ABSTRACT Flexible electronics (FEs) with excellent flexibility or foldability may find widespread applications in the wearable devices, artificial intelligence (AI), Internet of Things

(IoT), and other areas. However, the widely utilization may also bring the concerning for the fast accumulation of electronic waste. Green FEs with good degradability might supply a way to

overcome this problem. Starch, as one of the most abundant natural polymers, has been exhibiting great potentials in the development of environmental-friendly FEs due to its inexpensiveness,

good processability, and biodegradability. Lots of remarks were made this field but no summary was found. In this review, we discussed the preparation and applications of starch-based FEs,

FIBROIN-BASED FLEXIBLE ELECTRONICS Article Open access 06 May 2021 BOOSTING THERMOELECTRIC PERFORMANCE OF SINGLE-WALLED CARBON NANOTUBES-BASED FILMS THROUGH RATIONAL TRIPLE TREATMENTS

Article Open access 23 April 2024 INTRODUCTION More than 100 years have passed since human walked from vacuum tubes to transistors. Nowadays, electronic devices are transforming from solid,

durable, single-shape styles to wearable, versatile, high-performance, and multifunctional components, among which the flexible electronics (FEs) have already sprung up and gained extensive

attentions1,2,3,4. The appealing applications of FEs may at least include personalized mobile devices, health-care systems, human-machine interfaces, soft robots, electronic skin, artificial

intelligence (AI), and Internet of Things (IoT)5,6,7,8. To fabricate FEs, the soft, stretchable, foldable substrates are highly desired. In general protocols, electro-conducting materials

including metals, conducting polymers, or carbon-based materials are assembled on the flexible substrates, which involves polymer films such as polyethylene terephthalate (PET)9, polyimide

(PI)10,11, hydrogels12, textiles13, and elastomers such as polydimethylsiloxane (PDMS)14,15. Nevertheless, these substrates are usually non-degradable, which may result in accumulation of

electronic waste (e-waste). In estimation, the globe production of e-waste reached to 53.6 million tons by 2020, an average of 7.3 kg of one person. And this number might increase to 74.7

million tons by 203016. In 2019, only 17.4% of the e-waste was recycled due to the tedious process and imperfect policy. Green FEs with good degradability may supply a way to fundamentally

solve the problem. In recent years, a variety of environment-friendly, bio-degradable, and low-cost electronic products have emerged17,18, which were made of materials from nature, including

cellulose19,20,21,22,23,24, lignin25,26,27,28,29, proteins30,31,32, and starch33,34,35. These materials from nature have intrinsic advantages of abundant sources, low cost, good

degradability, acceptable biocompatibility, fine accessibility for chemical modification comparing with synthesized polymers. They can be safely and bio-compatibly used as implantable

devices for human healthcare and medical diagnose36. Furthermore, their utilization may be free of e-waste production to satisfy the requirements of the e-waste reduction. Among the reported

sustainable materials used in FEs, starch have advantages on the acceptable solubility in water, good property of film forming and relative low cost comparing with cellulose, lignin and

proteins. Therefore, a lot of efforts have been put into the developments of FEs based on starch37,38,39,40,41, in which the starch usually appeared as films or gels. The starch films were

often used as soft and transparent substrates because of the intrinsic good film-forming property of starch. The conductive materials can be easily coated or printed on the starch film to

fabricate FEs. On contrary, the starch hydrogel can be feasibly processed to various shapes with injection or 3D printing, which may further carbonize into conductive materials with

stereoscopic structures. The water containing system of the hydrogel may also benefit for their biomedical application. Though the starch films and gels have their own features, both were

versatile in the exploration of FEs, such as flexible conductive electrodes, sensors, supercapacitors, transistors, and resistive switching memory devices (Fig. 1). However, to the best of

our knowledge, no summary on this exciting aera was found. Therefore, here in this review, the fundamentals and advantage of the starch used for FEs were firstly discussed. Subsequently, the

starch-based devices were divided into two categories and successively summarized, i.e., FEs based on starch films vs FEs based on starch gels. The strategies of the preparation were

introduced and the role of the starch played in these FEs was highlighted. The challenges and future prospects were finally addressed to end the review. Starch is a natural-derived

polysaccharide comprising of glucose monomers, which can be abundantly harvested from the roots, stems, and seeds of the rice, corns, wheat, cassavas, potatoes, and other crops. According to

the molecular chain structure, the starch can be divided into amylose and amylopectin42,43,44. In the natural starch, amylose accounts for 20~26%, and the rest are amylopectin. The amylose

has a linear monomer at chain with only α-1,4 linkages thus possessing good extensibility in solution and can easily associate with some polar organic compounds by hydrogen bond. The

amylopectin is the branch form with α-1,4 linkages in the backbone and α-1,6 linkages at the branched points, which usually has much higher molecular weight45. In addition, starch molecules

