new starch capsules with antistatic, anti-wear and

RESEARCH ARTICLE

New starch capsules with antistatic, anti-wear andsuperlubricity properties

Nannan WANG1,2, Youbin ZHENG1,3, Yange FENG (✉)1,3, Liqiang ZHANG1,2, Min FENG1,2,

Xiaojuan LI1,2, and Daoai WANG (✉)1,3

1 State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,Lanzhou 730000, China

2 Center of Materials Sciences and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China3 Qingdao Center of Resource Chemistry and New Materials, Qingdao 266100, China

© Higher Education Press 2021

ABSTRACT: Adsorption of drug powder is caused by triboelectrification on the surfaceof starch capsule during filling process. Furthermore, high wear rate and poor waterlubricity also hinder the further practical applications of traditional starch capsule. Tosolve these problems, a glycerol-modified starch capsule with perfect anti-triboelec-trification and enhanced lubrication performance was fabricated. Hydrogen bondbetween glycerol and starch molecules could reduce the bound water content on thecapsule surface and thus realizes anti-triboelectrification. By adding glycerol, a three-tierstructure composed of starch-glycerol-water is formed through hydrogen bonding on thesurface of the starch film, which has been proven to be favorable for lubricationperformance. When 5% glycerol is added, the short-circuit current (Isc) of starch-basedtriboelectric nanogenerator (TENG) is reduced by 86%, and the wear volume of the starchfilm is reduced by 89%. Under water lubrication condition, the lubrication performance ofthe starch-glycerol film can reach the super lubricated level with a friction coefficient ofabout 0.005. This work provides a new route to obtain modified starch capsules withimproved anti-triboelectrification property, reduced wear rate and superlubricity property.

KEYWORDS: starch capsules; hydrogen bonds; anti-triboelectrification; anti-wear;superlubricity

Contents

1 Introduction2 Experimental

2.1 Materials2.2 Preparation of glycerol-modified starch film

(g-starch)2.3 Preparation of glycerol-modified starch capsule

(g-starch capsule)

2.4 Fabrication of starch and g-starch-based TENGs2.5 Characterization

2.5.1 Mechanical performance2.5.2 Triboelectric tests2.5.3 Tribological performance

3 Results and discussion3.1 Enhancement in mechanical and disintegrate

property of starch-glycerin film3.2 Enhancement in antistatic property of

starch-glycerin film3.3 Enhancement in tribological property of

starch-glycerin film

Received January 6, 2021; accepted March 11, 2021

E-mails: [email protected] (D.W.), [email protected] (Y.F.)

Front. Mater. Sci. 2021, 15(2): 266–279https://doi.org/10.1007/s11706-021-0555-7

4 ConclusionsAcknowledgementsReferencesSupplementary information

1 Introduction

Starch capsules are one kind of the best replacements forgelatin capsules because they are environmentally benign[1–3], able to degrade naturally [4–7], widely available[8–10], and low priced [11–14]. However, the starch filmcan easily generate triboelectric charges when contactingwith other materials, especially in the high-humidityenvironment, thereby causing problems in the productionof starch film-related fields. For example, dusts and othersmall particles are easily absorbed by the starch film in airdue to the presence of static electricity, thereby increasingthe defective rate [15–19]. Similarly, starch capsules canabsorb powder during the filling process, and drug powdersadsorbed on the starch capsule shell are usually removedby a mechanical brush, which results in waste. Therefore,solving the problem of electrostatic adsorption during thefilling process is a key factor in the application of starchcapsules. Meanwhile, starch capsules show wear scar dueto poor abrasion resistance during shell pulling and cutting,resulting in decreased transparency [20–21]. The poorlubricity of starch capsules also makes them difficult forpatients to swallow.When starch molecules are in contact with and separated

from other polymers or metals, contact electrification isvery easy to appear on the surface of the starch film[22–25]. Since the starch film has strong triboelectricpositivity, the whole starch film is positively chargedduring the contact–separate process. Furthermore, thetriboelectric positivity of starch becomes even strongerwhen the environment humidity increases. This situationoccurs because water molecules in the environment formhydrogen bonds with hydroxyl groups on the surface of thestarch film to fix water molecules, which have greatertriboelectric positivity [26]. As a result, the starch-watermolecule as a whole participates in the contact electrifica-tion. Thus, the contact electrification of starch capsulesunder a high-humidity environment is also worthy ofattention.To eliminate static electricity on the surface of the

polymer, researchers have conducted many studies,including mixing antistatic agents, removing free radicals,

and compounding quaternary ammonium salts. Ma et al.[27] used low temperature plasma (LTP) technology andacrylic treat polyester to reduce the charge density of fabricto achieve anti-static electricity. Baytekin et al. proved thatradical charge interplay allowed for controlling staticelectricity by doping common polymers [28] with smallamounts of radical-scavenging molecules. Kugimoto et al.[29] prepared coating solutions consisting of a quaternaryammonium salt, a urethane acrylate oligomer, a photo-initiator, and a mixture of solvents. The surface resistivityof the coating synthesized by this method was greatlyreduced, indicating excellent antistatic properties. Thesemethods of eliminating static electricity on the surface ofthe polymer are simple to implement and have obviouseffects. However, antistatic additives such as acrylic acidand quaternary ammonium salts are mostly harmful to thehuman body, which is why they cannot be mixed withstarch capsules as a medical dressing. Although theaddition of free radical scavengers such as vitamin E canhave a good antistatic effect, it does not help improve theanti-wear and lubrication property of starch capsules.Therefore, finding a strategy of preparing starch capsuleswith antistatic, anti-wear, and super-low friction ischallenging.We fabricated a new type of starch capsule with

