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。

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.

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.

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).

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).

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.

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.