contain lots of hydroxyl groups that are suitable for esterification46, etherification47, grafting48, cross-linking49, and other chemical modification50,51. Moreover, starch has outstanding

environmental degradability and good biocompatibility. These characteristics make it an important candidate as raw material for fabricating green FEs. Starch, is the most widely used

biodegradable materials for now. In Europe, 50% of the produced starch was used for non-food applications52, such as the composition in adhesives and paper binders, textiles, chemical

production, the feedstock for fermentation and other industrial products53. In addition, starch has physical, chemical, and functional properties, including good water solubility,

gelatinization, high temperature adhesion behavior, and easy modification. The utilization of starch as a platform for building FEs at least has following advantages: (1) Starch is one of

the most abundant renewable materials with extremely low cost. (2) Starch is degradable in water and soil without toxic residues thus being regarded as environmentally friendly materials.

(3) Starch is biocompatible and suitable for implantable electronics. (4) Starch is easily transformed into flexible, foldable films and gels. (5) Starch is light in weight. Comparing with

other biomass materials for FEs, the starch still has some advantage. For instant, it shows better solubility than cellulose and it holds lower cost than proteins53,54. FES BASED ON STARCH

FILMS Starch is easy to be transformed into a film through heating in the presence of water followed by casting and drying. However, the starch films prepared by pure natural starch are

usually fragile with low transparence. They are usually reinforced by compositing with other polymers, adding plasticizers, or modification through physical or chemical strategies38,55,56.

The modified starch films may obtain acceptable mechanical property57,58, good flexibility49, or other features such as electrical conductivity59, making them more favorable to be utilized

as soft substrates or the constituents of FEs. The data of FEs based on starch films were listed in Table 1. CONDUCTIVE FILMS DERIVED FROM STARCH Conductive films can be easily casted with

starch when electroconductive materials are encapsulated. The most frequently used conducting components include graphene, conducting polymers, ionic liquid, metals, and their alloys60.

Zheng et al.48 grafted starch molecules on the surface of graphene nanosheets (GN) in the presence of hydrazine hydrate. The conducting films were then fabricated with GN-starch filled

plasticized starch, which shown conductivity up to 9.7 × 10−4 S cm−1 when the content of GN-starch was only 1.774 wt%. The film also possessed excellent mechanical properties and moisture

resistance, because the GN was well dispersed with the assistance of starch. Prusty et al.61 prepared nanocomposites of functionalized single-walled carbon nanotubes (f-SWCNTs) and

polyacrylonitrile-co-starch (PAN-co-starch) using in-situ polymerization technique. The nanocomposites have excellent electrical conductivity up to 10−3 S cm−1. The starch nanocrystals

(SNCs) can be obtained after removing the amorphous zone by the treatment of enzyme, sulfuric acid, or hydrochloric acid62,63. The utilization of the starch nanocrystals to prepare

composites may not only improve the mechanical properties of materials, but also increase the biodegradability63,64. Zhu and co-workers65 extracted plate-like SNCs from natural waxy corns,

which were then used to improve the dispersion of graphene in the composite film based on soybean protein (Fig. 2a). The composite conductive film shown a good conductivity of 65.8 μS m−1,

enhanced mechanical strength, reduced water vapor and oxygen permeability as well as fine solvent resistance. The conductive films with good transparency can also be prepared from starch.

Miao and co-workers38 spray-coated the carbon nanotube (CNT), pristine graphene (PG), and poly(3,4-ethylenedioxythiophene) (PEDOT) on the composite film of starch and chitosan to fabricate

conducting film (Fig. 2b), which shown transmittance of 83.5% at 550 nm and low sheet resistance of 46 Ω sq−1 as well as good mechanical performance. The conducting film worked as a

transparent flexible electrode. Furthermore, it can be rapidly degraded in lysozyme solution at room temperature indicated it is a green electronic device. Similar transparent film recently

developed with graphene and potato starch, which achieved conductivity of 3.9 × 10−4 S cm−1 with good moisture barrier properties66. The most reported protocol to prepare starch films

involved dissolving in water, gelatinizing and casting67,68. Plasticizers such as polyols (glycerol, ethylene glycol) or citric acid have to be added to reinforced the films49,55,69.

Different from traditional-used plasticizers, ionic liquid (1-ethyl-3-methylimidazoliumacetate, [C2mim][OAc]) was employed as a functional plasticizer by Zhang and co-worker. to prepare

electroconductive and transparent film (Fig. 2c)59. The film had good electrical conductivity (>10−3 S cm−1) and was straightforwardly processed at moderate temperature (55–65 °C). Except

for solution casting as films, starch was used as a stabilizer in the preparation of conductive ink, which was then applied to fabricate conductive films. Cataldi et al.70 dispersed

commercial thermoplastic starch-based polymers (Mater-Bi, blends of thermoplastic starch and aliphatic polyester) and graphene nanosheets in an organic solvent to generate conductive inks,

which was sprayed on pure cellulose substrate (Fig. 2d), and then hot pressed to form flexible conductive film (Fig. 2e). The sheet resistance reached as low as 10 Ω sq−1 by regulating of

the content of the graphene nanoplatelets. It was noted that the composite film maintained its mechanical and electroconductive performance even under many severe folding events, which is

significant for flexible conductors. In the preparation of printable conductive ink, starch can also work as a reducing agent for metal ions. Bacalzo et al.71 synthesized an aqueous gold

nanoparticle (AuNP) ink suitable for inkjet printing using hydrolyzed starch as reducing agent and stabilizer (Fig. 2f). The AuNP ink can be printed and sintered at low temperature (<200 

