polymers
ArticleEnhanced Marine Biodegradation of Polycaprolactone throughIncorporation of Mucus Bubble Powder from Violet Sea Snail as
Protein Fillers
Koh Yoshida 1 , Sayaka Teramoto 2 , Jin Gong 1, * , Yutaka Kobayashi 1
1

2*

Citation: Yoshida, K.; Teramoto, S.;
Gong, J.; Kobayashi, Y.; Ito, H.
Enhanced Marine Biodegradation of
Polycaprolactone throughIncorporation of Mucus Bubble
Powder from Violet Sea Snail as
Protein Fillers. Polymers 2024, 16, 1830.
https://doi.org/10.3390/
polym16131830
Academic Editor: Beom Soo Kim
Received: 27 May 2024
Revised: 19 June 2024
Accepted: 25 June 2024
Published: 27 June 2024

Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.

and Hiroshi Ito 1, *Department of Organic Materials Science, Graduate School of Organic Materials Science, Yamagata University,
4-3-16 Jonan, Yonezawa 992-8510, Yamagata, Japan; t241691d@st.yamagata-u.ac.jp (K.Y.);
kobayashi.y@yz.yamagata-u.ac.jp (Y.K.)
Aquaculture Division, Iwate Fisheries Technology Center, 3-75-3 Heita, Kamaishi 026-0001, Iwate, Japan;
mochi.saya@gmail.com
Correspondence: jingong@yz.yamagata-u.ac.jp (J.G.); ihiroshi@yz.yamagata-u.ac.jp (H.I.);
Tel.: +81-238-26-3135 (J.G.); +81-238-26-3081 (H.I.)

Abstract: Microplastics’ spreading in the ocean is currently causing significant damage to organisms
and ecosystems around the world. To address this oceanic issue, there is a current focus on marine
degradable plastics. Polycaprolactone (PCL) is a marine degradable plastic that is attracting attention.
To further improve the biodegradability of PCL, we selected a completely new protein that has notbeen used before as a functional filler to incorporate it into PCL, aiming to develop an environmentallyfriendly biocomposite material. This novel protein is derived from the mucus bubbles of the violet
sea snail (VSS, Janthina globosa), which is a strong bio-derived material that is 100% degradable
in the sea environment by microorganisms. Two types of PCL/bubble composites, PCL/b1 andPCL/b5, were prepared with mass ratios of PCL to bubble powder of 99:1 and 95:5, respectively.
We investigated the thermal properties, mechanical properties, biodegradability, surface structure,and crystal structure of the developed PCL/bubble composites. The maximum biochemical oxygen
demand (BOD) degradation for PCL/b5 reached 96%, 1.74 times that of pure PCL (≈55%), clearly
indicating that the addition of protein fillers significantly enhanced the biodegradability of PCL.
The surface morphology observation results through scanning electron microscopy (SEM) definitely
confirmed the occurrence of degradation, and it was found that PCL/b5 underwent more significant
degradation compared to pure PCL. The water contact angle measurement results exhibited that all
sheets were hydrophobic (water contact angle > 90◦ ) before the BOD test and showed the changes insurface structure after the BOD test due to the newly generated indentations on the surface, which led
to an increase in surface toughness and, consequently, an increase in surface hydrophobility. A crystal
structure analysis by wide-angle X-ray scattering (WAXS) discovered that the amorphous regions
were decomposed first during the BOD test, and more amorphous regions were decomposed in
PCL/b5 than in PCL, owing to the addition of the bubble protein fillers from the VSS. The differential
scanning calorimeter (DSC) and thermal gravimetric analysis (TGA) results suggested that theaddition of mucus bubble protein fillers had only a slight impact on the thermal properties of PCL. In
terms of mechanical properties, compared to pure PCL, the mucus-bubble-filler-added composites
PCL/b1 and PCL/b5 exhibited slightly decreased values. Although the biodegradability of PCL wassignificantly improved by adding the protein fillers from mucus bubbles of the VSS, enhancing themechanical properties at the same time poses the next challenging issue.
Keywords: marine biodegradation; biochemical oxygen demand (BOD) test; polycaprolactone (PCL);
protein filler; mucus bubble; violet sea snail (Janthina globosa); biocomposite

This article is an open access article
distributed under the terms andconditions of the Creative Commons
Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction
The environmental pollution of plastics has been primarily a land-based concern fora long time, with little attention paid to the problem in the ocean [1]. However, nowadays,

Polymers 2024, 16, 1830. https://doi.org/10.3390/polym16131830

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Polymers 2024, 16, 1830

2 of 14from approximately 4.8 to 12.7 million tons of plastic are dumped into the world’s oceanseach year [2]. These microplastics spread throughout the ocean and do not disappear for
hundreds of years, causing major damage to organisms and ecosystems [3]. In response tothis problem in the ocean, marine degradable plastics are currently the focus of attention [4].
Among them, polycaprolactone (PCL) has attracted attention in recent years because of
its advantages such as a high biocompatibility, high biodegradability, and high electrospinning properties, which are applied in a wide range of fields such as biomedical, tissue
engineering, and medical implants. Its crystallization temperature is about 60 ◦ C and its
melting point is 59–64 ◦ C, making it easy to prepare and mold at a low temperature [5].
The properties of PCL can be improved by blending it with other fibers and polymers
to achieve better properties [6–9], and the addition of fillers impacts the mechanical andthermal properties of PCL, especially in the case of biocomposites [10–12]. PCL biocomposite materials have been reported by researchers, but the fillers they used were notmarine-derived proteins. Instead, they utilized agricultural waste such as wheat bran andcellulose nanofibers. Additionally, their primary focus was to improve the mechanicalproperties and water absorption properties of PCL [13–15]. Based on our search, there are
no reports on incorporating marine-derived proteins as functional fillers into PCL with theobjective of enhancing its biodegradability.
To further enhance the biodegradability of PCL, our approach is to incorporate theprotein fillers from mucus bubbles of the violet sea snail (VSS) into PCL. The VSS we usedin this work was Janthina globosa, Swainson, 1822, a marine mollusk, a species of thegenus Janthina in the family Epitoniidae (superfamily Epitonioidea). They float on thesurface of the open ocean suspended under a float constructed from bubbles of air coatedwith mucus [16]. All species of VSSs are cosmopolitan, living in temperate and tropicalseas and swarming. We selected mucus bubbles from VSS as protein fillers for two main
reasons. Firstly, VSS is a marine animal, and its mucus bubbles are marine proteins that are
expected to be fully decomposed in the sea environment by microorganisms. Secondly, themucus bubbles from VSSs are strong enough to support VSSs’ survival at sea. Therefore,
this strong bio-derived protein material attracted us to use it as a filler to enhance themarine biodegradation of PCL. Our objective was to develop an environmentally friendly
and functional biocomposite material through the incorporation of mucus bubble protein
fillers into PCL to create PCL/bubble composites. Two types of PCL/bubble composites,
PCL/b1 and PCL/b5, were prepared with mass ratios of PCL to bubble powder of 99:1
and 95:5, respectively. The thermal properties, mechanical properties, biodegradability,
surface structure, and crystal structure of the developed PCL/bubble composites were
investigated.
2. Materials and Methods
2.1. Preparation of PCL/Mucus Bubble Powder (PCL/Bubble) Sheets
2.1.1. Preparation of Dried Mucus Bubbles from the VSS
The snail specimens (Species: Janthina globosa, Swainson, 1822 (Mollusca: Gastropoda:
Epitoniidae); Collection place: Uehara Taketomi-cho, Yaeyama-gun, Okinawa, Japan,24◦ 23′ 43.4′′ N 123◦ 51′ 38.7′′ E) were washed with water, then the mucus bubbles wereseparated from the body and the egg and vacuum dried (degree of vacuum −100 kPa) atroom temperature for 24 h to prepare dried mucus bubble samples.
2.1.2. Preparation of Solvent-Cast Film-Shaped PCL Specimens
PCL pallets with an average molecular weight (Mn ) of 8000 were purchased from
Sigma-Aldrich, St. Louis, MO, USA. However, because pallet-shaped PCL cannot beground into powder even when using freeze pulverization, film-shaped PCL specimens
with a thickness of approximately 2 mm were prepared following the procedure below: 5 gof PCL was dissolved in 40 mL of tetrahydrofuran (THF). The mixture was then stirred
at room temperature for 24 h and left undisturbed for 24 h. Subsequently, the PCL films

