Seunghwan Choy - Academia.edu (original) (raw)

Papers by Seunghwan Choy

Biomaterials Research

Background Photodynamic therapy (PDT) is a promising strategy to promote antitumor immunity by in... more Background Photodynamic therapy (PDT) is a promising strategy to promote antitumor immunity by inducing immunogenic cell death (ICD) in tumor cells. However, practical PDT uses an intense visible light owing to the shallow penetration depth of the light, resulting in immunosuppression at the tumor tissues. Methods Herein, we propose an implantable micro-scale light-emitting diode device (micro-LED) guided PDT that enables the on-demand light activation of photosensitizers deep in the body to potentiate antitumor immunity with mild visible light. Results The micro-LED is prepared by stacking one to four micro-scale LEDs (100 μm) on a needle-shape photonic device, which can be directly implanted into the core part of the tumor tissue. The photonic device with four LEDs efficiently elicits sufficient light output powers without thermal degradation and promotes reactive oxygen species (ROS) from a photosensitizer (verteporfin; VPF). After the intravenous injection of VPF in colon tumor-...

npj Flexible Electronics

As one of conducting polymers, PEDOT:PSS, is commonly used in organic electronics, especially for... more As one of conducting polymers, PEDOT:PSS, is commonly used in organic electronics, especially for bioelectronics due to its advantages such as high electrical and ionic conductivity, solution-processability and biocompatibility. Creating bioelectronics with the PEDOT:PSS requires advanced techniques to obtain physical/chemical modification of the PEDOT:PSS for improved performance and various applications. To satisfy these demands, fibrillary gelation of PEDOT:PSS by injection to choline acetate, an ionic liquid, with a constant flow rate was used in this study to make a conductive fiber and improve characteristics of PEDOT:PSS. Conductive fibers by fibrillary gelation showed enhanced electrical conductivity of about 400 S cm−1 and volumetric capacitance of about 154 F cm−3 which would be strongly beneficial to be utilized for organic electrochemical transistors (OECTs), resulting in a high transconductance of 19 mS in a depletion-mode. Moreover, dedoping of the conductive fibers by...

Science

Bioelectronics needs to continuously monitor mechanical and electrophysiological signals for pati... more Bioelectronics needs to continuously monitor mechanical and electrophysiological signals for patients. However, the signals always include artifacts by patients’ unexpected movement (such as walking and respiration under approximately 30 hertz). The current method to remove them is a signal process that uses a bandpass filter, which may cause signal loss. We present an unconventional bandpass filter material—viscoelastic gelatin-chitosan hydrogel damper, inspired by the viscoelastic cuticular pad in a spider—to remove dynamic mechanical noise artifacts selectively. The hydrogel exhibits frequency-dependent phase transition that results in a rubbery state that damps low-frequency noise and a glassy state that transmits the desired high-frequency signals. It serves as an adaptable passfilter that enables the acquisition of high-quality signals from patients while minimizing signal process for advanced bioelectronics.

Carbohydrate Polymers, 2021

This study was conducted to analyze the difference in mechanically improved properties by disting... more This study was conducted to analyze the difference in mechanically improved properties by distinguishing α-chitin and β-chitin for Poly(L-lactic acid)(PLLA). First, dissolution of chitins was established by mixing polar solvents hexafluoroisopropanol (HFIP) and trifluoroacetic acid (TFA) in appropriate proportions. Under these conditions, the dissolved chitin was used for electrospinning with other polymers. The electrospun nanofibers of the PLLA and chitins were successfully produced. Compared to the pristine state, when chitin was added to PLLA, the tensile strength increased 1.41 times (α-chitin), by 1.61 times (βchitin), respectively. Based on this, it was confirmed that αand βchitin could be strategically used for different polymers. The results also suggest that chitin can be applied to various fields as good reinforcing material as well as electrospinning.