°C) to generate conductive film with resistance as low as 1.0 Ω sq−1. In this strategy, the starch solution was heated and hydrolyzed in NaOH solution, and then activated to reduce Au3+ in

HAuCl4 solution to Au0. Some starch molecules were hydrolyzed to stabilize the nanoparticles and control their further growth. This water-based ink can be used to print conductive patterns

on a variety of substrates, demonstrating the versatile possibility of application in FE devices development. SENSING ELEMENTS ON STARCH FILMS Flexible sensing devices have important

applications in the fields of human health monitoring, human-machine interface and wearable devices, which have attracted extensive attentions in recent year13,72,73. According to the

application of detection, the sensing devices can be divided into physical sensors74,75,76, chemical sensors77,78,79, and biological sensors80,81,82. The starch have been used to develop

degradable or recyclable flexible sensors because it is biocompatible and reproducible. STRAIN SENSOR Strain sensor is a device that transforms deformation into electrical signal, which is

widely used in wearable devices75,83,84,85. Liu et al.86 demonstrated a high-performance transparent strain sensor using starch to fabricate conductive matrix. The egg white was used as the

sacrificial layer to support the conducting networks of silver by electro-plating. The starch solution was then dip-coated on the surface of the silver mesh and generated self-supported

starch film with silver networks embedded (Fig. 3a), which shows low electro-resistivity (<1.0 Ω sq−1) and high transparency (82%) (Fig. 3b). As shown in Fig. 3c, the resistance of the

strain sensor changes following multiple bending cycles, which can be used to detect movements of human joints. The starch can be easily degraded and removed by water to yield an independent

Ag network, indicating that the devices is recyclable. CHEMICAL SENSOR Chemical sensors are used to detect chemicals in liquid or gas state, which have potentials in environment monitoring,

chemical diagnose, warning of hazardous gas. Jodar et al.87 modified glassy carbon electrodes with RGO, gold nanoparticles (GNPs), and potato starch (PS) to produce a stable RGO-GNPS-PS

conductive film as an electrochemical sensor for detection of estriol (Fig. 3d), which was a pollutant found in environment and aquatic organisms. The starch had good film-forming property

and chemical stability, which was demonstrated in good supportability and permeability, as well as to compensate for the surface roughness of the electrode. The linear response of estriol

was found in the range of 1.5–22 μmol L−1, and the detection limit was 0.48 μmol L−1. The RGO-GNPS-PS also effectively recycled 92.1–106% of estriol in natural water samples and synthetic

urine. Similar strategies were used to fabricate electrochemical sensors for the detection of various chemicals88,89,90,91,92, in which the starch played an important role such as the

substrates. GAS SENSOR The gas sensor is a device that detects various gas such as different organic vapors. It is also known as electronic nose79,93,94,95. Starch can be used to tune the

nanostructure of gas sensor to enhance its sensitivity and selectivity96. Kumar et al.97 investigated the use of amylose and amylopectin functionalized carbon nanotubes (CNT) to fabricate

electronic noses on microelectrodes by spraying them layer by layer to detect vapors of water, methanol and toluene. As shown in Fig. 4a, they adsorbed on the surface of carbon nanotubes in

helical or random conformation, respectively, which changed the junction gap of CNT. Interestingly, amylose increased the sensitivity of the CNT sensor to water, while amylopectin increased

the sensitivity to toluene (Fig. 4b). In another example, Peregrino et al.98 prepared gas sensor by assembling starch and graphene oxide (GO) layer by layer on a quartz substrate (Fig. 4c).

They found at least three important roles that the starch was played. At first, hydrogen bondings were established between −OH groups in starch and −C−O−C−, and =C=O from GO, which improved

the efficiency of film assembling. Secondly, the starch provided additional electron density during photoreduction of GO, making the reduction faster and more efficient. Thirdly, the

sensitivity of the device to humidity was improved since the starch is hygroscopic. Therefore, it is convincing that the starch may find more applications in fabricating such intelligent

device. MULTIFUNCTIONAL SENSOR The preparation of FEs with low cost and high precision detection of various stimuli are the significant challenges12,99,100. In our previous work37,

laser-induced porous carbon was transferred to the starch film producing a flexible multimodal sensor (Fig. 4d, e). The strain, humidity, temperature, and pressure can be detected with one