Polymers 2024, 16, 1830

3 of 14were obtained and transferred to a vacuum dryer, where they were vacuum dried at room
temperature for 24 h (degree of vacuum −100 kPa) to completely remove the THF solvent.
2.1.3. Preparation of PCL/Bubble Composite Powder via Freeze Grinding
The prepared dried mucus bubbles and film-shaped PCL were placed together in thegrinding jar and ground at 25 Hz for 2 min using a cryo milling machine (CryoMill, Retsch,
Verder Scientific Co., Ltd., Haan, Germany). Two types of PCL/bubble composite powders
were prepared with mass ratios of PCL to bubble of 99:1 (PCL/b1) and 95:5 (PCL/b5). Pure
PCL 100% (PCL) and mucus bubble powder were also prepared in the same manner ascomparative specimens.
2.1.4. Preparation of PCL/Bubble Composite Sheets by Heat Pressing
PCL/b1, PCL/b5, and pure PCL sheets were prepared by heat pressing their respective
powders prepared above. Hot pressing was conducted on a vacuum heat press machine(1824, Imoto Machinery Co., Ltd., Kyoto, Japan) at 70 ◦ C for 5 min under a pressure of
10 MPa, followed by cooling pressing using a cooling press machine (MP-WC, Toyo Seiki,Ltd., Tokyo, Japan) at 20 ◦ C for 5 min under a pressure of 0.5 MPa. The resulting hot-pressed
sheets had q thickness ranging from 250 to 380 µm and were used for biodegradation,mechanical properties, and other testing.
2.2. Characterization
2.2.1. Differential Scanning Calorimetry (DSC)
The melting and crystallization behavior of the PCL/bubble composites and PCL werestudied on a DSC measuring machine (M-DSC Q-2000, TA Instruments, New Castle, DE,
USA) operating under a nitrogen flow. Samples of about 5 mg were heated from 20 ◦ C to150 ◦ C at a heating rate of 10 ◦ C/min, held for 1 min, then cooled to −50 ◦ C at a cooling rateof 5 ◦ C/min and held for 1 min, and finally reheated again to 150 ◦ C at a rate of 10 ◦ C/min.
2.2.2. Thermal Gravimetric Analysis (TGA)
The thermal stability of the PCL/bubble composites, PCL, and mucus bubbles fromthe VSS was studied using a TGA (Q-50, TA Instruments, New Castle, DE, USA) at a heating
rate of 10 ◦ C/min from room temperature to 700 ◦ C in a nitrogen atmosphere. The sample
weight was more than 10 mg.
2.2.3. Tensile Testing
The mechanical properties of the specimens were determined by tensile tests usinga versatile tensile testing machine (AGX-X, SHIMADZU, Kyoto, Japan). The specimens
were dumbbell-shaped, punched out from hot-pressed 250–380 µm thick films, with dimensions of 35 mm × 6 mm × 2 mm (test specimen No. 7, JIS K6251 standard [17]). The
specimens were elongated at a constant rate of 20 mm/min until the specimens brokeat room temperature. The sheets of PCL/b1, PCL/b5, and PCL were measured. The
tensile strength at break and yield, elongation at break, Young’s modulus, and toughness
were evaluated from the tensile stress–strain curves. Testing was run on a minimum of
five specimens. The reported data were the average of the results of five specimens.
2.2.4. Wide-Angle X-ray Scattering (WAXS)
To determine the degree of crystallinity (Xc ), WAXS measurements were conducted on
an X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan). The sheets of PCL/b1, PCL/b5,
and PCL were measured before and after biodegradation testing. The radiation source wasnickel-filtered Cu Kα radiation with a wavelength of 1.5418 Å. The samples were scanned
from 3◦ to 50◦ at a scanning rate of 2◦ /min. The Xc was calculated using Formula (1).
Xc =

Ic
× 100
Ic + Ia(1)

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where Ic is the scattering intensity of the crystalline region and Ia is the scattering intensityof the amorphous region determined by the waveform separation.
2.2.5. Biodegradability Testing
Biochemical Oxygen Demand (BOD)
The BOD biodegradation was evaluated with a BOD instrumentation setup consistingof an OxiTop IDS A12 measuring head and a 250 mL BOD reactor (Central Scientific
Corporation, Tokyo, Japan). To enhance the initial activity, extracted seawater containing
an increasing number and diversity of bacteria was prepared by mixing 100 g of sedimentwith 600 mL of raw seawater, followed by stirring and sonication. The resulting mixture
was then added to the BOD reactor as an inoculum. The sample was placed into the BOD
reactor containing the extracted seawater, and the OxiTop IDS A12 measuring head was
attached to the head of a BOD reactor. The biodegradability testing was conducted at 20 ◦ C
for 56 days. To sustain the microbial activity, two nutrient solutions, 0.5 g/L of NH4 CL asa nitrogen source (solution N) and 0.1 g/L of KH2 (PO4 ) as a phosphorus source (solutionP), were added into the extracted seawater twice (after 14 days and after 28 days). Eachtime, 3.0 mL of solution N and 3.0 mL of solution P were added to 300 mL of extracted
seawater. In this study, we discussed the BOD biodegradation of PCL/b5 sheets and PCL
sheets. The BOD biodegradability (%) was defined with the following Formula (2).
BOD biodegradability (%) =BOD sample − BOD blank
× 100
ThOD

(2)

Here, BOD sample and BOD blank are the experimentally observed values of the oxygen
consumption of the sample and a blank medium, respectively. ThOD is a theoreticallycalculated value of oxygen demand of the sample, which was obtained by consuming thesample completely degraded into CO2 and H2 O. Since the composition of mucus bubbles
from the VSS was not clearly identified, we calculated ThOD by referencing the compositionof jellyfish mucin (known as qniumucin) [18,19].
Weight Loss in Extracted Seawater
The same extracted seawater used in the BOD test was also employed for conducting
biodegradation tests by measuring the weight loss of the PCL/b5 and PCL sheets. The
initial weight of these samples was measured using a precision electronic scale, then thesesamples were wrapped in PE mesh, soaked in 250 mL of the same extracted seawater, andthen incubated in a constant temperature and humidity chamber (KCL-2000W, EYELA,Tokyo, Japan) at 20 ◦ C with a humidity level of 50% RH. After a 56-day incubation period,
the samples were washed twice with distilled water, air dried, and their final weights were
measured following the same procedure as the initial measurements. The degradation was
estimated by calculating the percentage of weight loss. All experiments were performedin triplicate.
2.2.6. Scanning Electron Microscope (SEM)
The assessment of bio-erosion on the surfaces of the PCL/b5 and PCL sheets, both
before and after the BOD biodegradability testing, was visualized with a SEM (Miniscope
TM-1000, Hitachi High-Technologies Corporation, Tokyo, Japan) operated at accelerating
voltages of 10–15 kV. The samples were sputter-coated with platinum-palladium for SEM.
2.2.7. Contact Angle Measurement
To detect changes in surface hydrophilicity, contact angle measurements were carriedout with a drop shape analyzer system (DSA-25, KRÜSS, Hamburg, Germany) for the
PCL/b5 and PCL sheets before and after the BOD tests. Water was used as the test liquid,and contact angles were measured in a state of equilibrium with a minimum of three
replicates for each sample.