Carbohydrate Polymers, 2020

Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn (in wt%) alloys were prepared via melt centrifugal atomizatio... more Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn (in wt%) alloys were prepared via melt centrifugal atomization following powder compaction by uni-axial pressing at 6 GPa and 450 1C for 60 min. The obtained materials were porosity-free with good particle-to-particle contact. The ''composite-like'' microstructures were very fine, showing silicon particles distributed in an a-Al matrix and displaying nearly equiaxed intermetallic phases. The intermetallic phases were identified as b-Al 5 FeSi and a-Al 9.3 FeMn 1.4 Si 1.8 in the Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn alloys, respectively. The Vickers hardness and compressive yield strength were 182 HV5 and 540 MPa, respectively, for the Al-23Si-8Fe-1Cr alloy and 197 HV5 and 650 MPa, respectively, for the Al-23Si-8Fe-5Mn alloy. The thermal stability of the alloys was studied by mechanical testing after long-term annealing at 300-400 1C, by compressive testing at 200-300 1C and by creep testing at 300 1C and 250 MPa. The thermal stability of both alloys was significantly better when compared to the ''thermally stable'' casting Al-12Si-1Cu-1Mg-1Ni alloy that is commonly utilized in the automotive industry for thermally loaded components such as pistons for combustion or diesel engines.

Composites Science and Technology, 2019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACS Applied Materials & Interfaces, 2019

Figure S1. (a) TEM image and (b) SEM image of helical patterns in CM/Ti (yellow arrow, TiO 2 at s... more Figure S1. (a) TEM image and (b) SEM image of helical patterns in CM/Ti (yellow arrow, TiO 2 at surfaces; triangle, infiltrated Ti into collagen fibers).

Marine Drugs, 2019

Although collagens from vertebrates are mainly used in regenerative medicine, the most elusive is... more Although collagens from vertebrates are mainly used in regenerative medicine, the most elusive issue in the collagen-based biomedical scaffolds is its insufficient mechanical strength. To solve this problem, electrospun collagen composites with chitins were prepared and molecular interactions which are the cause of the mechanical improvement in the composites were investigated by two-dimensional correlation spectroscopy (2DCOS). The electrospun collagen is composed of two kinds of polymorphs, α- and β-chitin, showing different mechanical enhancement and molecular interactions due to different inherent configurations in the crystal structure, resulting in solvent and polymer susceptibility. The collagen/α-chitin has two distinctive phases in the composite, but β-chitin composite has a relatively homogeneous phase. The β-chitin composite showed better tensile strength with ~41% and ~14% higher strength compared to collagen and α-chitin composites, respectively, due to a favorable seco...

Nature Communications, 2019

Environmental and health concerns force the search for sustainable super engineering plastics (SE... more Environmental and health concerns force the search for sustainable super engineering plastics (SEPs) that utilise bio-derived cyclic monomers, e.g. isosorbide instead of restricted petrochemicals. However, previously reported bio-derived thermosets or thermoplastics rarely offer thermal/mechanical properties, scalability, or recycling that match those of petrochemical SEPs. Here we use a phase transfer catalyst to synthesise an isosorbide-based polymer with a high molecular weight >100 kg mol −1 , which is reproducible at a 1-kg-scale production. It is transparent and solvent/melt-processible for recycling, with a glass transition temperature of 212°C, a tensile strength of 78 MPa, and a thermal expansion coefficient of 23.8 ppm K −1. Such a performance combination has not been reported before for biobased thermoplastics, petrochemical SEPs, or thermosets. Interestingly, quantum chemical simulations show the alicyclic bicyclic ring structure of isosorbide imposes stronger geometric restraint to polymer chain than the aromatic group of bisphenol-A.

Carbohydrate Polymers, 2019

A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing ... more A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing critical heat flux (CHF); however, its instability over a long period of time due to sedimentation and aggregation has impeded its successful application in industry. In this study, lightweight negatively charged TEMPO-oxidized cellulose nanofibers (CNFs) were utilized as a nano-sized substance in water and examined to enhance both the CHF performance and thermal stability of nanofluids. Owing to low density of the CNFs and long range repulsion between negatively charged CNFs, there were no aggregation and sedimentation of CNFs with multiple boiling/ cooling cycles. In addition, with CNF concentrations of 0.01, 0.03, 0.05, and 0.10 wt%, CHF enhancement increases of 40.7%, 45.1%, 54.9%, and 69.4%, respectively, were achieved over that of pure water. The present results demonstrated the great potential of CNFs as eco-friendly and cost-effective nano-substances that can overcome the instability of nanofluids.