device, thus decreasing the price of sensor matrix. The sensor can detect strain with gauge factor (GF) of 134.2, response time of approximately 130 ms, and good durability more than 1000

cycles of bending-unbending, which can be used to monitor human motions including bending of fingers, wrists (Fig. 4f) and knees. The temperature change between 30 and 50 °C and pressure in

the range of 0–250 kPa were also detected by the sensor. The response time for pressure detection was ~0.15 s. Interestingly, the sensors in matrix pattern were easily fabricated with this

strategy, which precisely detected spatial pressure (Fig. 4g). Furthermore, multiple stimuli can be distinguished with the sensor, such as pressure coupled with temperature change. More

importantly, the sensor can be degraded in water thus being free of e-waste, so it is a typical green electronic device. SUPERCAPACITOR ON STARCH FILMS Traditional energy sources cannot

fulfill the dramatical increment of energy demands in nowadays, thus the alternate and renewable energy source attracted widely attentions. The electrolyte is one of the most important parts

of renewable energy device. Polymers including polyethylene oxide (PEO), poly (vinylidene fluoride) (PVDF), and poly (methylmethacrylate) (PMMA) were intensively investigated as host of

salt complex electrolytes in previous research101,102,103,104,105. Recently, people found the starch can meet the 3Es requirement of electrolytes that are (a) environment friendly, (b)

economical and (c) easy to mould in desired shape and size. For instance, Chauhan et al.106 prepared a highly transparent film with conductivity up to 10−2 S cm−1 using composite of corn

starch and NaClO4 after cross-linking with glutaraldehyde. The equivalent series resistance of the sample with thickness of 0.8 mm was ~6.252 Ω, the electrochemical stability window (ESW)

was ~2.4 V and the ion relaxation time was ~65 μs. Chen and co-workers107 synthesized ultrathin carbon nanosheet-supported Ni quantum dot hybrids (C-Ni-QDs) through hydrothermal method

followed by annealing process with the assistance of starch. The C-Ni-QDs was used to assemble a supercapacitor, which shown a high specific capacitance up to 1120 F g−1 at 2 A g−1 and a

capacitance retention of 97% after 2000 cycles. These results shown that the starch is a promising carbon sources for supercapacitors that is inexpensive and abundant. NANOGENERATOR ON

STARCH FILMS Nanogenerators based on triboelectric, piezoelectric, and electrostatic induction effects have been proposed to construct self-actuating systems that produce electricity from

various mechanical energies, including human motion108,109,110, vibration, and wind111,112. The flexible nanogenerators can provide a flexible and portable energy collection system or work

as self-powered sensors, which has recently become a hot research topic9,113,114,115,116. Triboelectric nanogenerators (TENGs) are made of two materials with different triboelectric

properties. The friction between the two films generate equal but opposite charges on both sides due to nanoscale surface roughness, while the friction potential layer generated in the

interface region drives the flow of electrons in the external load117,118. Bao et al.119 developed TENGs based on lignin-starch nanocomposites (Fig. 5a), in which the starch improved the

uniformity of the film. When the ratio of lignin to starch in the composite film was 3:7, the highest output was reached. The average short-circuit current was 3.96 nA cm−2, and the

open-circuit voltage was 1.04 V cm−2. Zhu et al.120 assembled TENGs based on a starch film, using the coupling effect between human skin and starch film to develop a self-powered sensor for

detecting human perspiration (Fig. 5b). Vela and co-workers121 described a starch-cellulose-based TENG using potato starch, and sandpaper as the substrate of microstructure. They found that

the output voltage depended on the thickness (50–200 μm) of the film. The electric output of 4 cm2 TENG reached up to 300 mV in the presence of the starch film. Further research by Vela

prepared a low-cost TENG using a starch film with micro-structure as a triboelectric dielectric layer (Fig. 5c)122. The 0.5% of CaCl2 was added into the film to improve the voltage output

from 0.4 to 1.2 V. Sarkar and co-workers40 developed TENGs using thermoplastic starch film as a positive electrode and PDMS as negative electrode (Fig. 5d). This bilayer TENG (b-TENG) can

generated open-circuit output voltage up to ~560 V and output current density of around ~120 mA m−2, which can light more than 100 LEDs. In addition, the TENG device can work as a

self-powered pedometer for walking and a gait analysis sensor for health evaluation (Fig. 5e, f). The good performance of the devices benefited from the surface morphology of the starch

film, which may open a window for the low cost and environmental-friendly self-powered electronic devices. ORGANIC TRANSISTOR ON STARCH FILMS Flexible organic field-effect transistor (OFET)

devices have attracted much attention as a display technology123,124,125,126,127. Intrinsically flexible OFET has significant advantages compared with inorganic thin film transistors on

accelerate technology update, simple production process and low cost. Shao et al.128 used starch as an ion-base gate dielectric for oxide thin film transistors (Fig. 6a). It was found that