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3. Results and Discussion
3.1. Thermal Properties of PCL/Bubble Composite Sheets and Powder
The sheets and powder for the PCL/bubble composites (PCL/b1 and PCL/b5) andpure PCL (PCL) were prepared according to the procedure shown in Figure 1a. The DSC
curves of dried mucus bubbles from the VSS (Figure 1b), the powder (Figure 1c), and the
sheet (Figure 1d) samples of PCL, PCL/b1, and PCL/b5 are also shown in Figure 1. The
crystallization temperature (Tc ) and melting point (Tm ) for these samples measured by
DSC are summarized and listed in Table 1.

Figure 1. The preparation procedure of sheets and powder for pure PCL (PCL) and PCL/bubble
composites (PCL/b1 and PCL/b5) (a), and the DSC curves of dried mucus bubbles from the VSS (b),the powder (c), and the sheet (d) samples of PCL, PCL/b1, and PCL/b5.
Table 1. The crystallization temperature (Tc ) and melting temperature (Tm ) of PCL, PCL/b1, PCL/b5,
and mucus bubble powder from the VSS measured by DSC.
T c (◦ C)

T m (◦ C)

31.7

54.7

PCL/b1

30.5

56.0

PCL/b5

31.7

55.4

Bubble

-

108.9

PCL

31.8

55.8

PCL/b1

29.5

54.0

PCL/b5

29.3

57.1

SamplePCL
PowderSheet

PCL/bubble
composite

PCL/bubble
composite

The dried mucus bubbles from the VSS showed a broad melting peak with a peakvalue of 108.9 ◦ C, but the Tc was not observed during the cooling process. For all powder
and sheet samples of PCL, PCL/b1, and PCL/b5, a clear single exothermal peak near 31 ◦ C
and a clear single endothermal peak near 55 ◦ C were observed, which belongs to the Tc
and Tm of PCL crystallites, respectively. No significant changes in either Tc or Tm wereobserved between PCL/bubble composites and pure PCL, which indicates that the additionof mucus bubble powder only had a slight impact on the Tc and Tm of PCL.

Polymers 2024, 16, x FOR PEER REVIEW

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Polymers 2024, 16, 1830

6 of 14and Tm of PCL crystallites, respectively. No significant changes in either Tc or Tm wereobserved between PCL/bubble composites and pure PCL, which indicates that the addition of mucus bubble powder only had a slight impact on the Tc and Tm of PCL.
Thethermal
thermalstabilitiesof PCL,
PCL/b1,
andsnailthe violetThestabilitiesof PCL,
PCL/b1,
PCL/b5,PCL/b5,
and the violet
mucus snail
bubblemucus bubble
powderwereassessedthrough
TGAa nitrogen
under aenvironment.
nitrogen environment.
Figure2 presents
powder wereassessedthrough
TGA under
Figure 2 presents
theweightloss vs.
temperatureplots (a) andtheir
derivative curves(b) obthe weightloss
vs. temperature
plots(a)corresponding
and their corresponding
derivative
curves (b)tainedfromfrom
TGA TGAmeasurements.
Detailed Detailed
results areresults
providedin provided
Table 2.
obtainedmeasurements.
arein Table 2.

Figureweightloss loss
vs. temperatureplots (a)plots
and theircorresponding
derivative curves(b)
Figure2.2.TheThe
weightvs. temperature(a) andtheir corresponding
derivative
curves (b)obtained from TGA measurements for PCL, PCL/b1, PCL/b5, and mucus bubble powder from theobtained from TGA measurements for PCL, PCL/b1, PCL/b5, and mucus bubble powder fromVSS.

the VSS.

Table 2. Thermal degradation temperatures obtained from TGA measurements for PCL, PCL/b1,
Table
Thermaldegradation
temperatures
PCL/b5,2.and
mucus bubble
powder fromthe VSS. obtained from TGA measurements for PCL, PCL/b1,

PCL/b5, and mucus bubble powder from the VSS.
Tonset(°C)
Sample
PCL
381.5
378.6
PCL PCL/b1
Powder PCL/bubble composite
PCL/b5381.2
PCL/b1
PCL/bubble
Powder
Bubble257.7
composite
SampleT5%(°C)
T onset
(◦ C)
360.3
364.5
381.5
360.2
378.6
-

T10%
(°C)
T 5%
(◦ C)
374.1
375.1
360.3
375.2
364.5
-

Tinflep
(°C)
T 10%
(◦ C)
408.8
406.0
374.1
404.5
375.1
276.1/292.6

T inflep
(◦ C)
408.8
406.0

PCL/b5
381.2
360.2
375.2
404.5
Tonset: onset temperatures, temperature at which the weight loss begins, T5%: temperature at which
there is a weight loss of 5 wt%,Bubble
T10%: temperature at which 257.7
there is a weight lossof 10 wt%,- Tinflep: 276.1/292.6
inflection
point, temperature at which the weight loss curve exhibits the greatest rate of change.
Tonset : onset temperatures, temperature at which the weight loss begins, T 5% : temperature at which there isa weight loss of 5 wt%, T10% : temperature at which there is a weight loss of 10 wt%, Tinflep : inflection point,
temperature at which the weight loss curve exhibits the greatest rate of change.

The TGA plot (Figure 2a) revealed two temperature ranges for the weight loss of the
mucus bubble. The first weight loss, below 150 ◦ C, beginning at 112.1 ◦ C with a loss of
88.6%, was attributed to the evaporation of water absorbed in the mucus bubble undernatural storage conditions, which indicated that the mucus bubble had a moisture content
of approximately 11%. The second weight loss between 200 and 500 ◦ C was due to thethermal degradation of the mucus bubble. Consequently, the TGA results indicated thatthe mucus bubble exhibited a thermal degradation onset temperature (Tonset ) at 257.7 ◦ Cwith a weight loss of approximately 4.5%.