ACS Applied Materials & Interfaces, 2019

Cellulose, 2018

Tumor-originated and undefined extracellular matrices (ECMs) such as Matrigel TM have been widely... more Tumor-originated and undefined extracellular matrices (ECMs) such as Matrigel TM have been widely used in three-dimensional (3D) cell and tissue culture, but their use is unacceptable in clinical cell therapies. In this study, we proposed a 3D cellulose nanofiber (CNF) hydrogel that has great potential as a defined tissue-engineering scaffold, especially for osteoblast culture. The CNF hydrogel showed attractive features as a cell scaffold material. It exhibited a * 1.4fold higher diffusion coefficient (* 2.98 9 10-7 cm 2 / s) of macromolecules such as bovine serum albumin than does Matrigel TM (\ 2.2 9 10-7 cm 2 /s) due to the former's higher porosity ([ 95%) and pore size (* 310.8 lm). Most pre-osteoblast cells that are encapsulated in the CNF hydrogel were immediately locked without sinking by instant hydrogen bond crosslinking between CNFs, whereas cells encapsulated in Matrigel TM sank to the bottom of the scaffold due to the slow sol-gel transition ([ 20 min). The elastic modulus of the cell-encapsulated CNF hydrogel could be reinforced by further calcium-mediated cross-linking without cytotoxicity. As a result, the pre-osteoblast cells in the CNF hydrogels were homogeneously distributed in the 3D structure, proliferated for 3 weeks, and successfully differentiated. Overall, CNFs showed that it has potential to be used in tissue engineering as a defined ECM component. Hyo Jeong Kim and Dongyeop X. Oh have contributed equally to this work and are considered first co-authors.

ACS Applied Materials & Interfaces, 2015

Mussels survive by strongly attaching to a variety of different surfaces, primarily subsurface ro... more Mussels survive by strongly attaching to a variety of different surfaces, primarily subsurface rocks composed of metal oxides, through the formation of coordinative interactions driven by protein-based catechol repeating units contained within their adhesive secretions. From a chemistry perspective, catechols are known to form strong and reversible complexes with metal ions or metal oxides, with the binding affinity being dependent on the nature of the metal ion. As a result, catechol binding with metal oxides is reversible and can be broken in the presence of a free metal ion with a higher stability constant. It is proposed to exploit this competitive exchange in the design of a new strategy for the formation, removal, and reformation of surface coatings and self-assembled monolayers (SAM) based on catechols as the adhesive unit. In this study, catechol-functionalized tri(ethylene oxide) (TEO) was synthesized as a removable and recoverable self-assembled monolayer (SAM) for use on oxides surfaces. Attachment and detachment of these catechol derivatives on a variety of surfaces was shown to be reversible and controllable by exploiting the high stability constant of catechol to soluble metal ions, such as Fe(III). This tunable assembly based on catechol binding to metal oxides represents a new concept for reformable coatings with applications in fields ranging from friction/wettability control to biomolecular sensing and antifouling.

Gluing dynamic, wet biological tissue is important in injury treatment yet difficult to achieve. ... more Gluing dynamic, wet biological tissue is important in injury treatment yet difficult to achieve. Polymeric adhesives are inconvenient to handle due to rapid cross-linking and can raise biocompatibility concerns. Inorganic nanoparticles adhere weakly to wet surfaces. Herein, an aqueous suspension of guanidiniumfunctionalized chitin nanoparticles as a biomedical adhesive with biocompatible, hemostatic, and antibacterial properties is developed. It glues porcine skin up to 3000-fold more strongly (30 kPa) than inorganic nanoparticles at the same concentration and adheres at neutral pH, which is unachievable with mussel-inspired adhesives alone. The glue exhibits an instant adhesion (2 min) to fully wet surfaces, and the glued assembly endures one-week underwater immersion. The suspension is lowly viscous and stable, hence sprayable and convenient to store. A nanomechanic study reveals that guanidinium moieties are chaotropic, creating strong, multifaceted noncovalent bonds with proteins: salt bridges comprising ionic attraction and bidentate hydrogen bonding with acidic moieties, cation−π interactions with aromatic moieties, and hydrophobic interactions. The adhesion mechanism provides a blueprint for advanced tissue adhesives.