the transistor performance is closely related to the specific capacitance and ionic conductivity of starch (Fig. 6b). Higher on/off ratio and field mobility can be achieved by glycerol

incorporated starch. The results demonstrated the potential application of starch in thin film transistors. In Jeong’s work33, a flexible OFET was fabricated on starch-PVA film by a

vapor-deposited method (Fig. 6c). The starch film has good transparency (Fig. 6d), remarkable mechanical strength, and good stability in non-polar solvent. The calculated field-effect

mobility was 0.013–0.37 cm2 (Vs)−1 with on/off ratio of 6.9 × 104–4.9 × 105 (Fig. 6e), which were comparable to that of other flexible OFETs. Interestingly, the device on the film can be

quickly degraded in fishbowl water by the fungi inside, suggesting it is eco-friendly biodegradable FEs with low cost. OTHER ELECTRONICS ON STARCH FILMS Resistive switching memories (ReRAMs)

are the major candidates for replacement of the state-of-the-art memory devices in the future due to their good scalability, ultrafast write and read access, and low power consumption129.

The ReRAM using polymer as the active layer has the advantages of good flexibility, low cost, good processability and fine compatibility with various substrate. Biopolymers such as

chitosan130 and cellulose131 have been considered in the electronic devices of synapses28,131,132. Due to the presence of loosely bound water molecules in the crystal network, the potato

starch has reasonable ionic conductivity133,134. Raeis-Hosseini et al.35 demonstrated a non-volatile, flexible and transparent ReRAMs device using potato starch and chitosan as active layer

(Fig. 7a). The device was prepared with Au/starch-chitosan/ITO on PET flexible substrate by spin coating and thermal evaporation. The inset images in Fig. 7a shows a magnified optical image

of the storage array on the flexible substrate. The resistance behavior of the device can be controlled effectively by the content of starch and chitosan as well as the chemical composition

of starch (amylose and amylopectin). Both Au/starch/ITO/PET and Au/starch-chitosan/ITO/PET device shown good set/reset performance (Fig. 7b, c). Furthermore, the progressive

setting/resetting behavior of this storage devices makes them potential in the application of neuromorphic devices such as simulation-based synaptic electronics. Fuel cells and redox flow

batteries provide considerable energy and power density that are more sustainable alternatives to lithium batteries. However, the perfluorosulfonic acid copolymer proton exchange membranes

in these cells are still an important environmental problem. In Alday’s work135, biopolymer electrolyte membranes (Bio-PEMs) were synthesized from starch, cellulose, and chitosan. The

starch-based Bio-PEMs were doped with ionic liquid, glycerol, or urea (Fig. 7d), which reached the highest ionic conductivity of 6.2 × 10−4 S cm−1 through a symmetrical cell test (Fig. 7e).

These Bio-PEMs are promising alternatives for conventional copolymer membranes in sustainable electrochemical systems and biodegradable devices. FES BASED ON STARCH GELS Gels derived from

biopolymers have attracted increasing attentions as a result of their good biocompatibility, innate biodegradability, and crucial biological functions136,137,138. Niu et al.139 reported a

gelatin/ poly (acrylic acid N-hydrosuccinimide ester) (PAA NHS ester)-based ionic conductive organohydrogel by introducing a glycerol water binary solvent system, which was used as wearable

health monitoring device. The starch was also widely used in the preparation of gels can readily generate a three-dimensional network _via_ physical entanglement of amylose and/or

amylopectin140. A large amounts of water or biological fluids can be retained due to their good hydrophilicity. Furthermore, the starch chains may be easily cross-linked through the abundant

hydroxyl groups. So, the starch-based gels were intensively investigated and shown great potential especially in biomedicine62,141,142,143,144,145,146. Gonzalez et al.147 prepared

starch-based hydrogels through the Diels-Alder cross-linking reaction between furan modified starch and water-soluble bismaleimide. The photograph and SEM images were shown in Fig. 8a.

Graphene nanoparticles were introduced to endow the antibacterial and electrical conducting property to the hydrogel, which might find prospects in biomedical application. Recently, Lin et

al.146 prepared an adhesive hydrogel from ionically crosslinked starch, which had good electrical conductivity, high tissue adhesion, strong antibacterial ability. The wound condition

diagnosis and management might be realized due to the conductivity of the hydrogel considering the wound condition can be transferred into electrical outputs. In addition, the application of

starch-based gels in smart window was also explored148. Wang et al. prepared smart window using thermosensitive starch gel as active layer, which shown an excellent solar modulation

property of 17.9% and average light transmittance of 57.8%. The details of the reported FEs based on starch gels were listed in Table 2. SENSING ELEMENTS BASED ON STARCH GELS Various sensors

were fabricated on starch gels to obtain biocompatible devices149,150. Kanaan et al.45 synthesized a biodegradable semi-interpenetrating polymer networks (S-IPNs) based on starch and