Polymers 2024, 16, 1830

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In comparison, according to the TGA results, PCL, PCL/b1, and PCL/b5 exhibiteda Tonset at around 380 ◦ C with a total weight loss of less than 0.5%. Additionally, PCL,
PCL/b1, and PCL/b5 displayed temperatures at which there were weight losses of 5 wt%(T5% ) and 10 wt% (T10% ) exceeding 360 ◦ C and approximately 375 ◦ C, respectively. Thesefindings clearly demonstrate that the addition of mucus bubble powder minimally affected
the thermal degradation behavior of PCL. In other words, the PCL composites, PCL/b1
and PCL/b5, exhibited a thermal stability level comparable to that of pure PCL.
Furthermore, the derivative peak (Figure 2b), representing the inflection point (Tinflep )denoting the greatest rate of change on the weight loss curve, revealed that, although the
Tinflep of the mucus bubble (276.1 ◦ C and 292.6 ◦ C) was lower than that of PCL, PCL/b1,
and PCL/b5 (approximately 405 ◦ C), the addition of the mucus bubble showed a minimal
impact on the Tinflep of PCL as well. Overall, the TGA results suggested that the bubbleadded PCL composites, PCL/b1 and PCL/b5, exhibited a high level of thermal durability
comparable to that of pure PCL. These results suggest that the impact of adding mucus
bubbles on thermal properties is smaller than that of other biocomposite materials using
natural fibers such as cellulose and macaiba [10,11].
3.2. Mechanical Properties of PCL/Bubble Composite Sheets
Tensile tests were conducted to evaluate the mechanical properties of the PCL/bubble
composites. The tensile stress–strain curves of PCL, PCL/b1, and PCL/b5 are presented in
Figure 3. The average values of the mechanical properties are given in Table 3. In Figure 3a,
the breaking strength, yield strength, breaking strain, toughness, and Young’s modulusof the PCL and bubble-added composites PCL/b1 and PCL/b5 were compared. Young’s
modulus was measured in the strain range of 0.05–0.25%, where stress is proportional tostrain. Compared to pure PCL, the mucus-bubble-added composites PCL/b1 and PCL/b5
exhibited a little lower breaking strength and breaking stain, and overall, their toughness
decreased from 117.0 MJ/m3 to 85.5 MJ/m3 . On the other hand, the yield strength of
PCL/b1 and PCL/b5 increased, notably, the Young’s modulus of PCL/b5 rose to 238.0 MPa,showing a 13% increase from PCL’s 210.2 MPa. Also, compared to pure PCL (Figure 3b),
variations in the physical properties for PCL/b1 (Figure 3c) and PCL/b5 (Figure 3d) wereobserved, suggesting that the addition of mucus bubbles also caused the generation ofPolymers 2024, 16, x FOR PEER REVIEW
8 of 16through SEM
structural heterogeneity in the bubble-added composites. This can be verified
micrographs of cross-sectional view of the sheet samples (Figure 4).

Figure 3.3.Tensilestress–strain
curvescurves
for PCL,for
PCL/b1,
PCL/b5. (a)Comparison
typicalcurvesFigureTensilestress–strain
PCL,and
PCL/b1,
and
PCL/b5.of(a)Comparisonof typicalfor PCL, PCL/b1, and PCL/b5, curves for (b) PCL, (c) PCL/b1, and (d) PCL/b5 obtained from fivecurvesfor
PCL,
PCL/b1,
and
PCL/b5,curves
for
(b)
PCL,
(c)
PCL/b1,
and(d)
PCL/b5
obtainedfromspecimens. The red line in (b), blue line in (c), and green line in (d) were selected for comparison in
five
specimens.
Theforredline
in (b),andblue
line inrespectively.
(c), and green line in (d) were selected for comparison(a) asa typical curve
PCL,
PCL/b1,
PCL/b5,in (a) as a typical curve for PCL, PCL/b1, and PCL/b5, respectively.
Table 3. Mechanical properties of PCL, and PCL/bubble composites, including PCL/b1 andPCL/b5.

SamplePCL

Breaking Strength
(MPa)
27.5

Yield Strength
(MPa)
11.2

Breaking Strain
(%)
757.2

Toughness(MJ/m3)
117.0

Young’s Modulus
(MPa)
210.2

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Table 3. Mechanical properties of PCL, and PCL/bubble composites, including PCL/b1 and PCL/b5.
SampleBreaking Strength
(MPa)

PCL
27.5
Polymers 2024, 16, x FOR PEER REVIEW
PCL/b1
20.9
PCL/b5
20.1

Yield Strength(MPa)

Breaking Strain(%)

Toughness(MJ/m3 )Young’s Modulus
(MPa)

11.2
11.3
12.5

757.2
648.8
626.6

117.0
85.5
85.5

210.2
199.8
238.0

9 of 16

Figure4. SEMmicrographsof cross-sectionaland PCL/b5
Figure4. SEMmicrographsof cross-sectionalview
viewforforPCLPCL (a1,a2),(a1,a2), PCL/b1
PCL/b1 (b1,b2),(b1,b2), andPCL/b5(c1,c2)sheets. (a1,b1,c1)
are low-magnification
pictures,(a2,b2,c2)
relativelyhigh-magnifi
high(c1,c2)
sheets.
(a1,b1,c1)
are low-magnification
pictures,and and(a2,b2,c2)
arearerelatively
magnification
pictures. Regions
insidered circles
in indicate(c2) indicate
agglomeration of
cationpictures. Regions
inside redcirclesin (c2)thetheagglomeration
ofmucusmucusbubble
bubble parti
particlesin
PCL/b5.
cles in PCL/b5.
Figure 4 shows cross-sectional views of the sheet samples observed with a scanning

3.3. electron
Degradabilityof PCL/Bubble Composites in Marine Environments
microscope (SEM) and the SEM micrographs. The cross-sectional surface wasrelatively
flat for PCL (Figure
while,composite
with the additionof mucus
it trans- by the
The degradabilityof the 4(a1,a2)),
PCL/bubble
PCL/b5sheetsbubbles,
was evaluatedformedintoaporousstructurewithparticlesofmucus
bubbleshavingadiameterof lessnumber
BOD biodegradation testing with extracted seawater containing an increasing
5 µm inside
the pores
for both4(b1,b2)) and(Figure4(c1,c2)).
andthandiversityof bacteria.
Pure
PCLPCL/b1
sheets(Figure
were evaluatedinPCL/b5the samemanneras the com
We also observed a slight increase in the agglomeration of mucus bubble particles inparison. The BOD biodegradation curves are shown in Figure 5. The BOD test results in
PCL/b5 with a higher addition amount of mucus bubbles (Figure 4(c2), regions inside red
dicatedthatthebiodegradationof bothagglomeration
PCL/b5 andwerepure
PCL took
place toincontwo steps
circles).
The
porousstructure and particle
primarily
considered
Initially,
the first
15–20
days,strain
quick
observed.
exhibited
tributewithinto the decreasein thebreaking
anddegradation
the increase inwas
the yield
strength,PCL/b5
especiallyfor PCL/b51.51
composites.
Other (6.9%
researchers
have alsosimilarresults
thatthe aggredegradation
times fasterper day)
thanreported
PCL (4.6%
perday).
Subsequently,
thegationoftheblendedfillers
or
a
large
size
of
these
fillerscancause
stressconcentrations,degradation rates for both samples slowed down around 25 days after the initiation of thefurther
resultingthe degradationof the mechanicalproperties
BOD
testing.
The indegreeof biodegradation
peakedafter [14,15,20–23].
approximately 15 days for