Loading a photocatalytic TiO 2 to organic carriers has been desired for volumetric TiO 2 incorpor... more Loading a photocatalytic TiO 2 to organic carriers has been desired for volumetric TiO 2 incorporation, facile retrieval, and sustainable utilization. Traditionally, suspended TiO 2 nanoparticles or its thin film on twodimensional substrate are popularly fabricated for pollutants decomposition without carriers; due to poor thermomechanical properties of the organic carriers. Herein, a combination of the chitin nanofiber carrier and atomic layer deposition proves relevance for formation of anatase TiO 2 thin layer so that photocatalytic decomposition in three-dimensional surface. Moreover, chitin nanofiber is capable of holding the TiO 2 nanoparticles for multiple cycles of photocatalysis. Those types of TiO 2 show characteristic degradation performance for gaseous (acetaldehyde) and aqueous pollutants (4-chlorophenol and rhodamine B). After catalytic reaction, chitin/TiO 2 is retrievable owing to carrier's robustness even in water without TiO 2 aggregation and loss. This work suggests that chitin-based photocatalyst is applicable to numerous pollutants through chitin's relatively high chemical resistance and stably wedged TiO 2 during photocatalytic reaction.

Carbohydrate Polymers, 2020

The major disadvantage of electrospun nanofibrous mats is their poor mechanical properties, which... more The major disadvantage of electrospun nanofibrous mats is their poor mechanical properties, which result from interfibrillar slips, porous structures, and the isotropic conformation of functional groups in fibers. In this work, we develop a tough electrospun mat without cost of both the stiffness and extensibility by combining two mutually exclusive polymers, i.e., generally “ductile” poly(vinyl alcohol) (PVA) and “stiff” α-chitin. The toughness of PVA/α-chitin is considerably higher (∼20 times) compared to PVA via intermolecular-fitted design and stoichiometric balance between hydrogen bonding donors and acceptors. Moreover, consistently oriented functional groups that are perpendicular to nanofibers improve mechanical properties. As a result, stiffness and extensibility are simultaneously increased by ∼19.3 and ∼3.8 times, respectively compared to PVA. The thermal stability with a 2.80-fold larger melting enthalpy of 823.95 ± 7.05 J g−1 than PVA. The great thermomechanical performance provides an insight for molecular design in electrospun nanofibers with chitin polymorphs.

Composites Science and Technology, 2019

A biobased and biocompatible thermoplastic elastomer (TPE) with superior 3D printability was demo... more A biobased and biocompatible thermoplastic elastomer (TPE) with superior 3D printability was demonstrated with great potential for customized manufacturing technologies and fabrication of biointegrated devices. The inherent structural and stereochemical disadvantages of biobased monomers, such as 2,5-furandicarboxylic acid, in comparison with today used petroleum based monomers like terephthalic acid generally lead to lower mechanical performance for the biobased replacement polymers. This is additionally enhanced by poor interfacial adhesion and fusion commonly encountered during customized manufacturing technologies like 3D printing. Herein, we demonstrate
that in-situ polymerization in the presence of trace amounts of nanocellulose can homogeneously distribute the nanofiller leading to dramatically strengthened thermally 3D-printable bio-furan-based TPE. This TPE exhibited a tensile strength of 67 MPa which is 1.5–7-fold higher than the values reported for silicone and thermoplastic urethane,
which are widely used in biomedical applications. In addition, the TPE had an impressive extensibility of 860% and negligible in vivo cytotoxicity; such properties have not been reported to date for bio-based or petrochemical TPEs. While a petrochemical 3D printed TPE counterpart retained only half of the tensile strength compared to the hot-pressed analogue, the 3D-printed biobased TPE in-situ modified with nanocellulose maintained 70–80% of its strength under the same experimental conditions. This is explained by inter-diffusion between interfaces facilitated by the nanocellulose and the furan rings. Using the ergonomic shape of a wrist as a 3D-printable design, we successfully
manufactured a wearable thermal therapeutic device from the nanocellulose modified biobased TPE, giving promise for wide variety of future applications.