improved antistatic, wear-resistant, and water lubricatingproperties by introducing a certain quality of glycerol.Glycerol is one of the most common used small moleculeplasticizers in the bio-based membrane materials prepara-tion industry owning to its nontoxic and harmless property.When starch is gelatinized with water as a solvent at a hightemperature (80–90 °C), the added small molecule polyol(glycerol) will accelerate the swelling process of the starchmolecules due to its strong polarity and form hydrogenbonds with the hydroxyl groups of the starch molecules onthe starch chains, thereby stably existing on the surface andinside of the starch film. The glycerol molecules on thesurface of the starch film that form hydrogen bonds withstarch molecules occupy part of the hydroxyl groups.Therefore, the content of bound water (water molecules inthe environment that form hydrogen bonds with thehydroxyl groups on the surface of the starch film) on thesurface of the starch film will decrease, which in turnreduces the amount of bound water that participates incontact electrification, thereby reducing triboelectrification.The glycerol molecules inside the starch film will formhydrogen bonds with water molecules entering the starchfilm to fix them, preventing water molecules from

Nannan WANG et al. New starch capsules with antistatic, anti-wear and superlubricity properties 267

migrating to the surface of the starch film and making itimpossible to form hydrogen bonds with hydroxyl groupsof the starch molecules on the surface of the starch film. Inaddition, we choose to use triboelectric nanogenerators(TENGs) to characterize the electrification properties ofstarch films in this article. TENG was first proposed byProf. Zhong Lin Wang et al. in 2012, which is a device thatcollects and converts the charges generated by the frictionof two polymers with different electrical polarity into astable current [30]. Compared with the traditional surfaceelectrostatic test method, using TENG to characterize theelectrification performance of the polymer is moreintuitive. Moreover, the test process is continuous, whichcan more accurately reflect the surface charge of thepolymer [31–32]. As a result, the content of surface boundwater is further reduced. The friction coefficient wasreduced by about 63% after 5% glycerol was addedcompared with the one without glycerol added, and thewear volume was reduced by about 90%. Output currentand voltage of the glycerol-starch based TENG werereduced by 86% and 89%, respectively. Compared with thetraditional starch capsule preparation method, this newstarch capsule preparation strategy has promising potentialapplications for reducing the defective rate in theproduction process and improving lubrication performancein the actual application, especially in high-humidityenvironments.

2 Experimental

2.1 Materials

The starch used in the experiment was Suimama tapiocastarch purchased from Thailand. Glycerol was purchasedfrom Aladdin. Distilled water was made in the laboratory.Polytetrafluoroethylene (PTFE) film (thickness: 0.1 mm),copper tape, and copper wire were purchased at a localsupermarket. Salts (LiCl, MgCl2, Mg(NO3)2, NaCl andKNO3) were purchased from Macklin, CP and used tocontrol the humidity to 15%, 35%, 55%, 75% and 95%.

2.2 Preparation of glycerol-modified starch film (g-starch)

The preparation process of starch films with differentglycerol contents was conducted according to the follow-ing method. First, starch powder (6 g) was mixed with100 mL of distilled water and different amounts ofglycerol, and gelatinized at 90 °C for 1 h. Then, the starch

paste was poured into a flat pan and dried at 50 °C for 4 h toform a smooth starch film. The starch film was peeled offand cut into a 4 cm � 4 cm size film with a thickness of0.1 mm.

2.3 Preparation of glycerol-modified starch capsule

(g-starch capsule)

The preparation process of starch capsules is as follows:mix 15 g tapioca starch, 0.3 g carrageenan and 0.2 g KClwith 100 mL distilled water, and then mechanically stir at atemperature of 90 °C for 1 h, and the mechanical stirringspeed is 1000 r$min–1. Then, pour the gelatinized starchglue into a beaker and keep it at 50 °C for 10 min to makethe bubbles disappear. Then, the capsule mold was dippedin the glue and dried at 50 °C for 1 h. Finally, pull out theshell and cut to get the starch capsule shell.

2.4 Fabrication of starch and g-starch-based TENGs

The square starch film was affixed with a conductivecopper electrode and a copper wire on the backside toobtain the starch film friction electrode. The PTFE film wascut into a size of 4 cm � 4 cm, and a copper electrode wasattached to the back with a copper wire taken out to preparea PTFE film electrode. The starch film electrode and thePTFE electrode were combined into a TENG.

2.5 Characterization

An interferometric noncontact surface profilometer (Micro-XAM-3D) was employed to observe the wear pattern andcalculate wear volume. Fourier transform infrared spectro-scopy (FTIR) results of starch-based composite film withdifferent glycerin contents were recorded on an FTIRspectrometer (Nicolet 6700, Thermo Fisher Scientific Inc.)from 4000 to 400 cm–1 at a resolution of 4 cm–1. Theultraviolet–visible (UV–Vis) transmittance of starch-basedcomposite film with different glycerol contents wasmeasured using a spectrophotometer with an integratingsphere (U-4100 HITACHI).

2.5.1 Mechanical performance

The mechanical properties of the starch film were measuredby using a microcomputer-controlled electronic universaltesting machine. The starch film to be tested was cut into40 mm � 15 mm rectangular strips and placed in adesiccator with a relative humidity (RH) of 50% for 24 h.

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During measurement, the initial clamping distance was20 mm. The stretching rate was 10 mm$min–1, and the filmthickness was measured with a micrometer. The tensilestrength and elongation at the break of the starch film wererecorded.

2.5.2 Triboelectric tests

In measuring the triboelectrification output performance, acommercial linear mechanical motor was used to drive thestarch film-based TENGs with a contact–separation mode.The output voltage was measured by using NationalInstruments PCIe-6259 DAQ card with a load resistance of100 MΩ, while the short-circuit current (Isc) was measuredby using an SR570 low-noise current amplifier (StanfordResearch System). The output of the starch film-basedTENGs under different humidity levels was tested by firstplacing the starch film triboelectrodes in a sealed box for 24h, with the humidity adjusted to 15%, 35%, 55%, 75% and95% by using different amounts of saturated salt solution.Then, the output performance in the same humidity controlbox was tested.