PCL/b5and aroundof 20
days forComposites
pure PCL.
The maximum
BOD degradation for pure PCL
3.3. Degradability
PCL/Bubble
in Marine
Environments
was aboutwhereasoffor
it reached
96%,
which
was was1.74evaluatedtimes that
The55%,
degradability
thePCL/b5,
PCL/bubble
composite
PCL/b5sheetsby of pure
PCL.
results clearlytesting
indicatedthat the seawater
inclusionof mucus
powdertheThese
BOD biodegradation
with extracted
containing
anbubbleincreasing
numberfrom theanddiversityofbacteria.
Pure
PCL
sheetswere
evaluated
in
the
same
manner
VSS in PCL/b5 resulted in a significantly higher BOD degradation comparedas
tothepure PCL
comparison.
The
BOD
biodegradation
curvesareshownin
Figure
5.
The
BOD
test
results
that is, the addition of protein fillers from mucus bubbles significantly enhanced the bio
indicated that the biodegradation of both PCL/b5 and pure PCL took place in two steps.
degradabilityof PCL.
Initially, within the first 15–20 days, quick degradation was observed. PCL/b5 exhibitedPolymers 2024, 16, 1830and diversity of bacteria. Pure PCL sheets were evaluated in the same manner as t
parison. The BOD biodegradation curves are shown in Figure 5. The BOD test re
dicated that the biodegradation of both PCL/b5 and pure PCL took place in tw
Initially, within the first 15–20 days, quick degradation was observed. PCL/b5 ex
9 of 14degradation 1.51 times faster (6.9% per day) than PCL (4.6% per day). Subsequendegradation rates for both samples slowed down around 25 days after the initiatio
degradation
1.51 times
faster (6.9%
day) than PCL (4.6%
per day).
the
BOD testing.
The degreeof perbiodegradation
peakedafterSubsequently,
approximately
15 ddegradation
rates
for
both
samplessloweddown
around
25
days
after
the
initiationof
PCL/b5 and around 20 days for pure PCL. The maximum BOD degradation for pu
the BOD testing. The degree of biodegradation peaked after approximately 15 days for
was about 55%, whereas for PCL/b5, it reached 96%, which was 1.74 times that
PCL/b5 and around 20 days for pure PCL. The maximum BOD degradation for pure
PCL.wasThese
clearlyfor
indicatedthat
the inclusion
mucus
bubblef
PCL
aboutresults
55%, whereas
PCL/b5, itreached96%, whichof
was
1.74 times
that powderofpure
These results
clearlythat the higher
inclusionBOD
of mucus
bubble powder
from to pu
VSS PCL.
in PCL/b5
resultedin indicateda significantly
degradationcomparedthe
PCL/b5
resulted
in a significantly
highermucus
BOD degradationcompared to pure
thatVSSis,intheadditionof protein
fillers frombubbles significantly
enhanced
PCL, that is, the addition of protein fillers from mucus bubbles significantly enhanced thedegradability of PCL.
biodegradability of PCL.

FigureBOD
testtest
resultsfor PCL/b5
sheets witha 95:5
mass
ratio of
PCL ratio
to mucus
bubblepure bubble
Figure5.5.
BOD
resultsfor PCL/b5
sheetswitha 95:5massof PCLtoand
mucus◦ C for 56 days.
PCL
at 20
PCLsheets
sheets
at 20°C for 56 days.

3.4. Weight Loss of PCL/Bubble Composites
The PCL/bubble composite PCL/b5 and pure PCL sheet samples were immersed in
the same extracted seawater used in the BOD test at 20 ◦ C. After 56 days, the samples werecollected, and the weight loss was calculated based on the weights of the sheets before andafter the test. The weight loss for each sample and average weight loss are summarizedin Table 4. PCL/b5 presented a 34.1% average weight loss, slightly higher than that ofpure PCL, which was 33.4%. It was also observed that the three PCL/b5 samples showedsimilar weight loss values, while there was greater variations in weight loss for three pure
PCL samples. This suggests that, in terms of weight loss, the biodegradation of PCL/b5
occurred slightly more easily and consistently compared to that of pure PCL.
Table 4. Values of weight loss and average weight loss for PCL/b5 and pure PCL sheets which were
measured at 20 ◦ C over a period of 56 days when they were immersed in the same extracted seawaterused in the BOD test.
Sample (a)Weight Loss (%)

Average Weight Loss (%)

PCL/b5

1
2
3

34.5
36.8
30.9

34.1

PCL

1
2
3

34.6
25.6
40.1

33.4

(a) Three samples (1.0 cm × 1.0 cm) were tested for each measurement. The average thickness of PCL/b5 sheets
was 250 ± 20 µm and that of PCL was 230 ± 20 µm.

3.5. Surface Structure of PCL/Bubble Composite Sheets before and after the BOD Test
The changes in surface morphology of the PCL/bubble composite (PCL/b5) sheets
and pure PCL sheets before and after the BOD test were observed with SEM. The SEM im-

Polymers 2024, 16, 1830

10 of 14ages of the surfaces of the PCL/b5 and PCL sheets before and after BOD test are presented
in Figure 6. For both PCL/b5 and pure PCL, compared to the smooth surface of the initialsheets (Figure 6(a1,b1)), after the BOD test, their surface became severely uneven and lots ofindentations with diameters of 10–200 µm on the surface were generated (Figure 6(a2,b2)).
According to the relatively high-magnification SEM images (Figure 6(a3,b3)), a large amountof fine particles (200 nm–5 µm) were detected inside the indentations for PCL/b5, whilea hollow network structure resembling a bird’s nest was observed for PCL. These observation results definitely confirmed the occurrence of degradation. Additionally, it was notedthat PCL/b5 underwent more significant degradation compared to pure PCL. This 11
wasPolymers 2024, 16, x FOR PEER REVIEW
of 16
evidenced by the considerable amount of fine particles observed, suggesting that PCL/b5
broke down into smaller pieces to a greater extent.

Figuresurface ofofPCL/b5
PCL/b5 (a1–a3)(a1–a3)andandPCL
PCL(b1–b3)(b1–b3)sheetsbeforeafter
BOD
Figure6.6.SEM
SEMimages
images ofof surface
sheetsbeforeandand
after
BOD
test.
sheets);(a2,b2):(a2,b2):after
afterthetheBOD
BODtesttest(soakedextracted
seawater
test.(a1,b1):(a1,b1):before
beforeBOD
BOD test
test (initial(initial sheets);(soakedin inextracted
seawater◦ Cforatat2020°C
relativelyhigh-magnification
high-magnificationpictures
pictures(a2,b2).
is evident
for56
56days);days); andand (a3,b3):(a3,b3): relatively
of of(a2,b2).
It isItevident
thatthatthesurfacesofboth
PCL/b5andPCL
sheetsunderwent
significant
decomposition.
the surfaces of both PCL/b5 and PCL sheets underwent significant decomposition.

Furthermore,
the changes
changesininsurface
surfacestructurewereinvestigatedby measuring
Furthermore, thestructurewerealso also
investigatedby measuring
the
water
contactangle
to determine
whetherthe hydrophobicityof the PCL/b5and pure
PCL
the
water
contactangle
to determine
whetherthe hydrophobicityof the PCL/b5andpure
sheetschangedbeforeand after
BOD
Figure7 shows7 theresults
the contact
PCL
sheetschangedbeforeandtheafter
thetest.
BOD
test. Figure
shows
theof
resultsof theangle
contact
measurement.
The averagecontact angles
beforethe BOD
test thewereBOD
95.6◦ testfor PCL/b5,
93.6◦ forangle
measurement.
The averagecontact
anglesbeforewere 95.6°◦◦
for PCL/b1,
for and
PCL,94.3°
and after
the BOD
were 101.9
for PCL/b5
and for
PCL/b5,93.6°andfor 94.3
PCL/b1,for PCL,and test,
afterthey
the BOD
test, they
were 101.9°◦ for PCL. All sheets exhibited a water contact angle of more than 90◦ , indicating that all
96.3
PCL/b5 and 96.3° for PCL. All sheets exhibited a water contact angle of more than 90°,
sheets were hydrophobic. It was also observed from the results that the surfaces of PCL/b5
indicating that all sheets were hydrophobic. It was also observed from the results that theand PCL became more hydrophobic after the BOD test. This was thought to be due to thesurfaces of PCL/b5 and PCL became more hydrophobic after the BOD test. This was
newly generated indentations on the surface after the BOD test, which led to an increase
thought to be due to the newly generated indentations on the surface after the BOD test,in the surface toughness and, consequently, an increase in the surface hydrophobility, as
which
led byto thean increasein the surface
toughnessand,consequently, an increase in theindicated
Wenzel roughness
in Wenzel’s
model[24–26].
surface hydrophobility, as indicated by the Wenzel roughness in Wenzel’s model [24–26].

