Introduction

Plastics are lightweight, moldable, and indispensable materials in modern life, with global production exceeding 400 million tons per year [1]. Most of these plastics are derived from petroleum, releasing CO₂ when incinerated and contributing to global warming. Biomass plastics, produced from renewable resources, can mitigate this issue since their combustion-derived CO₂ is fixed and offset by photosynthesis. In particular, plastics that are difficult to recycle and are inevitably incinerated should be replaced by biomass-based alternatives [2, 3].

Today, the rapid expansion of the Internet of Things (IoT) and artificial intelligence (AI) has led to an exponential increase in global data traffic [4]. The volume of electronic waste reached over 62 million tons in 2022 and is expected to exceed 80 million tons by 2030 [5]. Recycling is hindered by the tight integration of metals and plastics in electronic devices. Developing high-performance biomass-based plastics for modern electromagnetic technologies is, therefore, essential.

Polysaccharides represent one of the most abundant classes of renewable polymers and have a long history of industrial utilization even in the electromagnetic device industry. Cellulose acetate, for example, is widely used as a protective film for retardation plates in liquid crystal displays [6]. In addition, cellulose-based papers impregnated with insulating oils have long been used in electrical components, such as transformers, illustrating that polysaccharide-derived materials have historically played supporting roles in electrical insulation [7, 8].

Among the many electronics-related applications of plastics, this review focuses on their use in “communication devices”. Until the eighteenth century, long-distance information transfer relied primarily on the physical transportation of paper documents, such as letters. Alternative methods, including signal fires, flags, semaphore systems, and carrier pigeons, were also used, but these approaches allowed only limited information to be transmitted and required substantial time for delivery. A major transformation occurred in the nineteenth century with the invention of electrical communication technologies [9]. The practical implementation of the telegraph in the 1830s and the invention of the telephone in 1876 marked pivotal moments in the history of communication.

Since then, information has been transmitted through electromagnetic waves generated by time-varying voltages in metallic conductors. To ensure safe and reliable signal transmission, conductors must be coated with insulating materials. In the mid-nineteenth century, natural rubber was used as an insulating layer for submarine telegraph cables. In the early twentieth century, the emergence of synthetic resins derived from coal, such as phenolic resins, enabled the mass production of telephone and radio substrates and housings. Importantly, paper-based materials also played a central role in early electronic substrates. Because plastics alone often exhibited insufficient dimensional stability at elevated temperatures, paper was impregnated with resins and used as a structural component of electronic circuit boards. Paper-phenolic laminates, developed in the 1940s as some of the earliest printed circuit boards, were used for decades as low-cost substrates in consumer electronics [10]. However, their dielectric performance is inferior to that of glass–fiber-reinforced epoxy laminates, limiting their applicability in today’s high-frequency circuits [11, 12].

Modern communication systems are supported by two major electromagnetic technologies, namely, radio waves and light (Fig. 1) [13]. The first is wireless communication, which uses radio waves below 300 GHz. These signals propagate through air, enabling communication without physical cables; examples include radio broadcasting and mobile phones. The electronic circuits used in such systems rely on polymeric insulating layers that prevent short circuits while minimizing signal interference.

Fig. 1
Fig. 1
Full size image

Modern communication systems and the plastics used in their key components. Wireless communication (radio-wave-based) and optical communication (light-wave-based) both rely on petroleum-derived plastics: insulating polymers for electronic circuits and transparent polymers for optical waveguides

The second is optical communication, which uses light waves at frequencies of several hundred terahertz. Owing to their high frequency, light waves can transmit vastly larger volumes of data than radio waves. However, light cannot be transmitted efficiently through air over long distances, because it travels in straight lines. To address this, glass and plastic optical fibers were developed as waveguides that confine light through total internal reflection. While glass fibers dominate long-distance networks, plastic optical fibers are favored for short-distance communication and sensing applications due to their flexibility, toughness, and ease of processing.