2.5.3 Tribological performance

Tribological tests were performed on an oscillating frictionand wear tester (Optimol SRV-IV) under the back-and-forth test mode with starch film (50 mm � 10 mm) andsteel ball (φ = 6 mm) as friction pairs. First, the dry friction

coefficient of starch film with different glycerol contentswas tested. Then, a few drops of distilled water weredropped on the surface of the starch film to test the wetfriction coefficient. The testing amplitude was 1 mm, andthe duration was 2 min. All tests were performed at roomtemperature and the RH of 50%. The friction testingmachine was covered with a glass cover to test the frictioncoefficient under different humidity levels. In all frictiontests, the friction coefficient was recorded in successionthree times. After the friction test, the wear volumes of thedisks were estimated by using MicroXAM-3D.

3 Results and discussion

3.1 Enhancement in mechanical and disintegrate property

of starch-glycerin film

Figure 1 shows the preparation process and structuralcharacterization of starch-glycerol film. As revealed inFig. 1(a), starch powder, distilled water and glycerol weremixed in a round-bottom flask then placed in a constant-temperature water bath under mechanical agitation togelatinize the starch. The gelatinized starch paste waspoured into a flat-bottomed tray to obtain a flat and smoothstarch-glycerol film. Detailed fabrication process is listedin the experimental section. The glycerol molecule is asmall carbon chain polyhydroxy compound with smallsteric hindrance. Both starch molecules and glycerol

Fig. 1 (a) The preparation process of the starch-glycerol (GC) film. (b) Schematic of the formation of hydrogen bond between starchmolecule and glycerol molecule. (c) Comparison of infrared spectra of the blank starch film and the one with 5% glycerol.


molecules contain hydroxyl groups. Therefore, the glycerolmolecules will bind to the starch molecules in the form ofhydrogen bonds, as shown in Fig. 1(c).The infrared spectrum was tested to prove that the starch-

glycerol film was successfully prepared as shown inFig. 1(b). The blank starch film showed a hydroxylabsorption peak at 3336.1 cm–1, and the hydroxylabsorption peak of the starch-glycerol film with 5%glycerol appeared at 3404.3 cm–1, which indicates thatstarch-glycerol film was successfully prepared and theformation of hydrogen bonds caused the red shift of thehydroxyl absorption peak of the starch film [33–35].The infrared spectra of blank starch film and the one with

5% glycerol before and after complete drying at 90 °C for24 h were tested to verify the stability of the hydrogen bondformed by glycerol and starch film. As shown in Fig. 2(a),before drying, the hydroxyl absorption peak of the starchmolecule of the starch-glycerol film moved from 3336.1 to3404.3 cm–1 compared with the blank starch film, with

68.2 cm–1 red shift. Similarly, after drying, the hydroxyabsorption peak of the starch molecule of the 5% glycerol-added starch film moved from 3323.4 to 3367.5 cm–1 with43.2 cm–1 red shift compared with the starch film withoutadded glycerol. Interestingly, the hydroxyl absorptionpeaks of the infrared spectrum of the starch molecules ofthe blank starch film before and after drying appeared atdifferent positions (3336.1 and 3323.4 cm–1). This ismainly due to the starch molecules in the undried starchfilm form hydrogen bonds with the water molecules in theenvironment, causing the absorption peak of hydroxylgroups of the starch molecules to exhibit a red shift.Furthermore, the red shift wavenumber of the hydroxylabsorption peak of the starch molecule before drying islarger than that after drying. The water absorption of thestarch molecules and the glycerol molecules lead to thehydroxyl group of the starch molecule forming hydrogenbonds with the water molecules, thereby causing anincrease in red shift. The hydroxyl absorption peak of the

Fig. 2 (a) Comparison of infrared spectra between the blank starch film and the one with 5% glycerol before and after completely dryingat 90 °C. (b)(c) UV-absorption spectra and mechanical properties of the blank starch film and the ones with 1% and 5% glycerol.(d) Friability test of the blank starch capsule shell and the one with 5% glycerol. (e)(f)(g) Compression–recovery experiment of the blankstarch capsule and the one with 5% glycerol.

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starch molecules of the starch-glycerol film after completedrying showed a red shift, which is different with the blankstarch film. Therefore, high temperatures cannot break thehydrogen bond between glycerol and starch molecules,indicating that the starch-glycerol film is very stable underroom temperature.After glycerol was added, the transparency of the starch

film improved, as shown in Figs. S1(a)–S1(c). Glycerol isrich in hydroxyl groups, which can be combined with watermolecules in the form of hydrogen bonds during starchgelatinization. Therefore, the starch granule can betterexpand its starch chains during the water swell stage,reducing the presence of ungelatinized starch granules andfurther increasing the transparency of the starch film. TheUV-absorption spectra of starch films with differentglycerol contents were tested to further characterize thetransparency of starch films as shown in Fig. 2(b). Thetransparency of the starch film rises with the increase of theglycerol content, which is consistent with the SEMphotographs that the starch film with 5% glycerol issmoother than the blank starch film (Figs. S1(d) and S1(e)).In addition, adding 5% glycerin to the starch film can

