwood kraft pulp can also be used to prepare this elastic pulp foam materials. The strength of the resulting foam varies according to the type of pulp, however, under 100 compression cycles, all three types of pulp foams have good resilience and do not deteriorate depending on pulp type (Figure S10, Supporting Information).

Details are in the caption following the image
a) Sequential loading–unloading cycles of PLA@Pulp-10 from 10% to 90% strain amplitudes. The inset shows a magnified plot of a) up to 50% strain. Cyclic compression stress-strain curves of b) PLA@Pulp-10 and c) PLA@Pulp-20 at 50% strain amplitude during 3×104 tests. d) Height retention of PLA@Pulp-10 and PLA@Pulp-20 at 50% strain for 10 000 cycles. e) Maximum compressive stress and energy loss coefficient (△U/U) of PLA@Pulp-10 and PLA@Pulp-20. f) Comparison of the height retention, compressive strain, and cycle number of PLA@Pulp foam with those of other reported foams. g) Photographs of the PLA@Pulp-20 after 3×104 compression-release cycles at 50% strain.

2.3 Dielectric and Sensing Performance of PLA@Pulp Foam

The PLA@Pulp foam with low density (0.05 g cm−3), high porosity (> 96%), good mechanical elasticity, high compressibility and electrical insulation, could be expected to be used as a dielectric layer for capacitive sensing. To demonstrate the potential use of PLA@Pulp foam as a dielectric layer for capacitive sensors, we fabricated a parallel-plate capacitor using conductive copper tape as the conductive electrode and PLA@Pulp foam as the dielectric layer, as shown in the photographs and schematic in Figure 5a. Therefore, the sensing properties were further evaluated by changing the distance of parallel plate capacitors. Figure 5b shows a plot of dielectric constant () and loss tangent (tan δ) versus frequency for PLA@Pulp-10 under strain-free conditions. The dielectric constant and loss tangent of the foam drop sharply before 100 kHz and remain stable between 100 kHz and 1 MHz. The corresponding and tan δ, measured at 100 kHz, are 1.10 and 0.015, respectively. The sensing performance was tested at 100 kHz due to the low dielectric constant similar to air (air = 1).

Details are in the caption following the image
a) Photograph (top) and schematic (bottom) of a parallel-plate capacitive sensor using conductive tape as the conducting electrodes and PLA@Pulp foam as the dielectric layer. b) Plots of the dielectric constant and loss tangent (tan δ) as a function of frequency. c) Pressure-response curve of the sensor displaying its sensitivity from 0 to 80 kPa. d) Schematics illustrating PLA@Pulp foam capacitive sensing mechanism. e) Change in capacitance (ΔC/C0) as a function of different strain for PLA@Pulp-10. f) Plots of ΔC/C0 as a function of applied pressure by putting the weights or pressing onto the device with a tweezer for three times with increasing intensity. The inset in (f) shows the photograph of a weight (corresponding to the blue line, with different mass) and a tweezer (corresponding to the red line, with different force by hand) put on top of the device. g) Plots of ΔC/C0 as a function of applied noncontact touch by hovering a tweezer at close proximity to the device.

According to the relationship between the output C signals and the applied pressure stimuli, the calibrated curves of ΔC/C0 versus pressure for PLA@Pulp foam-based sensing devices are shown in Figure 5c. The pressure-sensing curves for PLA@Pulp sensors were divided into three fitting lines with progressively decreasing sensitivity: a high, medium, and low sensitivity range of 0.16, 0.07, and 0.03 kPa−1 (the curve slope directly reflects the sensitivity) in pressure ranges of 0–21.7, 21.7–51.5, and 51.5–80 kPa with linear correlation coefficients of 0.998, 0.992, and 0.990, respectively. Figure 5d illustrates the alteration in porosity of the foam as a dielectric layer during compression. Under low pressure, the foam displayed minor deformation with greater layer distance and higher porosity. With the increase of pressure, the foam underwent greater deformation, making the interlaminar fibers to come closer together. Figure 5e illustrates the cycle number relative to ΔC/C0 of a PLA@Pulp sensor at different compressive strains. ΔC/C0 increased as the compression strain increased from 10% (2.2 kPa) to 40% (8.4 kPa), and each cycle exhibited regularity. The response and recovery time of the sensor were 100 and 110 ms, respectively (Figure S11, Supporting Information). In addition, the signals can be recorded in real time without interruption during multiple sensing, which demonstrates the good stability of the sensors (Movie S3, Supporting Information). Similarly, sensors with different initial heights exhibited regular capacitance responses at 20% strain (Figure S12, Supporting Information). Figure 5f displays the ΔC/C0 in response to the application of pressure on the device using weights or a tweezer. It was observed that the weight/tweezer caused an initial dip in capacitance (≈−0.1), followed by a sudden spike when the weight/tweezer was pressed onto the device. The magnitude of the ΔC/C0 was determined by the intensity of the pressure, which compressed the PLA@Pulp foam and narrowed the distance between the two electrodes, thereby increasing capacitance. Upon release of the load, the compressed PLA@Pulp returned to its original shape and the capacitance returned to C0. As observed in Figure 5g, the device worked as a non-contact touch sensor, in which the capacitance was reduced as a tweezer approached the device. The reduction in capacitance was due to the absorption of the fringing electric field within the dielectric layer, causing a variation of ΔC/C0.[65] These results indicated that our PLA@Pulp pressure sensor exhibited good functional reliability.