In this focused review, we discuss polysaccharide-derived plastics from a materials design perspective, specifically targeting performance requirements for plastics used in radio-frequency and optical communication technologies. Although numerous studies have reported the use of polysaccharides in electronic applications [14,15,16,17,18,19], this review focuses on approaches that treat polysaccharides as “processable plastics”. Emphasis is placed on how molecular-structure control of polysaccharides can be exploited to transform them into processable materials while suppressing undesirable electromagnetic interactions, such as dielectric polarization and optical scattering.

Specifically, two representative examples developed in these several years are introduced: (1) the design of high-heat-resistant, low-dielectric polysaccharide esters for high-frequency electronic applications, and (2) the fabrication of transparent and mechanically robust pullulan-based fibers for optical communication and sensing. Through these examples, this review aims to clarify fundamental design principles that enable the use of renewable polysaccharides in advanced communication technologies.

High-heat-resistant and low-dielectric polysaccharide esters

Requirements and challenges for high-frequency circuit substrates

Insulating resins for electronic substrates must exhibit both high thermal resistance and favorable dielectric properties. For reliable soldering at temperatures above 200 °C, the polymer’s glass-transition temperature (Tg) should exceed 200 °C to avoid deformation. In addition, a low-dielectric constant (Dk) and low-dielectric loss (Df) are crucial for minimizing transmission loss of high-frequency signals. A higher Dk slows down signal propagation, while a higher Df causes energy dissipation due to polarization lag. As communication frequencies continue to increase to accommodate faster data transmission, polymers with low Dk and Df are increasingly required [20].

Conventional insulating polymers, such as polyimides and epoxy resins, are based on petroleum feedstocks. Polyimides typically show Tg values above 300 °C with Dk of 3–4 and Df around 0.010 [21]. Epoxy resins such as FR-4 exhibit Tg ≈ 200 °C, Dk of 3.8–4.3, and Df of 0.010–0.020. More recently, heat-resistant polymers with improved dielectric performance have been developed, including poly(phenylene ether) (PPE), which has Tg ≈ 200 °C, Dk = 2.6–2.8, and Df = 0.003–0.007 [22,23,24,25]. Fluorinated polymers can further lower the dielectric constant [26].

However, these polymers pose environmental concerns when disposed of by incineration, releasing CO₂ along with hazardous gases, such as aromatic hydrocarbons (BTEX) [27], nitrogen oxides (NOx) [28], and per- and polyfluoroalkyl substances (PFAS) [29]. To reduce such emissions, it is desirable to develop resins that achieve Tg > 200 °C and Dk < 3 with Df < 0.010 without relying on aromatic rings or fluorine atoms.

We, therefore, focused on polysaccharides, renewable polymers that possess cyclic structures in their backbones and can potentially offer thermal stability without aromaticity. Cellulose has been extensively studied as an electrical substrate by controlling its crystalline structure and through chemical modification [30,31,32,33]. Furthermore, in many previous studies, the abundant hydroxy groups and high hygroscopicity of cellulose have been intentionally exploited to develop high-dielectric materials rather than low-dielectric insulators [34,35,36,37,38]. Apparent reductions in dielectric constants have also been achieved by introducing a large volume fraction of air voids (Dk = 1) into cellulose-based films [39, 40]. Beyond conventional substrates, polysaccharide-based materials have enabled functionalities not readily achievable with existing plastics, including biodegradable communication substrates, high-dielectric capacitors, and devices such as antennas, memories, transistors, capacitors, and humidity sensors through hybridization with metals or conductive components [18, 41,42,43,44].

Nevertheless, polysaccharides are generally non-thermoplastic and insoluble in common organic solvents, which poses a major challenge for their application in conventional circuit board fabrication processes that rely on thermal lamination of insulating films. To enable practical processing, it is, therefore, necessary to impart thermoplasticity into polysaccharides. Esterification can overcome these limitations by converting them into processable thermoplastics. Cellulose acetate, for instance, is widely used as a protective film in liquid crystalline display polarizers but has Tg ≈ 180 °C, Dk ≈ 3.9, and Df ≈ 0.030, which remain insufficient for high-frequency substrates [21]. Achieving both high Tg and low Dk and Df has long been considered challenging for polysaccharides, and such materials had received limited attention until recent years [45,46,47,48,49].