copolymer of 2-hydroxyethyl methacrylate (HEMA) and 1-butyl-3-metaprazor-lium chloride (BVImCl) cationic (Fig. 8b). According to the relative composition of different monomers, the

electronic and ionic conductivity of the hydrogel was manipulated in range from 0.1 to 5.2 S cm−1, and the complex shear modulus was regulated in a range from 0.6 to 6.4 MPa. The flexible

hydrogel worked as a multi-stimuli-responsive platform that was sensitive to changes of relative humidity, ionic strength and current. Therefore, the adsorption/release capacity of the

hydrogel toward _L_-tryptophan was regulated through the DC voltage applied. Furthermore, the hydrogel shown good biocompatibility due to the existence of starch. It may find widely

applications in biological separation, sewage treatment systems, biomedical and electrochemical equipment. Liu et al.151 developed a flexible, ultra-low-cost conductive hybrid elastomer by

mixing polydimethylsiloxane (PDMS) and starch hydrogel, which had strong elasticity (56 kPa) and acceptable conductivity (10−2 S m−1). The composite elastomer can detect stress (gauge factor

0.71), strain (gauge factor 2.22) and humidity (sensitivity of 1.2 × 10−6 S m−1 RH %−1). Moreover, the sensor can be used for monitoring finger movements and bottle grasping (Fig. 8c, d),

demonstrating its potential in artificial skin and wearable devices. Noticeably, the volume ratio of the starch and PDMS in the composite was about 3:1, which is also significant for the

cost reduction. Xu et al.152 selected potato starch as the main network and introduced polyvinyl alcohol (PVA) and borax to improve the mechanical and conductive properties of hydrogels. Due

to the dual reversible interactions of hydrogen bonding and the boronic ester linkages, the hydrogels shown enhanced mechanical performance and super-fast self-healing ability. The hydrogel

was used as a strain sensor with GF = 1.02 at 110–200% strains, which can quickly perceive human movements with a response time of 180 ms, even small movements such as swallows and sounds.

The amylose and amylopectin may exhibit different mechanical property due to their structure difference. The linear chain of amylose makes it more resistant to processing and may show better

mechanical performance and less water sensitivity150. Liu et al.153 prepared flexible, transparent, ionically conductive hydrogel by simply mixing high-amylose starch with CaCl2 solution.

This material shown adjustable mechanical strength (500–1300 kPa), elongation at break (15–32%), Young’s modulus (4–9 MPa), toughness (0.05–0.26 MJ/m3) and suitable resistivity (3.7–9.2 

Ω·m). The hydrogel can be used as a sensor to detect strain, moisture and liquid with different pH value. The Amylopectin with dendritic structure may supply more physical interaction with

other materials thus is suitable to fabricate adhesive hydrogel. Gao et al.154 combined amylopectin with poly (acrylamide–acrylic acid) (P(AAm–AAc)) to prepare a hydrogel with good adhesion,

toughness and electrical conductivity. As a wearable sensor, the hydrogel shown high strain sensitivity (GF = 6.93) and stability, which was used to monitor various motions of human, such

as joint bending, walking, jumping, even talking. SUPERCAPACITOR BASED ON STARCH GELS Due to the carbon-rich nature and low price, starch has been used as a common precursor for producing

activated carbons which have wide applications such as electrode material155,156,157. Wang et al.34 reported a simple, rapid and scalable method for the production of the three-dimensional

flexible and porous graphene electrode materials, which simply dissolved starch in GO suspension followed by thermal carbonization and chemical activation. The electrode had a high specific

surface area of 1,519 m2 g−1, high energy density of 19.8 Wh kg−1 at the power density of 0.5 kW kg−1 and a high power density of 9.9 kW kg−1 at the energy density of 9.6 Wh kg−1. The

structural diagram and energy storage performance of the supercapacitor are shown in Fig. 9a. Furthermore, the flexible supercapacitors kept specific capacitance retention rate of 80% after

8000 cycles at 10 A g−1. The supercapacitor worked well even under large angle bending (138°), thus may find potential in the application of wearable equipment. Similarly, Liu et al.158

prepared three-dimensional interconnected reticular porous carbon electrode using corn starch as the carbon precursor (see protocol in Fig. 9b). The 3D electrode shown high specific

capacitance of 372 F g−1 at a current density of 0.5 A g−1 in 2 M KOH electrolyte with high energy density and long cycle life. The cyclic C–V diagram is shown in Fig. 9c. Considering the

high purity and low cost of corn starch and the excellent capacitance performance of 3D electrode, this study opens a way for the large-scale production of low-cost and high purity porous

carbon for supercapacitors. Willfahrt et al.159 prepared a screen printable hydrogel as electrolyte using corn starch and citric acid with ionic conductivity up to 2.30 ± 0.07 mS cm−1. The

hydrogel shown excellent printability and prolonged stability against degradation. The specific capacitance of the printed supercapacitor was up to 54 F g−1. Starch-based nanoparticle was