Biomaterials Research

Background Photodynamic therapy (PDT) is a promising strategy to promote antitumor immunity by in... more Background Photodynamic therapy (PDT) is a promising strategy to promote antitumor immunity by inducing immunogenic cell death (ICD) in tumor cells. However, practical PDT uses an intense visible light owing to the shallow penetration depth of the light, resulting in immunosuppression at the tumor tissues. Methods Herein, we propose an implantable micro-scale light-emitting diode device (micro-LED) guided PDT that enables the on-demand light activation of photosensitizers deep in the body to potentiate antitumor immunity with mild visible light. Results The micro-LED is prepared by stacking one to four micro-scale LEDs (100 μm) on a needle-shape photonic device, which can be directly implanted into the core part of the tumor tissue. The photonic device with four LEDs efficiently elicits sufficient light output powers without thermal degradation and promotes reactive oxygen species (ROS) from a photosensitizer (verteporfin; VPF). After the intravenous injection of VPF in colon tumor-...

npj Flexible Electronics

As one of conducting polymers, PEDOT:PSS, is commonly used in organic electronics, especially for... more As one of conducting polymers, PEDOT:PSS, is commonly used in organic electronics, especially for bioelectronics due to its advantages such as high electrical and ionic conductivity, solution-processability and biocompatibility. Creating bioelectronics with the PEDOT:PSS requires advanced techniques to obtain physical/chemical modification of the PEDOT:PSS for improved performance and various applications. To satisfy these demands, fibrillary gelation of PEDOT:PSS by injection to choline acetate, an ionic liquid, with a constant flow rate was used in this study to make a conductive fiber and improve characteristics of PEDOT:PSS. Conductive fibers by fibrillary gelation showed enhanced electrical conductivity of about 400 S cm−1 and volumetric capacitance of about 154 F cm−3 which would be strongly beneficial to be utilized for organic electrochemical transistors (OECTs), resulting in a high transconductance of 19 mS in a depletion-mode. Moreover, dedoping of the conductive fibers by...

Science

Bioelectronics needs to continuously monitor mechanical and electrophysiological signals for pati... more Bioelectronics needs to continuously monitor mechanical and electrophysiological signals for patients. However, the signals always include artifacts by patients’ unexpected movement (such as walking and respiration under approximately 30 hertz). The current method to remove them is a signal process that uses a bandpass filter, which may cause signal loss. We present an unconventional bandpass filter material—viscoelastic gelatin-chitosan hydrogel damper, inspired by the viscoelastic cuticular pad in a spider—to remove dynamic mechanical noise artifacts selectively. The hydrogel exhibits frequency-dependent phase transition that results in a rubbery state that damps low-frequency noise and a glassy state that transmits the desired high-frequency signals. It serves as an adaptable passfilter that enables the acquisition of high-quality signals from patients while minimizing signal process for advanced bioelectronics.

Carbohydrate Polymers, 2021

This study was conducted to analyze the difference in mechanically improved properties by disting... more This study was conducted to analyze the difference in mechanically improved properties by distinguishing α-chitin and β-chitin for Poly(L-lactic acid)(PLLA). First, dissolution of chitins was established by mixing polar solvents hexafluoroisopropanol (HFIP) and trifluoroacetic acid (TFA) in appropriate proportions. Under these conditions, the dissolved chitin was used for electrospinning with other polymers. The electrospun nanofibers of the PLLA and chitins were successfully produced. Compared to the pristine state, when chitin was added to PLLA, the tensile strength increased 1.41 times (α-chitin), by 1.61 times (βchitin), respectively. Based on this, it was confirmed that αand βchitin could be strategically used for different polymers. The results also suggest that chitin can be applied to various fields as good reinforcing material as well as electrospinning.