Recently, even polysaccharides composed of the same glucose unit have been reported to exhibit distinct properties depending on their glycosidic linkages [50]. For example, paramylon (β-1,3-glucan) derived from Euglena produces esters with superior flowability suitable for fiber spinning [51], while α-1,3-glucan esters synthesized enzymatically from sucrose show higher melting and glass-transition temperatures than cellulose esters [52]. α-1,6-Glucan esters can serve as strong adhesives for glass or wood [53]. Side-chain structures also have a pronounced influence: long alkyl chains lower Tg and enhance flexibility, while bulky or branched substituents increase Tg and stiffness [54,55,56].

We systematically investigated how both the polysaccharide backbone and the side-chain structure affect the thermal and dielectric properties of their esters [57,58,59,60]. The following sections present examples of thermoplastic polysaccharide esters that successfully combine high thermal stability and low-dielectric characteristics suitable for electronic substrates.

Polysaccharide esters with primary hydroxy-functional group

Synthesis

Three types of glucose-based polysaccharides—cellulose (β-1,4-glucan), paramylon (β-1,3-glucan), and α-1,3-glucan—were esterified with various linear, branched, and ring-shaped carboxylic acids (Fig. 2). Sterically bulky groups such as adamantane- and 2,2-dimethylpropionate could not be introduced into cellulose due to steric hindrance, whereas all other combinations yielded fully esterified products.

Fig. 2
Fig. 2
Full size image

Structures of three polysaccharide backbones and introduced ester side chains. Cellulose (β-1,4-glucan), paramylon (β-1,3-glucan), and α-1,3-glucan were modified with linear, branched, and ring-shaped carboxylic acids to investigate the effects of side-chain architecture on thermal and dielectric properties

Thermal properties

In general, increasing the carbon number of ester side chains leads to a decrease in Tg owing to an internal plasticizing effect. This trend was consistently observed for both linear and branched ester side chains across different polysaccharide backbones (Fig. 3). For example, cellulose propionate exhibits a Tg of 125 °C, whereas cellulose hexanoate shows a markedly lower Tg of 61 °C. Longer alkyl side chains possess higher conformational mobility and more effectively increase the free volume between polymer chains, thereby facilitating segmental motion of the main chain and reducing Tg.

Fig. 3
Fig. 3
Full size image

Relationship between Tg and side-chain carbon number for a cellulose, b paramylon, and c α-1,3-glucan esters. Ring-shaped side chains maintain or increase Tg; α-1,3-glucan-cyclohexanecarboxylate shows Tg of 205 °C

Branched side chains displayed a similar dependence on carbon number; however, their Tg values were slightly higher than those of the corresponding linear side-chain derivatives. The branched bulky structures limit the main-chain mobility causing higher Tg [54,55,56, 61, 62].

In contrast, cyclic ester side chains exhibited a distinctly different behavior. For cellulose cyclic esters, Tg values were maintained within a relatively narrow range (approximately 125–136 °C), showing only a weak dependence on ring size [57]. When compared with linear or branched esters having a similar number of carbon atoms, cyclic esters consistently exhibited higher Tg values (for side chains with six carbon atoms: cellulose hexanoate, 61 °C; cellulose 2-methylvalerate, 50 °C; cellulose cyclopropane carboxylate, 125 °C) [54,55,56]. These results indicate that bulky cyclic substituents do not function as effective internal plasticizers to the same extent as long linear alkyl chains. Their restricted conformational freedom limits side-chain mobility, thereby reducing their ability to enhance segmental motion of the polymer backbone. Notably, α-1,3-glucan cyclohexane carboxylate (α-1,3-glucan-CH) exhibited a Tg of 205 °C, which is sufficiently high to withstand soldering processes required for circuit substrate fabrication [57].

It is reasonable to consider that Tg of polysaccharide esters is governed by the combined influence of multiple competing factors: (1) an internal plasticization effect, in which side chains increase the distance between polymer backbones and enhance free volume, leading to a decrease in Tg and (2) a steric hindrance effect, in which bulky substituents restrict main-chain mobility and result in an increase in Tg. The balance between these effects appears to be essential for designing polysaccharide-based plastics with tailored Tg values.