significantly enhance its mechanical properties. As shownin Fig. 2(c), as the glycerol content increases, the strengthof the starch film decreases and the elongation at the breakincreases, thereby indicating that the flexibility of thestarch film increases as the glycerol content increases.Figure 2(d) and Video S1 show the friability test of starchcapsules without glycerol and starch capsules with 5%glycerol. Starch capsules with different glycerol contentswere placed in a desiccator under 50% humidity for 24 h.Then, the capsules were placed on a wooden board with athickness of 2 cm, and afterwards, a 20 g PTFE weight hitthe capsule freely through a glass tube with a length of20 cm, and whether the capsule was broken or not wasobserved. The starch capsule without glycerol was crushedby weight, whereas the one with 5% glycerol was not,thereby indicating that the flexibility of the starch capsuleincreases and the friability decreases after glycerol wasadded, which is consistent with the above experimentalresult. The compression–recovery experiment of blankstarch capsule and the one with 5% glycerol wasconducted, as shown in Figs. 2(e)–2(g). A metal weightwith a mass of 500 g was pressed on the starch capsules todeform them. Then, the shape of the capsules was restoredby hand, and the indentation was observed. Compared withstarch capsules with 5% glycerol, the blank starch capsuleshave more obvious indentation, indicating that the addition

of glycerol can increase the flexibility of starch capsulesand avoid the appearance of indentation caused byextrusion during storage or transportation.Figures S2(a)–S2(d) show the disintegration experiment

of capsules. Starch capsules often become stuck in thethroat when swallowed, causing discomfort, because oftheir poor water lubrication performance. An outstandinglubrication performance will allow starch capsules to reachthe stomach faster and then disintegrate, as shown inFig. S2(a). Figures S2(b)–S2(d) and Video S2 show asimulation of starch capsule disintegration in the stomach.Capsule shells with different glycerol contents were filledwith talc powder and placed in two hanging baskets withbaffles pressing on them. The hanging baskets were placedin a 1000 mL beaker containing 700 mL of distilled water,and the beaker was placed in a disintegrator at a constanttemperature of 37 °C. The motor was driven to make thehanging baskets move up and down to simulate thesqueezing action in the stomach, and the leakage of talcfrom the starch capsules was observed. The talc powder inthe starch capsule with 5% glycerol started to leak at 2 min,while the talc powder in the blank starch capsule began toleak 1 min later, showing the excellent water solubility ofstarch capsule containing glycerol. Moreover, starch glueremained on the baffle pressed against the blank starchcapsule shell after disintegration, while no starch glueremained on the baffle pressed on the starch capsule shellwith 5% glycerol, indicating that the presence of glycerolcan accelerate the dissolution process. Therefore, starchcapsules could disintegrate in the stomach more quicklyand accelerate the release of drugs.In order to test the release of the model drug in the

capsule shell, the dissolution profile of the capsule shellfilled with the drug was tested, as shown in Fig. S3. Putticagrelor powder into 3 gelatin capsule shells and 3 starchcapsule shells respectively, then place the capsules in thehanging basket, and test the dissolution amount of thepowder in the ZRS-8G dissolution tester. The dissolutionmedium is 0.2% Tween-80 and 80% aqueous solution.Take samples every 10, 15, 20, 30, 45 and 60 min to test theUV absorbance of the solution, and the maximumabsorption wavelength is 300 nm. The dissolution amountof the drug is calculated by the following equation:

Cotent=% ¼A

E1%1 cm

� 1

100� V � D

m� 100 (1)

where A is the absorbance of the test solution; E1%1 cm is the


percentage absorption coefficient of the tested product; V isthe volume of the test product prepared in the first time(mL); D is dilution ratio of the tested product; and m is thesample size of the test. The results show that the dissolutionrate of the drugs filled in the two capsule shells is around95% within 60 min, but in the range of 10–50 min, thedissolution rate of the drugs filled in the starch capsule shellis significantly higher than that of the gelatin capsule.Moreover, the basic performance and stability of starchcapsules were tested, as shown in Tables S1–S4. Theresults show that the various indicators of starch capsulesand gelatin capsules meet the requirements, but thedisintegration speed of starch capsules is significantlyfaster than that of gelatin capsules.

3.2 Enhancement in antistatic property of starch-glycerin

film

In order to determine the position of starch films withdifferent glycerin content in the triboelectric sequence, thecurrent peak shape and value of TENG composed ofstarch-nylon was tested. During the test, the nylon

electrode was used as the positive electrode of TENG,and the starch film was used as the negative electrode.Therefore, if the electric positiveness of nylon is larger thanthat of starch, the current peak shape will be up-down peak.And the larger the current value of TENG, the larger theelectronegativity of the glycerol-starch film. During thetest, the humidity was stable at 35% RH. After each test,the surface of the nylon film was blown by an ion blowerfor 20 min to ensure that there is no charge accumulationon the surface. Figure 3(a) shows the triboelectric sequenceof starch films with different glycerol contents. The starch-based electrodes with different glycerol contents werecontacted-separated from the nylon-11-based electrode,and then the values of Isc were collected. The shape oftriboelectricity signal when the starch film is rubbed againstnylon 11 is the up-down peak, and a higher glycerolcontent corresponds to a larger Isc value, indicating that thetriboelectric positivity of the starch film decreases and thetriboelectirc positivity increases as the glycerol contentincreases. Therefore, the triboelectric sequence of starchfilms with different glycerol contents and nylon 11 film isnylon 11> starch film> starch film with 1% GC> starch

Fig. 3 (a) Triboelectric sequence of starch films with different glycerol contents. (b) Short-circuit current (Isc), (c) output voltage (Uo),(d) integrated charge density, and (e) changes in current with voltage of blank starch-based TENG and the ones with 1% and 5% glycerol.(f) Stability test of blank starch-based TENG and the one with 5% glycerol at 35% RH.