Carbohydrate Polymers, 2020

Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn (in wt%) alloys were prepared via melt centrifugal atomizatio... more Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn (in wt%) alloys were prepared via melt centrifugal atomization following powder compaction by uni-axial pressing at 6 GPa and 450 1C for 60 min. The obtained materials were porosity-free with good particle-to-particle contact. The ''composite-like'' microstructures were very fine, showing silicon particles distributed in an a-Al matrix and displaying nearly equiaxed intermetallic phases. The intermetallic phases were identified as b-Al 5 FeSi and a-Al 9.3 FeMn 1.4 Si 1.8 in the Al-23Si-8Fe-1Cr and Al-23Si-8Fe-5Mn alloys, respectively. The Vickers hardness and compressive yield strength were 182 HV5 and 540 MPa, respectively, for the Al-23Si-8Fe-1Cr alloy and 197 HV5 and 650 MPa, respectively, for the Al-23Si-8Fe-5Mn alloy. The thermal stability of the alloys was studied by mechanical testing after long-term annealing at 300-400 1C, by compressive testing at 200-300 1C and by creep testing at 300 1C and 250 MPa. The thermal stability of both alloys was significantly better when compared to the ''thermally stable'' casting Al-12Si-1Cu-1Mg-1Ni alloy that is commonly utilized in the automotive industry for thermally loaded components such as pistons for combustion or diesel engines.

Composites Science and Technology, 2019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACS Applied Materials & Interfaces, 2019

Figure S1. (a) TEM image and (b) SEM image of helical patterns in CM/Ti (yellow arrow, TiO 2 at s... more Figure S1. (a) TEM image and (b) SEM image of helical patterns in CM/Ti (yellow arrow, TiO 2 at surfaces; triangle, infiltrated Ti into collagen fibers).

Marine Drugs, 2019

Although collagens from vertebrates are mainly used in regenerative medicine, the most elusive is... more Although collagens from vertebrates are mainly used in regenerative medicine, the most elusive issue in the collagen-based biomedical scaffolds is its insufficient mechanical strength. To solve this problem, electrospun collagen composites with chitins were prepared and molecular interactions which are the cause of the mechanical improvement in the composites were investigated by two-dimensional correlation spectroscopy (2DCOS). The electrospun collagen is composed of two kinds of polymorphs, α- and β-chitin, showing different mechanical enhancement and molecular interactions due to different inherent configurations in the crystal structure, resulting in solvent and polymer susceptibility. The collagen/α-chitin has two distinctive phases in the composite, but β-chitin composite has a relatively homogeneous phase. The β-chitin composite showed better tensile strength with ~41% and ~14% higher strength compared to collagen and α-chitin composites, respectively, due to a favorable seco...

Nature Communications, 2019

Environmental and health concerns force the search for sustainable super engineering plastics (SE... more Environmental and health concerns force the search for sustainable super engineering plastics (SEPs) that utilise bio-derived cyclic monomers, e.g. isosorbide instead of restricted petrochemicals. However, previously reported bio-derived thermosets or thermoplastics rarely offer thermal/mechanical properties, scalability, or recycling that match those of petrochemical SEPs. Here we use a phase transfer catalyst to synthesise an isosorbide-based polymer with a high molecular weight >100 kg mol −1 , which is reproducible at a 1-kg-scale production. It is transparent and solvent/melt-processible for recycling, with a glass transition temperature of 212°C, a tensile strength of 78 MPa, and a thermal expansion coefficient of 23.8 ppm K −1. Such a performance combination has not been reported before for biobased thermoplastics, petrochemical SEPs, or thermosets. Interestingly, quantum chemical simulations show the alicyclic bicyclic ring structure of isosorbide imposes stronger geometric restraint to polymer chain than the aromatic group of bisphenol-A.