Dielectric properties

The dielectric constant (Dk, measured at 1 GHz) decreased as the number of carbon atoms in the side chain increased (Fig. 4). For cellulose esters, Dk decreased from 3.9 (acetate) to 3.1 (propionate) and 2.7 (hexanoate). Similar trends were observed for paramylon and α-1,3-glucan esters, attributed to the reduced proportion of polar ester groups in longer side chains. When side chains contained five or more carbon atoms, Dk fell below 3.0, lower than those of epoxy or polyimide resins. Interestingly, Dk was scarcely affected by backbone linkage or side-chain geometry (linear, branched, or ring-shaped).

Fig. 4
Fig. 4
Full size image

Dependence of dielectric constant (Dk at 1 GHz) on side-chain length for a cellulose, b paramylon, and c α-1,3-glucan esters. Dk decreases with increasing side-chain carbon number; values below 3.0 are achieved for side chains containing five or more carbons

A particularly notable feature was the simultaneous achievement of high Tg and low Dk in ring-shaped esters. α-1,3-Glucan-CH showed Tg = 205 °C, Dk = 2.7, and Df = 0.013, comparable to or superior to common substrate polymers. Further annealing increased crystallinity and improved dielectric properties to Dk = 2.48 and Df = 0.007 (Table 1; Fig. 5). These values outperform those of polyimide and epoxy, demonstrating the potential of polysaccharide esters for high-frequency substrate applications.

Table 1 Thermal and dielectric properties of polysaccharide esters and reference polymers
Full size table
Fig. 5
Fig. 5
Full size image

a Cast film of α-1,3-glucan-CH and b relationship between crystallinity and dielectric properties. Annealing increased crystallinity, lowering Dk and Df to 2.48 and 0.007, respectively

Polysaccharide esters without primary hydroxy-functional group

Thermal properties

Primary hydroxy-functional group (–CH₂OH) tends to orient under external electric fields, contributing to dielectric loss in the MHz–GHz range [31, 63, 64]. To suppress this effect, we examined esters of polysaccharides lacking a primary hydroxy-functional group, α-1,6-glucan (dextran) and xylan (Fig. 6). Both were esterified with propionic acid (Pr), and α-1,6-glucan was also modified with cyclohexanecarboxylic acid (CH).

Fig. 6
Fig. 6
Full size image

Chemical structures of a α-1,6-glucan esters and b xylan esters. These polysaccharides lack a primary hydroxy-functional group in the side chains

α-1,6-Glucan-Pr showed Tg = 121 °C and Tm = 275 °C, while α-1,6-glucan-CH exhibited Tg = 141 °C and Tm = 255 °C. Although their Tg values were below 200 °C, the melting points above 250 °C suggest sufficient dimensional stability for soldering. In contrast, xylan-Pr had Tg = 68 °C, indicating the need for further improvement through blending or crosslinking.

Dielectric properties

Dk of xylan-Pr (2.52) was lower than those of α-1,6-glucan-Pr (2.97) and cellulose-Pr (3.08), likely due to the smaller number of hydroxy-derived polar groups in the xylan backbone. Meanwhile, α-1,6-glucan-CH exhibited Dk = 2.58, comparable to cellulose-CH (2.61). These results suggest that both the carbon number of the side chain and the number of hydroxy-functional groups in the repeating unit strongly influence Dk.

Regarding dielectric loss, α-1,6-glucan-Pr and xylan-Pr each showed Df = 0.008, roughly half that of cellulose-Pr (0.015). Because these polymers lack primary hydroxy-derived ester side chains, they likely exhibit reduced orientational polarization at high frequencies. Remarkably, α-1,6-glucan-CH achieved Df = 0.003, the lowest among all polysaccharide esters examined (Fig. 7). Its dielectric performance (Dk = 2.58, Df = 0.003) matched that of the best non-fluorinated polymer, cyclo-olefin polymer (COP, Dk = 2.5, Df = 0.003) while maintaining crystallinity and Tg = 141 °C. These results highlight α-1,6-glucan-CH as a promising non-aromatic, non-fluorinated resin combining thermal stability with exceptional dielectric properties.