2.4 Recyclability, Biodegradability, and Environmental Impact of the PLA@Pulp Foam

Figure 6 illustrates the recyclability, temperature stability, and biodegradability of the PLA@Pulp foams. The recyclability properties of the foam were analyzed by their appearance and mechanical strength. As shown in Figure 6a, the discarded foam was fragmented and submerged in water. Then, it was dispersed under agitation and foamed at a high stirring speed. The recycled PLA@Pulp foam can still be effectively formed after filtering and drying, and the foam can be returned to the recycling process. Figure 6b shows the compressive strength of the foam at 50% strain after seven recycling cycles. The strength gradually decreased with the increase of recycling times, which may be due to the trace loss of additives and fine fibers in water. Notably, the foam maintains 90% of its initial strength after three recycles, which is 12.6, 12.4, and 11.9 kPa, respectively, showing great recyclability and strength retention. In order to make foam have good strength, we newly added 2 wt% additives, calculated based on the dry weight of foam recycled 6 times, in the seventh recycle process. As expected, the strength of the obtained foam increased back to 12.85 kPa, which was 97% of the initial strength.

Details are in the caption following the image
a) The demonstration of the recyclability of the PLA@Pulp foam. b) The compressive stress of PLA@Pulp foam after 7 reuse cycles. c) Stress-strain curves of PLA@Pulp-10 after 3 reuses under ten consecutive compressions. d) Biodegradation tests and weight change of the PLA@Pulp foam under composting degradation environment. e) Comparison of the thermal stability of PLA@Pulp foam and plastic foam at different temperatures. f,g) Comparison of current manufacturing routes of petroleum-based foam (g) to our bio-based foam. h) Comparison of the economic cost of our bio-based foam to commercial polymer foams. i) Comparison of greenhouse gas (GHG) emissions of our bio-based foam to petroleum-based foams according to life cycle assessment (LCA).

Figure 6c shows 10 cycles of compression of the foam after 3 recycling cycles, and the almost coincident stress-strain curves demonstrated its excellent resilience even after recycling. Since no chemical crosslinking agent was introduced in the PLA@Pulp foam, it was expected that the foam could retain the biodegradability characteristic of bio-based fibers (pulp and PLA). The biodegradability of the foam was tested in a composting environment as shown in Figure 6d, where the weight of the PLA@Pulp foam was reduced by 85% after 50 days. There are abundant bacteria, actinomycetes, fungi, and other microbes in composting soil. Under aerobic conditions, biodegradable PLA and pulp fibers can be converted to stable humus.[66, 67] Figure 6e shows the thermal stability of our bio-based PLA@Pulp foam and common petroleum-based foams. When the temperature reached up to 120 °C, the bio-based PLA@Pulp foam showed good dimensional stability, which was in sharp contrast to the polymer-based foam with severe shrinkage. Besides, given that the foam was prepared at 60 °C with a limited quantity of bound water, an additional analysis was conducted to examine the size and strength of the foam at different temperatures. As the temperature increased from 80 to 150 °C, the height of the foam remained unaltered and the diameter exhibited a slight reduction (Figure S13, Supporting Information), while the strength increased from 13.2 to 25.0 kPa (Figure S14, Supporting Information). This result can be attributed to the evaporation of the bound water as a result of the elevated temperature, which led to a more compact pore structure and consequently an increase in foam density. Importantly, PLA@Pulp foam has excellent mechanical strength and resilience when the ambient relative humidity was above 90% (Figure S15, Supporting Information). In summary, the PLA@Pulp foam displays remarkable stability in a broad range of temperature and humidity conditions.