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film with 5% GC. Then, Isc and Uo of TENG composed ofstarch-based electrodes with different glycerol contents andPTFE-based electrode were tested. To make the experi-mental conditions fit the actual working conditions moreaccurate, the environment humidity during the test iscontrolled at 60%. As shown in Figs. 3(b) and 3(c), whenglycerol was not added, Isc and Uo of starch-based TENGwere 8.8 μA and 236.2 V, respectively. After the addition of1% glycerol, Isc and Uo decreased to 4.3 μA and 137.6 V,respectively. The starch-based TENG with 5% glycerolcontent had Isc and Uo of 1.4 μA and 26.7 V, which were84% and 88% lower than those of the blank starch-basedTENG, respectively. As a result, the addition of glycerolplays an important role in reducing the triboelectricity ofstarch film. Then, the short-circuit current is integrated toobtain the integrated charge, as shown in Fig. 3(d), whichshows the same tendency with Isc. In order to make therelationship between current and voltage with the addedglycerin amount clearer, a histogram is drawn as shown inFig. 3(e). According to the image, both Isc and Uo valuesshow a decrease trend with the increase of the glycerinamount. The stability experiments of the starch-basedTENG and the one with 5% glycerol are shown in Fig. 3(f).These two TENGs were tested for 10000 cycles at 60%RH, and the values of Isc are both stable, indicating that themethod of glycerol addition to achieve anti-triboelectrifica-tion is stable and reliable.The characteristics of triboelectric charging of the starch

film are reflected not only in the low-humidity environment(humidity less than 50%) but also in a high-humidityenvironment with humidity greater than 50%, in which thestarch film is more likely to be triboelectrically charged[26]. Therefore, the study of anti-triboelectrification in ahigh-humidity environment is also critical for starch film.Isc and Uo values of the starch-based TENG and the starch-glycerol-based TENG at different humidity levels weretested to verify that the addition of glycerol can reduce thetriboelectric charge generate on starch film. As shown inFigs. 4(a) and 4(b), when the humidity is constant at 15%,the Isc value of the starch-based TENG is 6.2 μA, while Iscof the starch-glycerol-based TENGwith 5% added glycerolis 5.1 μA. As the content of glycerol increases, the Isc valueof the starch-based TENG increases, whereas the Isc valueof the starch-glycerol-based TENG decreases. When thehumidity reaches 95%, Isc of the starch-based TENGincreases to 15.8 μA, while Isc of the starch-glycerol-basedTENG decreases to 1.6 μA. Similarly, Uo of the starch-based TENG increases with the rise in humidity, while Uo

of the starch-glycerol-based TENG decreases with theincrease in humidity. Interestingly, Isc of the starch-basedTENG is approximately the same as that of the starch-glycerol-based TENG at 15% humidity, while the differ-ence between them increases when the humidity increases.This finding indicates that the bound water content on thesurface of the starch film is the key to determine theelectrical output of the starch-based TENG. Subsequently,Isc values of the starch-based TENG and the starch-glycerol-based TENG under dynamic humidity weretested. As shown in Figs. 4(c) and 4(d), in a closed boxwith continuous water vapor, starch-based electrodes withdifferent glycerol contents are driven to contact andseparate with the PTFE electrode alternatively, and thereal-time Isc was collected at the same time. The resultsshow that as the humidity increased from 35% to 60%, thereal-time Isc of the starch-based TENG continued toincrease, while the starch-glycerol-based TENG showeda completely opposite trend, which is consistent with theresults above. The change trend of Isc under dynamichumidity indicates that the starch-glycerol based TENGlost the performance of high humidity resistance but gainantistatic property, especially under high humidity.Since the hydroxyl groups on the surface of the starch

film will form hydrogen bonds with water molecules in theenvironment to fix water molecules participating in thetriboelectrification, the output of the starch-based TENGincreases with humidity [26]. So, the amount of boundwater on the starch film surface will affect the electricaloutput value of the starch-based TENG, in other words,reducing the bound water content of the starch film surfacewill effectively reduce the triboelectric charge of the starchfilm. The reason why the addition of glycerin can reducethe triboelectricity of the starch-based TENG is that whenstarch and glycerol were gelatinized together, hydrogenbonds could form between the glycerol and starchmolecules and exist stably, resulting in the even distribu-tion of glycerol molecules on the surface and inside thestarch film. Due to the resistance value of glycerol is muchsmaller than that of the starch film (Fig. S4), the presence ofglycerol molecules on the surface will reduce the surfaceresistivity of the starch film (Fig. S5), which proves thatthere are glycerol molecules bound to starch molecules inthe form of hydrogen bonds on the surface of the starchfilm. Water molecules in the environment will pass throughpores on the surface of the starch film and enter the interior,forming hydrogen bonds with glycerol molecules inside, asshown in Fig. 4(e). These water molecules captured by


glycerol will exist in the form of bound water inside thestarch film, which makes them unable to move freely to thesurface to form hydrogen bonds with surface hydroxyl,thereby reducing the bond water content on the surface. Itshould be noted that in a high-humidity environment with a

high content of water molecules, the internal glycerinmolecules play little role, so it is a secondary factoraffecting electrical output in a high humidity environment.However, this influencing factor cannot be ignored in a lowhumidity environment.

Fig. 4 Changes of (a) Isc and (b)Uo for the blank starch-based TENG and the one with 5% glycerol with humidity. Real-time changes ofIsc for (c) the blank starch-based TENG and (d) the one with 5% glycerol with humidity. Schematics of (e) hydrogen bonds betweenglycerol inside the starch film and water molecules, (f) hydroxyl sites on the surface of the starch film occupied by glycerol, and(g) glycerol molecules on the surface of the starch film forming hydrogen bonds with water molecules in the environment.(h)(i)(j) Comparison of powder adsorption of starch capsules with different glycerol contents.