Fig. 7
Fig. 7
Full size image

a Cast film of α-1,6-glucan-cyclohexanecarboxylate and b comparison of dielectric constants and losses among various polymers. α-1,6-Glucan-CH (Dk = 2.58, Df = 0.003) shows dielectric performance comparable to cyclo-olefin polymer (COP) while maintaining crystallinity and higher thermal stability c Frequency dependence of transmission loss (10 MHz–67 GHz) for coplanar lines fabricated on α-1,6-glucan-B1H2 substrates compared with epoxy-based FR-4

This chapter systematically examined the thermal and dielectric properties of polysaccharide esters with different glycosidic linkages and side-chain structures. The results demonstrated that the introduction of cyclic side chains, such as cyclohexane carboxylate groups, enables the simultaneous achievement of high Tg and low-dielectric properties, likely due to restricted backbone mobility and a reduced effective contribution of dipolar polarization. In addition, crystallization was found to be effective in further reducing Df by suppressing dipolar relaxation.

Polysaccharides lacking a primary hydroxy-functional group, as well as those bearing longer ester side chains, tended to exhibit lower Dk and Df. This behavior is attributed to the reduced contribution of primary hydroxy-derived dipolar relaxation, which is known to be active in the high-frequency region.

Beyond thermoplastic esters, low-dielectric thermosetting resins have also been developed by introducing both unsaturated and saturated ester groups into polysaccharides, such as cellulose, paramylon α-1,3-glucan, and α-1,6-glucan [58, 60, 65]. Polysaccharide unsaturated esters prepared using a mixture of plant oil-derived 2-butenoic acid (B) and hexanoic acid (H) at a molar ratio of 1:2 exhibited thermoplastic behavior and could be thermally molded at 150 °C without inducing crosslinking reactions. Upon heating to approximately 200 °C, the unsaturated moieties underwent crosslinking, leading to an increase in Tg to around 230 °C. This two-step thermal behavior enabled the fabrication of materials with sufficient thermal resistance to withstand soldering processes.

After crosslinking, these thermoset resins exhibited dielectric constants of 2.5–2.6 and dielectric losses in the range of 0.009–0.014, and were either outperforming or comparable to conventional polymers, such as epoxy resins [24, 66,67,68]. Notably, α-1,6-glucan unsaturated esters ( α-1,6-glucan-B1H2)  retained thermoplastic processability prior to curing, allowing lamination with copper foil and subsequent formation of copper circuitry. Circuit boards fabricated using this material transmitted high-frequency signals up to 67 GHz, with transmission loss approximately 40% lower than that of epoxy-based substrates at 67 GHz (Fig. 7c). These results demonstrate the feasibility of polysaccharide-based resins as low-loss substrates for high-frequency communication applications.

Transparent and strong pullulan optical fibers

Requirements and limitations of plastic optical fibers

Plastic optical fibers (POFs) are typically fabricated from amorphous petroleum-based polymers, such as polymethyl methacrylate (PMMA), which can be melt-drawn into flexible and transparent fibers. Compared with glass fibers, PMMA fibers are less brittle, easier to handle, and cheaper to produce, making them suitable for short-distance communication and sensing applications [69, 70].

However, PMMA fibers suffer from several limitations. Their Tg is around 100 °C, restricting their use in high-temperature environments such as automotive systems or chemical plants operating at 150–200 °C [71, 72]. Applying a high-temperature polymer coating could extend the usable range, but such multi-material constructions are difficult to recycle, and incineration remains the common disposal route [73, 74]. Moreover, uncoated PMMA fibers used outdoors can fragment into microplastics, posing environmental concerns [75,76,77]. Developing optical fibers that combine heat resistance, optical performance, and environmental compatibility is, therefore, an urgent goal.