Polymers
FOR PEER REVIEWPolymers 2024,
2024, 16,
16, x1830

1211ofof 16
14

Figure 7. Changes in water contact angle of PCL/b5, PCL/b1, and PCL sheets before and after the
Figure 7. Changes in water contact angle of PCL/b5, PCL/b1, and PCL sheets before and after the
BOD test.
test. (a,b):(a,b): PCL/b5;
PCL/b5;(c):(c):PCL/b1;
PCL/b1;(d,e):(d,e):PCL;
PCL;(a,c,d):(a,c,d):before
beforeBOD
BODtesttest(initial(initialsheets);
sheets);(b,e)(b,e) after
after
BOD
◦ C for 56 days).
BOD
test(soakedinextracted
seawaterat
20
BOD test (soaked in extracted seawater at 20 °C for 56 days).

3.6. Crystal Structure of PCL/Bubble Composites
3.6. Crystal Structure of PCL/Bubble Composites
The degradation of polyester is said to depend on the chemical composition, molecThe degradation of polyester is said to depend on the chemical composition, molecular weight, hydrophobicity/hydrophilicity, crystallinity, and surface area of the sample,
ular weight, hydrophobicity/hydrophilicity, crystallinity, and surface area of the sample,
etc. [27–29]. Aiming to discover whether crystals affected the degradability of the PCL/
etc. [27–29]. Aiming to discover whether crystals affected the degradability of the
bubble composites, the crystal structure of the PCL/bubble composites, including PCL/b5
PCL/bubble
thePCL,
crystalthe PCL/bubble
composites,
includingand PCL/b1,composites,
as well as pure
wasstructure
examinedofusing
WAXS at room
temperature,and thePCL/b5
andPCL/b1, as(Xwellas pure PCL, was examined using WAXS at room temperadegree ofcrystallinityc ) was calculated basing on the WAXS results. The sheet samples
ture,
and
degreeof crystallinity(𝑋𝑐 ) was calculated
basingon thethe WAXSWAXS patterns
results. The
beforeandtheafter
the BOD
test were measured.
Figure 8A,B
present
andsheetsamples
before
and
after
the
BOD
test
were
measured.
Figure
8A,B
the Xc of PCL/b5, PCL/b1, and PCL sheets before and after the BOD tests. present the
WAXS
patterns
and the𝑋𝑐 of PCL/b5,
PCL/b1,and(Figure
PCL sheets
before5),and
after
the BODFor
all samplesof PCL/b5,
PCL/b1,and PCL
8A, Table
three
peaks
were
◦
◦
◦
tests.
observed at diffraction angles of 2θ ≈ 21.1 , 21.7 and 23.5 on the WAXS patterns, which
For all samples(Figure8A, Table 5),This
threepeaks werecorresponded
to a d ofofPCL/b5,around PCL/b1,
4.2 Å, 4.1and
Å, PCLand 3.8
Å, respectively.
impliedthere
observedatdiffraction
anglesof2θ
≈21.1°,
21.7°
and
23.5°
on
the
WAXS
patterns,
which
was no obvious difference in the crystal structure with or without the addition of thecorresponded
a d of around4.2 Å,
Å, andÅ, respectively.
This
impliedthere wasmucus bubbletoproteinfillers fromthe4.1
VSS,
both3.8
beforeand after the
BOD
test. Regardingnoobviousdifferenceinthecrystalstructure
withorwithout
theaddition
of
the
mucus
Xc (Figure 8B), before the BOD test, PCL/b5 exhibited a value of 44%, higher thanthebubbleproteinfillers
from
the
VSS,
both
before
and
after
the
BOD
test.
Regarding𝑐
42% observed for PCL/b1 and PCL, suggesting that the addition of the bubble fillers𝑋at(Figure8B), before
PCL/b5
exhibited
a
value
of
44%,
higherthan
the42%
5 wt% resulted
in atheriseBOD
in Xtest,
.
After
the
BOD
test,
both
PCL/b5andpure
PCL
showedcobserved
for
PCL,
that the additionof tothebefore
bubblefillers
5 wt%
a higher X
reachingand
47%
andsuggesting
46%, respectively,
comparedtheBODattest.
Inc , PCL/b1
resultedin thea risein 𝑋structure
.
After
the
BOD
test,
both
PCL/b5andpure
PCL
showedahighersummary,
crystalremainedlargely
unchanged,
whileX
increased
after
the
𝑐
c
𝑋BOD
47%be
and
46%, respectively,
comparedregions
to before
thedecomposed
BOD test. Infirstsummary,
test. It canconcludedthat the amorphous
were
during𝑐 , reachingthestructureremainedlargely regions
unchanged,
𝑋𝑐 increased
after theBOD
the crystal
BOD test,
and more
amorphous
werewhile
decomposedin PCL/b5
thanin test.
PCL
It
can bethat
regions
were
decomposed
first during
the BOD
owing
toconcluded
the addition
ofthe
theamorphousbubble protein
fillers.
These
protein fillers
are supposedtotest,
moreforamorphous
regions attracting
were decomposedin PCL/b5
than inPCL
owing
to theserveandas food
microorganisms,
them to thesurroundingarea
andfacilitating
decomposition.
Therefore,
the additionof bubble
proteinpromotedtoandaccelerated
additionof the bubble
proteinfillers. These
proteinfillersfillers
are supposedserve
as foodthemicroorganisms,
biodegradation ofattracting
PCL.
forthem to the surrounding area and facilitating decomposition. Therefore, the addition of bubble protein fillers promoted and accelerated the biodegradation of PCL.

Polymers
FOR PEER REVIEWPolymers2024,
2024, 16,
16, x1830

13 of 16
12 of 14

Figure 8.
8. WAXS
WAXSpatterns
patterns(A)(A)andand thethe degree
degree ofof crystallinity
crystallinity X
PCL/b1,and
andPCL
PCL
Figure𝑋𝑐c (B)(B) ofof PCL/b5,
PCL/b5, PCL/b1,
sheetspeaks(I, (I,II andIII,III,
as as
shown
in A)
detected
for allsheetsbefore
beforeandandafterafterthetheBOD
BODtest.
test.Three
Three
peaks
II andshown
in wereA) weredetectedforsamples.
all samples.