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Another reason for the decrease in the bound watercontent of the starch film surface is that the glycerolmolecules on the surface of the starch film will formhydrogen bonds with hydroxyl groups on the surface tooccupy the site of hydroxyl groups. As a result, the watermolecules in the environment cannot be fixed by forminghydrogen bonds with the hydroxyl groups on the surface,thereby further reducing the surface bound water content.The schematic of hydroxyl sites on the surface beingoccupied by glycerol molecules is shown in Fig. 4(f). Sincethe hydroxyl groups on the surface of the starch film areoccupied by glycerin molecules, the water molecules in theair that could have formed hydrogen bonds with the surfacehydroxyl groups will lose the priority of binding to thesurface hydroxyl groups, which will result in a reduction inthe surface bound water content. These water moleculesthat cannot form hydrogen bonds with the hydroxyl groupson the starch surface will bind to the glycerol moleculesfixed on the surface, and they will not be able to form anintegral part with the starch molecules to participate intriboelectrification, as shown in Fig. 4(g). Therefore, thiswater film layer should be considered as free water in theenvironment, which will accelerate the dissipation oftriboelectric charges to reduce triboelectricity. As shownin Fig. S6(a), the water absorption rate of glycerol is farfaster than that of the starch film (the water absorption rateof glycerol is about 12 times that of starch film), therefore,water molecules in the environment will first be adsorbedby glycerol, which will cause the long-term existence of thewater film on the surface of the starch-glycerol film. As theenvironmental humidity increases, the glycerol moleculeson the surface of the starch film will adsorb more watermolecules, thereby increasing the area of the water film andin turn accelerating the dissipation of triboelectric charge.The increase of the environmental humidity will reduce theelectrical output of the starch-glycerol-based TENG toachieve anti-triboelectrification performance under highhumidity. Meanwhile, the water film will also lead to anincrease in the dissipation of triboelectricity. Thus, theincrease in the electrical output of the starch-based TENGcaused by the bound water on the starch film surface willnot be dominant and the competition between the increaseand the dissipation in triboelectric charges caused by themfinally results in a decrease trend of the triboelectric outputof the starch-glycerol film.Figures 4(h)–4(j) and Video S3 show a comparison of

the powder adsorption of starch capsules with differentglycerol contents. The starch capsules were rubbed with

PTFE in the same way to make them charged and thenmoved slowly towards the polyethylene terephthalate(PET) powder for adsorption. With the addition of glycerol,the capsule absorbed less PET powder, thus proving thatthe addition of glycerol can effectively achieve anti-triboelectrification.

3.3 Enhancement in tribological property of starch-

glycerin film

In addition to the perfect anti-triboelectrification perfor-mance of the starch-glycerol film, the tribological proper-ties also improved. As shown in Fig. 5(a), the dry frictioncoefficient of the blank starch film is 0.32, while those ofthe starch film with 1% and 5% glycerol are 0.2 and 0.13,respectively. That is, as the glycerol content increases, thedry friction coefficient of the starch film graduallydecreases. Figures 5(b), 5(d) and 5(e) compare wearvolumes and three dimensional (3D) topographies of wearscars on the starch film with different glycerol contents. Aninteresting detail is that the addition of only 1% glycerolreduced the wear volume by more than 76% compared withthat of the blank starch film, whereas around 89% reductionin wear volume was achieved by adding 5% glycerol. Thus,the starch-glycerol film exhibited better lubrication perfor-mance than that of the blank starch film. The 3Dtopographies given in Figs. 5(d)–5(f) show that the wearscar became smaller and shallower after glycerol wasadded. In addition, the friction coefficients of starch filmswith different glycerol contents in the presence of waterwere tested, as shown in Fig. 5(c). Similarly, as the glycerolcontent increases, the friction coefficient of the starch filmdecreases, and when the amount of glycerol added is 5%,the friction coefficient of the starch-glycerol film is reducedto 0.005, which reached the super-lubricated level. Starchcapsules have poor water lubricity, which is why they oftenbecome stuck in one’s throat and cause discomfort when aperson is taking medicine. Starch capsules that reach thesuper-lubricated level under water lubrication will greatlycompensate for this deficiency and shorten the time forstarch capsules to reach the stomach from the oral cavity toimprove the dissolution efficiency of a drug. The bondwater film above the glycerol layer will cause a significantreduction in the friction coefficient due to the lubricity ofwater. Another reason for the reduction in the frictioncoefficient of starch-glycerol films is that glycerol itself hasoutstanding lubricity property. As shown in Fig. S6(b), thefriction coefficient of the starch film is reduced by 90% in


the presence of glycerol compared with that withoutglycerol. Thus, these two reasons will result in areduction in the dry friction coefficient of the starch-glycerol film.

4 Conclusions

In summary, we demonstrated a new type of starchcapsules with improved antistatic, wear-resistant, andwater lubricating properties by introducing a certain qualityof glycerol. The glycerol molecules inside and on thesurface of the starch capsule can form hydrogen bonds withwater molecules in the environment and occupy thehydroxyl sites on the surface to reduce the bound watercontent, thereby reducing the triboelectric charge. Further-more, bound water film on glycerol molecules caneffectively reduce the friction coefficient and acceleratedissipation of triboelectric charges in a high-humidityenvironment. When 5% glycerol was added, dry frictioncoefficient and wear volume were reduced by 60% and89%, and friction coefficient under water lubricationreached the super-lubricated level with a friction coefficientof about 0.005. The short-circuit current and the outputvoltage decreased 10 and 12 times in 95% humidityenvironment, respectively. This work provides a new wayto obtain starch capsules with anti-static, wear resistanceand super-low friction properties, which also provideguidance for improving antistatic property of ones that are

easy to become triboelectrically charged under high-humidity environment.

Acknowledgements Thanks for the financial support of the Program forTaishan Scholars of Shandong Province (No. ts20190965), the National KeyResearch and Development Program of China (2020YFF0304600), theNational Natural Science Foundation of China (Grant No. 51905518), andthe Innovation Leading Talents Program of Qingdao (19-3-2-23-zhc) inChina.

References

[1] Oladzadabbasabadi N, Ebadi S, Mohammadi Nafchi A, et al.