Regenerated cellulose fibers, such as viscose rayon, have long been used as strong, low-cost materials [78]. Recently, their potential use as optical waveguides has been demonstrated. For example, regenerated cellulose fibers coated with cellulose acetate as a cladding layer can guide light [79,80,81], and carboxymethylcellulose fibers can transmit light even without a cladding layer [82]. These cellulose fibers are expected to serve as novel sensing materials owing to their intrinsic functionality and thermal stability above 100 °C, in contrast to PMMA fibers, which soften near this temperature [83,84,85,86].

Nevertheless, the optical performance of cellulose fibers remains inadequate. Their crystallinity scatters light, resulting in opaque fibers with transmission lengths of only about 10 cm. Furthermore, during viscose processing, cellulose molecules undergo oxidation and yellowing, which absorb blue light and hinder visible-range transmission [87, 88].

To overcome these limitations, other biopolymers, such as agar and silk, have also been investigated [89]. Another promising candidate is pullulan, an amorphous, water-soluble polysaccharide consisting of maltotriose units linked by α-(1 → 6) bonds (Fig. 8a). Produced by the microorganism Aureobasidium pullulans, pullulan has been used in food coatings and pharmaceutical capsules due to its oxygen barrier properties [90]. Although acetylated pullulan films exhibit extremely low birefringence and high optical transparency, a practical fiber-spinning method for pullulan has not yet been established [91]. Recently, we developed a fully aqueous gel-spinning process that produces tough and transparent pullulan fibers without using organic solvents or thermal treatments. This section introduces the fabrication process and optical performance of these fibers [92].

Fig. 8
Fig. 8
Full size image

a Chemical structures of pullulan and borax, b borax crosslinked pullulan hydrogel, c dried pullulan fiber, and d fiber surface micrograph. The reversible borate crosslinking enables an all-aqueous gel-spinning process that yields transparent, defect-free pullulan fibers without organic solvents

Fabrication of transparent pullulan fibers via gel spinning

Most polysaccharides are poorly thermoplastic due to dense hydrogen bonding, and thus are typically processed by wet spinning. Yet many require strong alkali or ionic liquids for dissolution, leading to colored products and solvent-handling issues. Because pullulan dissolves readily in water, it provides a promising route for environmentally benign fiber formation.

However, when pullulan aqueous solutions are coagulated using nonsolvents, such as ethanol, phase separation proceeds slowly, causing pores and light scattering. To avoid this, we applied a gel-spinning technique, often used for ultra-high-molecular-weight polyethylene [93].

Specifically, a 20–40 wt% pullulan solution was mixed with a 5 wt% aqueous borax solution to form a colorless, transparent hydrogel (Fig. 8b). In water, borax generates borate ions that reversibly crosslink the hydroxy-functional group of pullulan. The hydrogel was extruded through a syringe to form >1 m continuous gel filaments, which were then dried at room temperature for 24 h under fixed ends to yield transparent solid fibers (Fig. 8c). Microscopic observation confirmed smooth surfaces and defect-free cross sections (Fig. 8d). Wide-angle X-ray diffraction showed no crystalline peaks, confirming that the fibers were amorphous. The absence of crystalline domains accounts for their high optical transparency.

This process requires no coagulation bath, no organic solvent, and no acid or base treatment. The fibers form simply by evaporating water, making the process both simple and sustainable. The method can also be extended to other water-soluble polysaccharides or hydroxy-containing derivatives, opening opportunities for broader polysaccharide-based fiber materials.

Mechanical and thermal properties

Despite being amorphous, the pullulan fibers exhibited mechanical strength comparable to or exceeding that of crystalline polysaccharide fibers. The tensile strength reached approximately 200 MPa (Fig. 9a), surpassing PMMA (70 MPa) and polycarbonate (150 MPa) [21, 94]. The fibers became stiffer as the borax content increased, suggesting that crosslinking density controlled rigidity.

Fig. 9
Fig. 9
Full size image

a Stress–strain curve and b dynamic mechanical properties of pullulan fibers. The fibers exhibit a tensile strength around 200 MPa and maintain an elastic modulus up to 200 °C, demonstrating outstanding toughness and heat resistance for an amorphous polymer

While the fibers dissolved in water within a few hours, they were entirely insoluble in chloroform, even after 24 h of immersion, and retained their tensile strength. This solvent resistance indicates potential for applications, where conventional POFs would fail. For applications exposed to water, a hydrophobic coating would be required. For example, using a hydrophobic resin with a refractive index lower than that of pullulan could serve not only as a protective layer but also as a cladding, potentially improving the optical transmission performance.