The production of foams is a process that must consider two key issues: cost-effectiveness and sustainability. In order to gain insight into the relative merits of bio-based and petroleum-based foams, we conducted an economic analysis and life cycle assessment (LCA)[68, 69] of the two types of foams (Figure 6f,g). As shown in Figure 6h, the cost of PLA@Pulp foam is lower than PU and PDMS foam (Table S2, Supporting Information), which gives it an advantage in economic benefits. Importantly, LCA was carried out to further quantify the environmental impact of the PLA@Pulp foam, which was compared with four common fossil-based foams, namely PS, PE, PU, and PDMS foam (Figure S16 and Table S3, Supporting Information). The greenhouse gas emissions of our bio-based foams were evaluated according to the Global Warming Potential (GWP) impact category. As shown in Figure 6i, our PLA@Pulp foam has a cradle-to-gate GWP value of 1.04 kg CO2 equiv kg−1, and much lower than those of other fossil-based materials such as EPS foam (4.23 kg CO2 equiv kg−1), EPE foam (2.32 kg CO2 equiv kg−1), PU foam (4.95 kg CO2 equiv kg−1), and PDMS foam (16.69 kg CO2 equiv kg−1). Additionally, as one of the most worrisome impacts, the toxicity of PLA@Pulp foam is significantly lower compared to fossil-based foams for terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic, and noncarcinogenic toxicity (Figure S16, Supporting Information). In brief, from an LCA perspective, this PLA@Pulp foam was found to be a promising bio-based material to displace the fossil-derived foam materials for high-value applications such as pressure sensing.

3 Conclusion

In this work, a highly porous and low-density super-elastic PLA@ pulp foam was successfully developed for the first time using a typical process of wet foaming, ionic cross-linking, and oven drying. The PLA@Pulp foam exhibited excellent elasticity and durability, maintaining 91% height after compression and 72% maximum stress at 50% strain for 30000 cycles. Moreover, the foam was axial dimensionally stable from −20 to 150 °C and the compression strength could be enhanced at 120–150 °C, while the temperature does not affect the high resilience of the foam. There was no statistically significant degradation in resilience performance over three cycles when the PLA@Pulp foam was recycled without new adding of any chemical additives. Importantly, these foams have a low dielectric constant (similar to that of air) owing to their high porosity and insulating nature of these bio-fibers. These properties make the PLA@Pulp foam ideal for multiple-forms capacitive sensing applications. Additionally, a variety of sensing capabilities, including non-contact touch and contact touch, were demonstrated in a parallel-plate capacitive sensor using PLA@Pulp foam as the dielectric layer with very good working performance. Taken together, the PLA@Pulp foam represents a new type of sustainable, reusable, super-elastic foam with less environmental impact, which has great potentials in various advanced applications such as intelligent pressure sensors, monitoring device, and beyond

4 Experimental Section

Materials

Bleached softwood kraft pulp (containing 85.5% cellulose and 13.5% hemicellulose), bleached hardwood kraft pulp (containing 83.3% cellulose and 15.7% hemicellulose), bleached bamboo kraft pulp (containing 82.1% cellulose and 17.2% hemicellulose), and unbleached softwood kraft pulp (containing 70.3% cellulose, 10.9% hemicellulose and 17.5% lignin) were kindly gifted by Mudanjiang Hengfeng Paper Co., Ltd. (China). PLA fiber (length 5 mm) was purchased by Shenzhen Esun Industrial Co., Ltd. (China). Sodium alginate (SA, medium viscosity), CaCl2 (assay ≥ 99.0%), sodium dodecyl sulfate (SDS, assay ≥ 99.0%), were bought from Sinopharm Group Chemical Reagent Co., Ltd. (China) and used as received without further purification.

Preparation of the Elastic PLA@Pulp Foam

The fabrication method of the elastic PLA@Pulp foam was similar to the previous work with slight modifications.[31, 44] First, the PLA fiber (2.7 g), deionized water (200 g), the cellulose pulp (0.3 g), SA (12 mL, 2 wt%) were sequentially added into a high-speed blender (Royalstar, RSD-800, China). After the above substances were mixed well, 1.5 wt% of SDS (based on oven dried pulp) was added for bubble generation (1000 rpm for 1 min). Finally, CaCl2 (12 mL, 2 wt%) was added in the bubble mixture with mechanical stirring (1000 rpm) for 1 min at room temperature (25 °C). The wet foam was poured into a bottomless tempered cylindrical glass mold with a 50-mesh wire gauze at the bottom and kept for 20 min at room temperature for drainage (to remove excess filtrate), and then the wet foam was pressed and shaped into a desired height. After that, the molded sample was dried at 60 °C overnight to obtain the PLA@Pulp foam. Finally, the dried foam was pre-compressed to maintain consistency in shape. The foam with different pulp content on the weight of oven-dried fiber was denoted as PLA@Pulp-x, where x was the pulp fiber content of the foam in wt%. In addition, the process for preparing PLA@Pulp foam for recycling followed the same procedure as above, without extra addition of any additives during the first 6 recycles, but simply by mechanically breaking in water and pouring it into mold for shaping.