Table
5. TheThelocations
locationsofof
diffraction
peaks
corresponding
interplanar
d ofTable 5.
diffraction
peaks
and andcorresponding
interplanar
crystalcrystal
spacingspacing
d of PCL/b5,
PCL/b5,and
PCL calculated
theresults.
SAXS results.
PCL/b1,PCL/b1,
and PCLcalculated
from thefrom
SAXS

SampleSample

PCL/b5
PCL/b5

PCL/b1

PCL/b1PCL

PCL

After BOD
After BOD
BeforeBOD
BeforeBOD
Before BOD
BeforeAfter BOD
BOD
Before
BOD
After BOD
Before BOD

Peak I

Peak I2θ (°)
2θ (◦ )
21.14
21.14
21.08
21.08
21.21
21.21
21.03
21.20
21.03

d1 (Å)d1 (Å)
4.20
4.20
4.21
4.21
4.19
4.19
4.22
4.19
4.22

21.20

4.19

4. Conclusions

Peak II (Shoulder Peak)2θ (°)d2 (Å)
2θ (◦ )d2 (Å)
21.65
4.10
21.65
4.104.09
21.73
21.73
4.094.07
21.81
21.81
4.074.10
21.67
21.83
21.67
4.104.07

Peak III
Peak III2θ(°)
d3 (Å)
2θ (◦ )d3 (Å)
23.45
3.79
23.45
3.79
23.42
3.80
23.42
3.80
23.55
3.78
23.55
3.78
23.35
3.81
23.54
3.78
23.35
3.81

21.83

23.54

Peak II (Shoulder Peak)

4.07

3.78

We prepared biocomposite sheets by adding protein fillers from mucus bubbles of
4. Conclusions
the VSS into PCL in order to further improve the biodegradability of marine-degradableWe
prepared
biocomposite
by addingproteinreachedfillers frombubblesofplastic
PCL.
The maximum
BOD sheets
degradation
for PCL/b5
96%, mucus
1.74 times
that ofthe VSS into PCL in order to further improve the biodegradability of marine-degradable
pure PCL (≈55%), clearly indicating that the addition of protein fillers significantly enplastic PCL. The maximum BOD degradation for PCL/b5 reached 96%, 1.74 times thathanced the biodegradability of PCL. The surface morphology observation results throughof pure PCL (≈55%), clearly indicating that the addition of protein fillers significantly
SEM definitely confirmed the occurrence of degradation. Compared to a hollow network
enhanced the biodegradability of PCL. The surface morphology observation results throughstructure resembling a bird’s nest observed for PCL, a large amount of fine particles rangSEM definitely confirmed the occurrence of degradation. Compared to a hollow network
ing from 200 nm to 5 μm were detected inside the indentations for PCL/b5, suggesting
structure resembling a bird’s nest observed for PCL, a large amount of fine particles ranging
that PCL/b5 underwent more significant degradation compared to pure PCL. The waterfrom 200 nm to 5 µm were detected inside the indentations for PCL/b5, suggesting that
contact angle measurement results exhibited that all sheets were hydrophobic (water conPCL/b5 underwent more significant degradation compared to pure PCL. The water contact
tact angle > 90°) before the BOD test and showed the changes in surface structure after theangle measurement results exhibited that all sheets were hydrophobic (water contact angle
BOD◦test due to the newly generated indentations on the surface, which led to an increase> 90 ) before the BOD test and showed the changes in surface structure after the BOD testin surface toughness and, consequently, an increase in surface hydrophobility. The crystal
due to the newly generated indentations on the surface, which led to an increase in surface
structure analysis by WAXS discovered that the amorphous regions were decomposedPolymers 2024, 16, 1830

13 of 14

toughness and, consequently, an increase in surface hydrophobility. The crystal structure
analysis by WAXS discovered that the amorphous regions were decomposed first during
the BOD test, and more amorphous regions were decomposed in PCL/b5 than in PCL
owing to the addition of the bubble protein fillers. These protein fillers are supposed toserve as food for microorganisms, attracting them to the surrounding area and facilitating
decomposition. Therefore, the addition of protein fillers from mucus bubbles of the VSS
promoted and accelerated the biodegradation of PCL.
The DSC and TGA results suggested that the addition of mucus bubble protein fillers
had only a slight impact on the thermal properties of PCL. From these results, it canbe concluded that the PCL/bubble composite material was a material that had a highmarine biodegradability while maintaining its thermal properties. In terms of mechanicalproperties, compared to pure PCL, the mucus-bubble-filler-added composites PCL/b1 andPCL/b5 exhibited a slightly lower breaking strength and breaking stain, and overall, their
toughness decreased; on the other hand, their yield strength increased. The degradationof the mechanical properties was considered to be due to the aggregation of the blended
fillers or the large size of the fillers. Although the biodegradability of PCL was significantly
improved by adding the protein fillers from mucus bubbles of the VSS, enhancing themechanical properties at the same time poses the next challenging issue. It is supposed thatthe compatibility between plastic PCL and protein fillers is not good. Firstly, conducting
a detailed structure analysis of the interface between the matrix PCL and filler protein to
obtain clues to further enhance the mechanical properties of PCL/bubble composites seems
important. Then, proposals such as functionalizing bubble powder for better dispersion,
enhancing matrix/powder interfaces, and minimizing the bubble powder size to increase
the surface area would be very helpful. Furthermore, we have initiated research involving
the addition of marine proteins derived from other marine organisms, such as jellyfish andhagfish, as protein controls to verify the advantage, necessity, and importance of protein
fillers from VSS. We hope to share our findings on this topic in our next upcoming report.
Author Contributions: Conceptualization, J.G. and H.I.; methodology, K.Y., Y.K., J.G. and H.I.; formal
analysis, K.Y. and J.G.; investigation, K.Y., S.T., Y.K. and J.G.; resources, S.T.; data curation, K.Y. andJ.G.; writing—original draft preparation, K.Y., J.G. and H.I.; writing—review and editing, K.Y., J.G.
and H.I.; supervision, J.G. and H.I.; project administration, J.G. and H.I.; funding acquisition, J.G. and
H.I. All authors have read and agreed to the published version of the manuscript.
Funding: This work was partially supported by NEDO [Moonshot R&D–Millennia Program] Grant,
grant number NEDO JPNP18016.
Institutional Review Board Statement: The animal study protocol was approved by the Ethics
Committee of Yamagata University (protocol codes R2165, R3123, R4057, R5054 and R6052, withapproval dates of 2021/1/19, 2021/3/8, 2022/3/7, 2023/3/6, and 2024/3/6, respectively).
Data Availability Statement: The data presented in this study are available within the article.
Conflicts of Interest: The authors declare no conflicts of interest.

References
1.
2.
3.
4.

5.

Wang, G.; Huang, D.; Ji, J.; Völker, C.; Wurm, F.R. Seawater-Degradable Polymers—Fighting the Marine Plastic Pollution. Adv.
Sci. 2021, 8, 2001121. [CrossRef] [PubMed]
Jambeck, J.R.; Geyer, R.; Wilcox, C.; Siegler, T.R.; Perryman, M.; Andrady, A.; Narayan, R.; Law, K.L. Plastic Waste Inputs fromLand into the Ocean. Science 2015, 347, 768–771. [CrossRef] [PubMed]da Costa, J.P.; Santos, P.S.M.; Duarte, A.C.; Rocha-Santos, T. (Nano)Plastics in the Environment—Sources, Fates and Effects. Sci.
Total Environ. 2016, 566–567, 15–26. [CrossRef] [PubMed]
Lu, B.; Wang, G.-X.; Huang, D.; Ren, Z.-L.; Wang, X.-W.; Wang, P.-L.; Zhen, Z.-C.; Zhang, W.; Ji, J.-H. Comparison of PCL
Degradation in Different Aquatic Environments: Effects of Bacteria and Inorganic Salts. Polym. Degrad. Stab. 2018, 150, 133–139.
[CrossRef]
Ilyas, R.A.; Zuhri, M.Y.M.; Norrrahim, M.N.F.; Misenan, M.S.M.; Jenol, M.A.; Samsudin, S.A.; Nurazzi, N.M.; Asyraf, M.R.M.;
Supian, A.B.M.; Bangar, S.P.; et al. Natural Fiber-Reinforced Polycaprolactone Green and Hybrid Biocomposites for Various
Advanced Applications. Polymers 2022, 14, 182. [CrossRef] [PubMed]Polymers 2024, 16, 1830

6.
7.