Dynamic mechanical analysis revealed that the storage modulus remained nearly constant up to 200 °C, with no softening observed (Fig. 9b). Hence, the fibers can maintain dimensional stability in high-temperature environments, where PMMA deforms.

Optical transmission properties

The pullulan fibers exhibited high transmittance from the visible to near-infrared region (350–1300 nm) (Fig. 10a). When blue (405 nm) and red (637 nm) laser light was coupled into the fibers, they successfully transmitted light over distances exceeding 50 cm without any cladding (Fig. 10b, c). The fibers could be bent or looped while maintaining total internal reflection, indicating stable waveguiding. Absorption peaks near 1400 nm were attributed to water, consistent with pullulan’s hygroscopic nature.

Fig. 10
Fig. 10
Full size image

a Optical transmittance spectrum of a 4 cm pullulan fiber and photographs showing b blue-light (405 nm) and c red-light (637 nm) waveguiding over ~50 cm. The fibers guide visible light by total internal reflection without cladding, achieving transmission losses of 1.15 dB cm−1 (blue) and 1.42 dB cm−1 (red)

Previously reported regenerated cellulose fibers could not guide blue light and showed transmission losses of 5.9 dB/cm even at optimal wavelengths [79]. In contrast, pullulan fibers achieved transmission losses of 1.15 dB/cm at 405 nm and 1.42 dB/cm at 637 nm, representing a remarkable improvement. These low losses can be attributed to the absence of crystalline scattering and to the mild, oxidation-free processing conditions that prevent chromophore formation.

The ability to guide blue light enables new applications, including biosensing, bioimaging, photocatalysis, and micro-pattern exposure using short-wavelength light [95,96,97,98]. Moreover, their transparency and flexibility make them promising candidates for optical interconnects in compact electronic systems [99].

Comparison and potential applications

Although commercial PMMA fibers still achieve lower transmission losses (10−3 dB/cm range), theoretical studies suggest that cellulose fibers could reach similar performance if extrinsic scattering and absorption are eliminated [100]. Given pullulan’s amorphous structure and glucose-based composition, there remains potential to match or surpass PMMA performance by optimizing purity, cross-sectional geometry, and by applying cladding coatings.

Considering both optical and mechanical aspects, pullulan fibers are unique: they are transparent and amorphous yet mechanically robust (~200 MPa), thermally stable up to 200 °C, and capable of transmitting light across the visible and NIR regions (Fig. 11). PMMA fibers, while optically excellent, soften near 100 °C. Aromatic polymers such as polyimide or polyamide-imide withstand heat but absorb visible light due to π–π interactions and charge-transfer complexes. Regenerated cellulose fibers are opaque due to crystallinity and oxidation-induced yellowing. Pullulan fiber thus combines attributes that were previously incompatible: transparency, strength, and heat resistance.

Fig. 11
Fig. 11
Full size image

Comparison of tensile strength and thermal stability (Tg or softening point) among polymeric fibers. Pullulan fibers combine transparency, high strength, and stability up to 200 °C, properties previously difficult to achieve simultaneously in amorphous polymers. Open circle, colorless and transparent amorphous polymer, black triangle, colored amorphous polymer

Potential applications include optical sensing and communication in high-temperature or solvent-rich environments, such as automotive engines or chemical process monitoring. In addition, pullulan exhibited absorbance in the 1200–1400 nm range due to water molecules within the fibers, and the magnitude of this absorbance depended on the ambient humidity. This property could be exploited for optical humidity sensing, because environmental humidity can be monitored by transmitting optical signals through the pullulan fiber and measuring the resulting change in absorption [92].