also used in the preparation of supercapacitors. For example, starch nanocrystals prepared by hydrolysis of starch have been widely used in many fields62,63,65,160. Chen et al.41 used

graphene oxide (GO) nanosheets to combine with dialdehyde starch nanocrystals to fabricate porous, strong, compressible aerogels that were used as supercapacitor electrodes and highly

efficient adsorbents (Fig. 9d). The starch nanocrystal prevented self-stacking between graphene nanopores to improve the electrical properties of aerogels, resulting in the increment of the

specific capacitance from 198 to 316 F g−1. The good mechanical properties and high specific surface area of the aerogels promised their application in dyes removing. Therefore, the hybrid

aerogel might be used as a flexible electrode of supercapacitors or candidate materials for biomedical and environmental cleaning. NANOGENERATOR BASED ON STARCH GELS A common limitation of

the sensing system is that the most sensors cannot work without external power source. TENGs can act as an active sensor to detect pressure without the use of an external power

source9,110,111. In addition, complex circuits can also be avoided in these self-powered devices. Chen et al.39 proposed a self-powered, flexible, triboelectric sensor (SFTS) composed of

starch-based hydrogel, polydimethylsiloxane (PDMS) and silicone rubber. The two-dimensional SFTS (2D-SFTS) with a grid structure can track continuous sliding information on the fingertip,

such as trajectory, velocity, and acceleration. 3D motion detection (Figs. 9e) and 3D robotic manipulation (Fig. 9f) were also realized by combining with a one-dimensional (1D) SFTS. By

introducing starch, the cost was decreased and environmental friendliness was improved for the composite elastomer. DEGRADABILITY AND BIOCOMPATIBILITY OF STARCH-BASED ELECTRONICS The natural

polymers including cellulose, lignin and starch belong to biodegradable polymer materials, which can be degraded by bacteria, mold, algae, and other microorganisms through enzyme or

chemical hydrolysis. The main molecular chains are firstly broken resulting into the reduction of molecular weight. They eventually decomposed to small molecules or metabolized into carbon

dioxide and water161. The polymers with higher molecular weight may takes a longer time to be degraded. A greater microbial diversity may also accelerate the degradation, such as the

activated sludge. The natural polymers such as starches usually have to be blended with synthetic polymers to reach the mechanical requirements in practical application. The degradation of

the composite of starch and synthetic polymers may be accelerated because the starch might be firstly decomposed by microorganisms, which makes the hybrids porous and increases the specific

surface area, thus facilitating further enzymatic hydrolysis or decomposition in the nature162. As known the starch is derived from nature and should be degradable, lots of experimental

protocols have been established to confirm the degradation of FEs based no starch. One intuitive way is recording the decomposing process of the device. For instance, the conductive starch

film developed by Miao and co-workers38 was found to be quickly dissolved and disappeared in the buffer of acetic acid and sodium acetate (pH = 4.5) with 3wt % lysozyme after about 8 min, as

shown in Fig. 10a. In another examples, the starch device was gradually covered with fungus until it was completely degraded when it was soaked in fish tank water (Fig. 10b)33. In our

previous work, the starch-based sensor was broken in the flush water generated by the shower nozzle and then completely decomposed in about 10 min, indicating a rapid degradation under water

flushing (Fig. 10c)37. Besides, the starch-based devices were also demonstrated to be degradable in static water37,120. It should be pointed out that the treatment of the other materials in

these devices were usually ignored, including the graphene, conducting polymers, or metals. The reason may be ascribed to their non-hazardous feature and low content of these conductive

materials in FEs. However, it may be a risk when the usage of the starch-based FEs is large enough. The non-degradable components may be recycled by filtration or flotation after the

degradation of starch. But solid protocols need to be addressed in the future research. Except for degradability, the biocompatibility of the starch-based FEs was also concerned especially

for medical applications, which can be confirmed through cytotoxicity tests45,163. The cell viability and the cell proliferation assays of the starch-based hydrogels were checked using

Balb/3T3 fibroblasts cell in Kanaan’s work (Fig. 10d, e)43. The cell viabilities (∼100%) for all starch-containing hydrogels after 48 h direct contacting were around 100%. The cell

proliferation increased approximately 20% for the sample in the presence of starch. These results suggest that the FEs based on starch are degradable in natural environments and

biocompatible in human body, thus may be free of e-waste and find applications in biomedical therapies. CONCLUSIONS AND OUTLOOKS In summary, comparing with traditional flexible materials

such as PET or PI films, starch has significant advantages including low cost, controllable biodegradability, easy processability and acceptable biocompatibility, which make it versatile in

future green electronics. Starch can work as flexible substrates, electrodes, carbon source, dispersants, stabilizers, or modifier of conductive materials. In the most of its applications,

starch appears in the form of thin films or gels. The electronic products may be included but not limited as flexible electrodes, sensors, supercapacitors and nanogenerators. However, there