Functional properties of dually modified sago starch/κ-carragee-

nan films: An alternative to gelatin in pharmaceutical capsules.

Carbohydrate Polymers, 2017, 160: 43–51

[2] Misic Z, Muffler K, Sydow G, et al. Novel starch-based PVA

thermoplastic capsules for hydrophilic lipid-based formulations.

Journal of Pharmaceutical Sciences, 2012, 101(12): 4516–4528

[3] Watts P, Smith A. TARGITTM technology: Coated starch capsules

for site-specific drug delivery into the lower gastrointestinal tract.

Expert Opinion on Drug Delivery, 2005, 2(1): 159–167

[4] Vilivalam V D, Illum L, Iqbal K. Starch capsules: An alternative

system for oral drug delivery. Pharmaceutical Science &

Technology Today, 2003, 3(2): 64–69

[5] Jankowski T, Zielinska M, Wysakowska A. Encapsulation of

lactic acid bacteria with alginate/starch capsules. Biotechnology

Techniques, 1997, 11(1): 31–34

[6] Fakharian M H, Tamimi N, Abbaspour H, et al. Effects of κ-

carrageenan on rheological properties of dually modified sago

Fig. 5 Comparison of (a) friction coefficient, (b) wear volume, and (c) friction coefficient under water lubrication. (d)(e)(f) 3Dtopographies of wear scars on the starch film with different glycerol contents. Applied load: 2 N; frequency: 5 Hz.

276 Front. Mater. Sci. 2021, 15(2): 266–279

starch: Towards finding gelatin alternative for hard capsules.

Carbohydrate Polymers, 2015, 132: 156–163

[7] Bae H J, Cha D S, Whiteside W S, et al. Film and pharmaceutical

hard capsule formation properties of mungbean, waterchestnut,

and sweet potato starches. Food Chemistry, 2008, 106(1): 96–105

[8] Zhang N, Liu H, Yu L, et al. Developing gelatin-starch blends for

use as capsule materials. Carbohydrate Polymers, 2013, 92(1):

455–461

[9] Gullapalli R P, Mazzitelli C L. Gelatin and non-gelatin capsule

dosage forms. Journal of Pharmaceutical Sciences, 2017, 106(6):

1453–1465

[10] Liu X X, Wang Y F, Zhang N Z, et al. Morphology and phase

composition of gelatin-starch blends. Chinese Journal of Polymer

Science, 2014, 32(1): 108–114

[11] Kenyon C J, Cole E T, Wilding I R. The effect of food on the in-

vivo behavior of enteric-coated starch capsules. International

Journal of Pharmaceutics, 1994, 122(3): 207–213

[12] Baier G, Musyanovych A, Dass M, et al. Cross-linked starch

capsules containing dsDNA prepared in inverse miniemulsion as

“nanoreactors” for polymerase chain reaction. Biomacromole-

cules, 2010, 11(4): 960–968

[13] Zhang L, Wang Y, Liu H, et al. Developing hydroxypropyl

methylcellulose/hydroxypropyl starch blends for use as capsule

materials. Carbohydrate Polymers, 2013, 98(1): 73–79

[14] Gohil U C, Podczeck F, Turnbull N. Investigations into the use of

pregelatinised starch to develop powder-filled hard capsules.

International Journal of Pharmaceutics, 2004, 285(1–2): 51–63

[15] Shukur M F, Ibrahim F M, Majid N A, et al. Electrical analysis of

amorphous corn starch-based polymer electrolyte membranes

doped with LiI. Physica Scripta, 2013, 88(2): 025601

[16] Bin-Dahman O A, Rahaman M, Khastgir D, et al. Electrical and

dielectric properties of poly(vinyl alcohol)/starch/graphene nano-

composites. Canadian Journal of Chemical Engineering, 2018,

96(4): 903–911

[17] Shukur M F, Kadir M F Z. Electrical and transport properties of

NH4Br-doped cornstarch-based solid biopolymer electrolyte.

Ionics, 2015, 21(1): 111–124

[18] Shukur M F, Ithnin R, Kadir M F Z. Electrical characterization of

corn starch-LiOAc electrolytes and application in electrochemical

double layer capacitor. Electrochimica Acta, 2014, 136: 204–216

[19] Hazrol M D, Sapuan S M, Ilyas R A, et al. Electrical properties of

sugar palm nanocrystalline cellulose reinforced sugar palm starch

nanocomposites. Polimery, 2020, 65(5): 363–370

[20] WuM,Wang Y,WangM, et al. Effect of SiO2 nanoparticles on the

wear resistance of starch films. Fibres & Textiles in Eastern

Europe, 2008, 16(4): 96–99

[21] Biresaw G, Kenar J A, Kurth T L, et al. Investigation of the

mechanism of lubrication in starch-oil composite dry film

lubricants. Lubrication Science, 2007, 19(1): 41–55

[22] Dias A B, Müller C M O, Larotonda F D S, et al. Biodegradable

films based on rice starch and rice flour. Journal of Cereal Science,

2010, 51(2): 213–219

[23] Calado V M A, Ramos A. Applications of starch nanocrystal-

based blends, composites and nanocomposites. In: Visakh P M,

Yu L, eds. Starch-based Blends, Composites and Nanocomposites.