8.

9.
10.
11.
12.

13.
14.
15.
16.
17.
18.

19.
20.
21.
22.
23.

24.
25.
26.
27.

28.
29.

14 of 14Ebrahimifar, M.; Taherimehr, M. Evaluation of In-Vitro Drug Release of Polyvinylcyclohexane Carbonate as a CO2 -Derived
Degradable Polymer Blended with PLA and PCL as Drug Carriers. J. Drug Deliv. Sci. Technol. 2021, 63, 102491. [CrossRef]
El-Naggar, M.E.; Shalaby, E.S.; Abd-Al-Aleem, A.H.; Abu-Saied, M.A.; Youssef, A.M. Synthesis of Environmentally Benign Antimicrobial Dressing Nanofibers Based on Polycaprolactone Blended with Gold Nanoparticles and Spearmint Oil Nanoemulsion.
J. Mater. Res. Technol. 2021, 15, 3447–3460. [CrossRef]
Herrero-Herrero, M.; Alberdi-Torres, S.; González-Fernández, M.L.; Vilariño-Feltrer, G.; Rodríguez-Hernández, J.C.; Vallés-Lluch,
A.; Villar-Suárez, V. Influence of Chemistry and Fiber Diameter of Electrospun PLA, PCL and Their Blend Membranes, Intended
as Cell Supports, on Their Biological Behavior. Polym. Test. 2021, 103, 107364. [CrossRef]
Doganci, M.D. Effects of Star-Shaped PCL Having Different Numbers of Arms on the Mechanical, Morphological, and Thermal
Properties of PLA/PCL Blends. J. Polym. Res. 2021, 28, 11. [CrossRef]
Ouled Ltaief, A.; Ghorbel, N.; Benhamou, K.; Arous, M.; Kaddami, H.; Kallel, A. Impact of Cellulose Nanocrystals Reinforcement
on Molecular Dynamics and Dielectric Properties of PCL-based Polyurethane. Polym. Compos. 2021, 42, 2737–2750. [CrossRef]
dos Santos Filho, E.A.; Siqueira, D.D.; Araújo, E.M.; Luna, C.B.B.; de Medeiros, E.P. The Impact of the Macaíba Components
Addition on the Biodegradation Acceleration of Poly (E-Caprolactone) (PCL). J. Polym. Environ. 2022, 30, 443–460. [CrossRef]
Reis, R.S.; Souza, D.D.H.S.; Marques, M.D.F.V.; da Luz, F.S.; Monteiro, S.N. Novel Bionanocomposite of Polycaprolactone
Reinforced with Steam-Exploded Microfibrillated Cellulose Modified with ZnO. J. Mater. Res. Technol. 2021, 13, 1324–1335.
[CrossRef]
AlMaadeed, M.A.; Kahraman, R.; Noorunnisa Khanam, P.; Madi, N. Date Palm Wood Flour/Glass Fibre Reinforced Hybrid
Composites of Recycled Polypropylene: Mechanical and Thermal Properties. Mater. Des. 2012, 42, 289–294. [CrossRef]
Herzele, S.; Veigel, S.; Liebner, F.; Zimmermann, T.; Gindl-Altmutter, W. Reinforcement of Polycaprolactone with Microfibrillated
Lignocellulose. Ind. Crops Prod. 2016, 93, 302–308. [CrossRef]
Hejna, A.; Sulyman, M.; Przybysz, M.; Saeb, M.R.; Klein, M.; Formela, K. On the Correlation of Lignocellulosic Filler Composition
with the Performance Properties of Poly(ε-Caprolactone) Based Biocomposites. Waste Biomass Valor 2020, 11, 1467–1479. [CrossRef]
Beu, A.G. Evolution of Janthina and Recluzia (Mollusca: Gastropoda: Epitoniidae). Rec. Aust. Mus. 2017, 69, 119–222. [CrossRef]
JIS K 6251; Rubber, Vulcanized or Thermoplastic Determination of Tensile Stress-Strain Properties. Japanese Standards Association:
Tokyo, Japan, 2023.
Ushida, K.; Sato, R.; Momma, T.; Tanaka, S.; Kaneko, T.; Morishita, H. Jellyfish Mucin (Qniumucin) Extracted with a Modified
Protocol Indicated Its Existence as a Constituent of the Extracellular Matrix. Biochim. Biophys. Acta (BBA)—Gen. Subj. 2022, 1866,
130189. [CrossRef] [PubMed]
Masuda, A.; Baba, T.; Dohmae, N.; Yamamura, M.; Wada, H.; Ushida, K. Mucin (Qniumucin), a Glycoprotein from Jellyfish, andDetermination of Its Main Chain Structure. J. Nat. Prod. 2007, 70, 1089–1092. [CrossRef]
Svärd, A.; Brännvall, E.; Edlund, U. Modified and Thermoplastic Rapeseed Straw Xylan: A Renewable Additive in PCL
Biocomposites. Ind. Crops Prod. 2018, 119, 73–82. [CrossRef]
Hejna, A.; Formela, K.; Saeb, M.R. Processing, Mechanical and Thermal Behavior Assessments of Polycaprolactone/Agricultural
Wastes Biocomposites. Ind. Crops Prod. 2015, 76, 725–733. [CrossRef]
Jayathilaka, L.P.I.; Ariyadasa, T.U.; Egodage, S.M. Development of Biodegradable Natural Rubber Latex Composites by Employing
Corn Derivative Bio-fillers. J. Appl. Polym. Sci 2020, 137, 49205. [CrossRef]
Jeon, H.; Kim, M.-S.; Park, S.B.; Kim, S.; Lee, M.; Park, S.-A.; Hwang, S.Y.; Koo, J.M.; Oh, D.X.; Park, J. Improved Mechanical
Properties of Biodegradable Polycaprolactone Nanocomposites Prepared Using Cellulose Nanocrystals. Cellulose 2023, 30,
11561–11574. [CrossRef]
Wenzel, R.N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988–994. [CrossRef]
Wenzel, R.N. Surface Roughness and Contact Angle. J. Phys. Chem. 1949, 53, 1466–1467. [CrossRef]
Cheng, B.K.; Naccarato, B.; Kim, K.J.; Kumar, A. Theoretical Consideration of Contact Angle Hysteresis Using Surface-EnergyMinimization Methods. Int. J. Heat Mass Transf. 2016, 102, 154–161. [CrossRef]
Ponjavic, M.; Nikolic, M.S.; Nikodinovic-Runic, J.; Jeremic, S.; Stevanovic, S.; Djonlagic, J. Degradation Behaviour of
PCL/PEO/PCL and PCL/PEO Block Copolymers under Controlled Hydrolytic, Enzymatic and Composting Conditions. Polym.
Test. 2017, 57, 67–77. [CrossRef]
Yang, K.-C. A Self-Reinforcing Biodegradable Implant Made of Poly (3-Caprolactone)/Calcium Phosphate Ceramic Composite
for Craniomaxillofacial Fracture Fixation. J. Cranio-Maxillofac. Surg. 2016, 44, 1333–1341.
Sung, H.-J.; Meredith, C.; Johnson, C.; Galis, Z.S. The Effect of Scaffold Degradation Rate on Three-Dimensional Cell Growth andAngiogenesis. Biomaterials 2004, 25, 5735–5742. [CrossRef]

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