are still some problems need to be resolved despite abundant recent breakthroughs have been made. Firstly, the mechanical property of the starch films/gels need to be strengthened in some

occasions when the mechanical robustness is required. Some physic and chemical strategies were developed to date. For an example, small molecules such as ethylene glycol was usually used as

plasticizers of the starch film to improve the toughness37. PVA with cross-linkers was added into the starch film to increase the tensile strength and solvent resistance of starch substrate

for FEs33. In addition, the molecular chains of the starch can be grafted with polymers, which may also achieve high mechanical strength. But the chemical modification may also bring other

problem, such as the degradability. The synthesized degradable polymers such as polylactic acid (PLA) or poly (butyleneadipate-co-terephthalate) (PBAT) may be good choices to reinforce the

starch-based materials164,165,166. Secondly, the additional components in starch-based electronics may increase the concerns for their biocompatibility and hinder their biomedical

applications. For instance, the metals, nanowires, or conductive polymers that were used to improve the electrical conductivity. The investigations on the toxicology of such materials in FEs

need to be promoted urgently, in which the biologist should be involved. Some metals and metal alloys have been found to be bioabsorbable and widely used in temporary biomedical

implants167. On the other hand, if the conductive materials were chemical modified or enveloped with biocompatible molecules, the toxicity may be significantly lowered down to the acceptable

level, thus meeting the requirements of diagnose or therapy. Thirdly, the starch is degradable in the nature environment, but the complete degradation of FEs may not be practical. One way

to reach the totally degradation may be the using of organic conductive polymers. Otherwise, the non-degradable materials including metals, graphene, carbon nanotubes need to be recycled in

the future when the starch-base FEs are widely used. Zhou and coworkers168 recycled liquid metal form FEs after dissolving the soft substrate, which supplied a clue for the recycle of the

non-degradable component in the FEs. Fourthly, starch-based electronics can be gradually degraded in water or biological fluid, also implying that they might be impacted by external

environment during working. In general, the life-time of the FEs may be not too long. How to make their reliable life-time be controlled remains an open question. Spiridon et al.169 studied

the degradation behavior of PVA/starch composite film, finding that the enzyme and nanoparticles influenced the degradation rate. Further investigation is highly desired in the future to

explore more methods to manipulate the degradation of starch-based FEs. Last but not the least, the self-powered systems or devices that do not require external power sources, may provide

the possibility for reliable outdoor electronics. Starch-based self-powered electronics can provide independent, continuous power to the FEs. For instance, Sarkar et al.24,26. prepared

triboelectric nanogenerator based on thermoplastic starch39,40. Such products are increasingly favored by more and more applications, but their development is still in the infancy stage.

More portable and sensitive self-powered devices need to be further explored. The work is far from done, however, it is convincing that more and more practical FE products based on starch

will be developed in the future with further efforts putting into this exciting area, since the great potentials have been widely recognized. It is also of the great significance to the

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93, 1884–1890 (2008). Article  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS The authors would like to thank the financial supports from the National Natural Science Foundation

of China (No.21805178), Chinese Postdoctoral Science Foundation (2019M653734), Key Research and Development Program of Shaanxi Province (2021GY-305), Natural Science Foundation of Shaanxi

Provincial Department of Education (20JS062), Foundation for Selected Oversea Chinese Scholar in Shaanxi Province (No. 2017016), and the advanced research fund of SUST (2016GBJ-14). AUTHOR

INFORMATION AUTHORS AND AFFILIATIONS * Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, College of Bioresource Chemical and Materials Engineering,

Shaanxi University of Science & Technology, Xi’an, 710021, P.R. China Huacui Xiang, Zhijian Li & Hanbin Liu * Ningbo Institute of Materials Technology and Engineering, Chinese

Academy of Sciences, Ningbo, 315201, P.R. China Tao Chen * School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an, 710021, P. R. China Hongwei Zhou * Frontiers

Science Center for Flexible Electronics, Xi’an Institute of Flexible Electronics (IFE), Northwestern Polytechnical University, Xi’an, 710072, China Wei Huang Authors * Huacui Xiang View

author publications You can also search for this author inPubMed Google Scholar * Zhijian Li View author publications You can also search for this author inPubMed Google Scholar * Hanbin Liu

View author publications You can also search for this author inPubMed Google Scholar * Tao Chen View author publications You can also search for this author inPubMed Google Scholar *

Hongwei Zhou View author publications You can also search for this author inPubMed Google Scholar * Wei Huang View author publications You can also search for this author inPubMed Google

Scholar CONTRIBUTIONS H. Xiang wrote original draft and collected the data. H. Liu. conceptualized, supervised, and edited the draft. Z. Li, T. Chen, and H. Zhou reviewed the manuscript and

provided scientific comments and suggestions. W. Huang provided overall supervision of the work. CORRESPONDING AUTHORS Correspondence to Hanbin Liu or Wei Huang. ETHICS DECLARATIONS

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