RSC Green Chemistry Series, 2016, 37: 143–216

[24] Wang W, Yang H, Cui M. Effects of additives on the properties of

starch. In: Visakh P M, Yu L, eds. Starch-based Blends,

Composites and Nanocomposites. RSC Green Chemistry Series,

2016, 37: 403–432

[25] Donnadio A, Pica M, Taddei M, et al. Design and synthesis of

plasticizing fillers based on zirconium phosphonates for glycerol-

free composite starch films. Journal of Materials Chemistry, 2012,

22(11): 5098–5106

[26] Wang N, Zheng Y, Feng Y, et al. Biofilm material based

triboelectric nanogenerator with high output performance in 95%

humidity environment. Nano Energy, 2020, 77: 105088

[27] Ma C, Zhao S, Huang G. Anti-static charge character of the plasma

treated polyester filter fabric. Journal of Electrostatics, 2010,

68(2): 111–115

[28] Baytekin H T, Baytekin B, Hermans T M, et al. Control of surface

charges by radicals as a principle of antistatic polymers protecting

electronic circuitry. Science, 2013, 341(6152): 1368–1371

[29] Kugimoto Y, Wakabayashi A, Dobashi T, et al. Preparation and

characterization of composite coatings containing a quaternary

ammonium salt as an anti-static agent. Progress in Organic

Coatings, 2016, 92: 80–84

[30] Fan F R, Tian Z Q, Wang Z L. Flexible triboelectric generator.

Nano Energy, 2012, 1(2): 328–334

[31] Jin Y, Xu W, Zhang H, et al. Complete prevention of contact

electrification by molecular engineering. Matter, 2021, 4(1): 290–

301

[32] Li X, Zhang L, Feng Y, et al. Solid–liquid triboelectrification

control and antistatic materials design based on interface

wettability control. Advanced Functional Materials, 2019,

29(35): 1903587

[33] Joseph J, Jemmis E D. Red-, blue-, or no-shift in hydrogen bonds:

A unified explanation. Journal of the American Chemical Society,

2007, 129(15): 4620–4632

[34] Kannan P P, Karthick N K, Mahendraprabu A, et al. Red/blue

shifting hydrogen bonds in acetonitrile-dimethyl sulphoxide

solutions: FTIR and theoretical studies. Journal of Molecular

Structure, 2017, 1139: 196–201

[35] Ryu I S, Liu X, Jin Y, et al. Stoichiometric analysis of competing

intermolecular hydrogen bonds using infrared spectroscopy. RSC

Advances, 2018, 8(42): 23481–23488


Supplementary information

Fig. S1 (a)(b)(c) Transparency photos of starch films withdifferent glycerol contents. SEM images of (d) the blank starchfilm and (e) the one with 5% glycerol content.

Fig. S2 Disintegration experiment of capsules with differentglycerol contents: (a) schematic diagram of capsule administra-tion process; (b) disintegration demonstration experiment ofblank starch capsule and glycerol-starch capsule; comparison ofresidue on baffle between (c) blank starch capsule and(d) glycerol-starch capsule after disaggregation.

Fig. S3 Dissolution curves of gelatin and starch capsules filledwith drugs.

Fig. S4 Surface resistivities of starch films with different glycerolcontents.

Fig. S5 Resistance values of (a) glycerol and (b) blank starch film.

Fig. S6 (a) Comparison of water absorptions of starch film andglycerol at 95% RH. (b) Comparison of friction coefficientsbetween the starch film coated with glycerol and the one withoutglycerol.

278 Front. Mater. Sci. 2021, 15(2): 266–279

Video S1 Comparison of friability between the blank starch capsule and

the one with 5% glycerol. During the test, the environment humidity is

always maintained at 50%. This video can be found at https://doi.org/

10.1007/s11706-021-0555-7.

Video S2 Comparison of disintegration performance between the blank

starch capsule and the one with 5% glycerol. This video can be found at

https://doi.org/10.1007/s11706-021-0555-7.

Video S3 Comparison of adsorption property between the blank starch

capsule and the one with 5% glycerol after contact with electricity. This

video can be found at https://doi.org/10.1007/s11706-021-0555-7.

Table S1 Performances comparison of gelatin and starch capsulesSP CT DOT FB LOD/% DT/min

Gelatin capsule Pale yellow transparency, elastic 0/10 0/50 12.82 8–10

Starch capsule Colorless and transparent, elastic 0/10 0/50 10.13 3–5

Notes: SP, sample; CT, character; DOT, degree of tightness; FB, friability; LOD, loss on drying; DT, disintegration time.

Table S2 Effect of high temperature on properties of gelatin and starch capsulesSP Day 0 Day 5 Day 10

CT DOT FB LOD/% DT/min CT DOT FB LOD/% DT/min CT DOT FB LOD/% DT/min

A1 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 8.79 12–15 ▲ 0/10 0/50 7.11 11–14

A2 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 11.57 9–13 ▲ 0/10 0/50 7.80 10–12

B1 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 7.51 4–7 ▼ 0/10 0/50 6.39 5–7

B2 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 8.95 5–8 ▼ 0/10 0/50 6.86 6–8

Notes: ▲ — pale yellow transparency, elastic; ▼ — colorless and transparent, elastic; SP — sample; CT — character; DOT — degree of tightness; FB — friability;LOD — loss on drying; DT — disintegration time.

Table S3 Effect of high humidity on properties of gelatin and starch capsulesSP Day 0 Day 5 Day 10


A1 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 22.36 6–10 ▲ 0/10 0/50 21.57 8–12

A2 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 19.65 8–12 ▲ 0/10 0/50 19.62 9–11

B1 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 15.03 3–6 ▼ 0/10 0/50 15.48 3–5

B2 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 11.13 2–5 ▼ 0/10 0/50 12.05 4–6


Table S4 Effect of strong light on properties of gelatin and starch capsulesSP Day 0 Day 5 Day 10


A1 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 9.96 12–14 ▲ 0/10 0/50 11.71 12–15

A2 ▲ 0/10 0/50 12.82 8–10 ▲ 0/10 0/50 11.10 10–13 ▲ 0/10 0/50 12.08 11–13

B1 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 6.43 4–6 ▼ 0/10 0/50 6.02 3–5

B2 ▼ 0/10 0/50 10.13 3–5 ▼ 0/10 0/50 7.27 5–7 ▼ 0/10 0/50 7.30 4–6



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