To verify the universally applicability of “pulp”, bleached hardwood kraft pulp, bleached bamboo kraft pulp, and unbleached softwood kraft pulp with 10 wt% pulp content were also used to prepare PLA@Pulp foam under the same preparation conditions.

Fabrication of Sensor

The PLA@Pulp-10 was used as the dielectric layer and adhesive copper conductive tape was used as conducting electrodes. Conductive electrodes were cut into different sizes and adhered to the upper and lower surfaces of the dielectric layer to make capacitive pressure sensors of different sizes. Sensors with a diameter of 50 mm and different heights were used to investigate the effect of distance on sensing performance, while 15 × 15 × 2 mm sensors were used for contact and non-contact sensing tests.

Characterization

The size of PLA fiber and pulp fiber was characterized using a fiber quality analyzer (Valmet FS5). The morphologies of pulp fiber, PLA fiber, and PLA@Pulp foam samples were observed using optical Microscope (NSZ818 M, China) and a scanning electron microscope (SEM, Hitachi S-4800, Japan) with the acceleration voltage of 3 kV. Attenuated Total Reflection – Fourier transform infrared spectra (ATR-FTIR) of pulp foams were collected using a Nicolet 5700 spectrometer (Thermos fisher scientific, USA) in a wavenumber range of 4000–800 cm−1. The PLA@Pulp foam with diameter of 50 mm and height of 30 mm was scanned by micro-computed tomography (micro-CT) (Nikon XTH 320) with a tube voltage of 40 kV, a tube current of 50 µA, a scanning time of 2 h, and a resolution of 4 µm. In situ scanning electron microscopy (in situ SEM, TESCAN MIRA4) was used to observe the microstructure of foam under different strain states. The compressive stress of foam samples was performed by an electronic tensile machine (MTS Systems, CMT 6503, China) with a compression rate of 60 mm min−1, and each sample was tested for at least three times. The strain of 10%–90% was chosen to evaluate the compressibility and elasticity of foam samples over a large deformation interval. The thermal stability was evaluated by detecting the dimensional changes of the foams, and the density of the PLA@Pulp foam, PE foam, and PS foam used was 50 mg/cm3 and the dimension was 20 mm × 20 mm × 15 mm. Composting degradation of PLA@Pulp foam was tested according to ISO 20 200:2015 standard and their morphology changes were checked.

The dielectric and capacitance properties of the various foams was measured characterized using a two-point measurement with the aid of an LCR meter (Tonghui TH2830) by following the well-known Cp-D function. Varying compressive strains of the foams were tested using Tonghui TH2830 LCR meter dielectric test fixture controlled by a micromanipulator with an accuracy of 10 µm. The electronic tensile machine (MTS Systems, CMT 6503, China) with a force gauge was used to apply external pressure for compression measurements on the sensor. In the parallel plate capacitance model, dielectric permittivity was measured over a frequency of 10 Hz to 100 kHz. The vary compressive strains of foams was measured using a moving fixture along with the acrylic sheet to apply a uniform force. The applied forces were recorded using a force gauge. All the electromechanical characterizations were carried out at 1 V at 100 kHz frequency under room temperature unless stated otherwise.

Life-Cycle Assessment (LCA)

The LCA analysis was conducted by Simapro 9.5 based on ISO14040. A functional unit was setup to produce 3.24 g of PLA@pulp foam for comparing the environmental impact of different foaming materials. The system boundaries included raw material purchase, transportation, and material production. The life cycle impact assessment was conducted using the ReCiPe2016 method, which provides 18 environmental impact categories. The detailed process of lifecycle assessment is shown in the Supporting Information.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. U22A20423, 22208358, 22478405, 22408384, and W2412117), Heilongjiang Natural Science Foundation for Outstanding Young Scholars (No. JQ2023C004), International Partnership Program of Chinese Academy of Sciences (No. 323GJHZ2023019MI), Iran National Science Foundation (No. 4020841), Qingdao Science and Technology Demonstration Project (Nos. 24-1-8-smjk-18-nsh and 24-1-8-cspz-6-nsh), and Fundamental Research Funds for Central Universities of China (No. 2572021CG04).

    Conflict of Interest

    The authors declare no conflict of interest.