Cellulose fibers are inherently hygroscopic, and immersion in water leads to measurable changes in the characteristics of guided optical signals. By exploiting this property, cellulose fibers have been reported to function as standalone water-absorption sensors without the need for additional functional layers [79, 81, 82]. For example, integration of such fibers into textiles has been proposed for monitoring perspiration or human respiration. However, despite these demonstrations, sensing of ambient environmental humidity using cellulose fibers alone has not yet been achieved.

In contrast, pullulan, a water-soluble polysaccharide, is expected to exhibit higher hygroscopicity than cellulose. Changes in the internal water content of pullulan fibers in response to ambient humidity have been reported [92]. Such hygroscopic behavior is typically regarded as undesirable for optical fibers, because it degrades signal transmission characteristics. From an alternative perspective, however, this response can be interpreted as an intrinsic humidity-sensitive function [101]. By deliberately exploiting this property, pullulan fibers can be applied as single-material optical humidity sensors that do not require external coatings or composite structures.

From an application standpoint, pullulan-based optical humidity sensors could be applied to humidity monitoring during the manufacture and storage of food products, chemical materials, and construction materials [102,103,104]. In addition, the chemical structure of polysaccharides, characterized by a high density of hydroxy groups, is fundamentally different from that of conventional plastic optical fibers, such as PMMA. By chemically modifying these hydroxy groups to introduce selective recognition sites for target analytes, it may be possible to construct fiber-type biosensors based on polysaccharide optical fibers. A further noteworthy aspect is that, unlike conventional plastic optical fibers, pullulan fibers are likely to exhibit biodegradability [105]. Although systematic evaluation of their biodegradation behavior remains necessary, this characteristic suggests potential applicability in disposable or difficult-to-recover sensing applications, such as those encountered in agricultural environments. With respect to durability, it may be possible to tune the lifetime of pullulan fibers through surface treatments or related modifications [106]. Accordingly, material design tailored to the intended operating environment and sensing system will be required.

Conclusion

This review presented recent progress in the development of polysaccharide-based plastics designed for use in advanced communication devices. By focusing on the relationship between molecular structure and functional performance, we demonstrated that renewable polysaccharides can be engineered to exhibit properties comparable to or exceeding those of petroleum-derived polymers used in modern electronics and photonics.

First, the design of thermoplastic polysaccharide esters was explored for application as insulating materials in high-frequency circuit substrates. Systematic variation of the polysaccharide backbone and side-chain structures revealed that bulky ring-shaped side chains can provide both high glass-transition temperatures and low-dielectric constants without incorporating aromatic rings or fluorinated units. Among the studied polymers, α-1,3-glucan-cyclohexanecarboxylate (α-1,3-glucan-CH) achieved Tg = 205 °C, Dk = 2.65, Df = 0.013, values surpassing conventional epoxy resins. Further annealing improved the dielectric performance to Dk = 2.48 and Df = 0.007. Moreover, α-1,6-glucan-CH, a polysaccharide ester without a primary hydroxy-functional group, exhibited outstanding dielectric properties (Dk = 2.58, Df = 0.003), comparable to the best non-fluorinated commercial materials, such as cyclo-olefin polymer (COP). These results indicate that both the nature of the glycosidic linkage and the presence of primary hydroxy-functional group govern dielectric relaxation behavior in polysaccharide esters.

Second, we introduced transparent and mechanically strong pullulan fibers produced through a simple all-aqueous gel-spinning method using borax as a reversible crosslinker. The process requires no organic solvents, acids, or high-temperature steps, yielding amorphous fibers with exceptional clarity and strength (~200 MPa). The fibers maintained their modulus up to 200 °C and could guide blue-to-near-infrared light (350–1300 nm) over 50 cm without cladding. These properties enable potential applications in optical communication and sensing systems operating under high-temperature or solvent-exposed conditions, where PMMA fibers fail.

The combination of molecularly tunable structure, renewable origin, and high functional performance establishes polysaccharides as promising candidates for sustainable materials in next-generation information and communication technologies. As data transmission demands continue to grow, addressing both device performance and environmental sustainability will become increasingly critical. Continued research into the chemistry, processing, and structure–property relationships of polysaccharides will expand their utility beyond traditional packaging and textile uses, enabling their integration into advanced electronic and optical systems.