Carbohydrate Polymers, 2019

A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing ... more A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing critical heat flux (CHF); however, its instability over a long period of time due to sedimentation and aggregation has impeded its successful application in industry. In this study, lightweight negatively charged TEMPO-oxidized cellulose nanofibers (CNFs) were utilized as a nano-sized substance in water and examined to enhance both the CHF performance and thermal stability of nanofluids. Owing to low density of the CNFs and long range repulsion between negatively charged CNFs, there were no aggregation and sedimentation of CNFs with multiple boiling/ cooling cycles. In addition, with CNF concentrations of 0.01, 0.03, 0.05, and 0.10 wt%, CHF enhancement increases of 40.7%, 45.1%, 54.9%, and 69.4%, respectively, were achieved over that of pure water. The present results demonstrated the great potential of CNFs as eco-friendly and cost-effective nano-substances that can overcome the instability of nanofluids.

ACS Applied Materials & Interfaces, 2019

Cellulose, 2018

Tumor-originated and undefined extracellular matrices (ECMs) such as Matrigel TM have been widely... more Tumor-originated and undefined extracellular matrices (ECMs) such as Matrigel TM have been widely used in three-dimensional (3D) cell and tissue culture, but their use is unacceptable in clinical cell therapies. In this study, we proposed a 3D cellulose nanofiber (CNF) hydrogel that has great potential as a defined tissue-engineering scaffold, especially for osteoblast culture. The CNF hydrogel showed attractive features as a cell scaffold material. It exhibited a * 1.4fold higher diffusion coefficient (* 2.98 9 10-7 cm 2 / s) of macromolecules such as bovine serum albumin than does Matrigel TM (\ 2.2 9 10-7 cm 2 /s) due to the former's higher porosity ([ 95%) and pore size (* 310.8 lm). Most pre-osteoblast cells that are encapsulated in the CNF hydrogel were immediately locked without sinking by instant hydrogen bond crosslinking between CNFs, whereas cells encapsulated in Matrigel TM sank to the bottom of the scaffold due to the slow sol-gel transition ([ 20 min). The elastic modulus of the cell-encapsulated CNF hydrogel could be reinforced by further calcium-mediated cross-linking without cytotoxicity. As a result, the pre-osteoblast cells in the CNF hydrogels were homogeneously distributed in the 3D structure, proliferated for 3 weeks, and successfully differentiated. Overall, CNFs showed that it has potential to be used in tissue engineering as a defined ECM component. Hyo Jeong Kim and Dongyeop X. Oh have contributed equally to this work and are considered first co-authors.

ACS Applied Materials & Interfaces, 2015

Mussels survive by strongly attaching to a variety of different surfaces, primarily subsurface ro... more Mussels survive by strongly attaching to a variety of different surfaces, primarily subsurface rocks composed of metal oxides, through the formation of coordinative interactions driven by protein-based catechol repeating units contained within their adhesive secretions. From a chemistry perspective, catechols are known to form strong and reversible complexes with metal ions or metal oxides, with the binding affinity being dependent on the nature of the metal ion. As a result, catechol binding with metal oxides is reversible and can be broken in the presence of a free metal ion with a higher stability constant. It is proposed to exploit this competitive exchange in the design of a new strategy for the formation, removal, and reformation of surface coatings and self-assembled monolayers (SAM) based on catechols as the adhesive unit. In this study, catechol-functionalized tri(ethylene oxide) (TEO) was synthesized as a removable and recoverable self-assembled monolayer (SAM) for use on oxides surfaces. Attachment and detachment of these catechol derivatives on a variety of surfaces was shown to be reversible and controllable by exploiting the high stability constant of catechol to soluble metal ions, such as Fe(III). This tunable assembly based on catechol binding to metal oxides represents a new concept for reformable coatings with applications in fields ranging from friction/wettability control to biomolecular sensing and antifouling.

Gluing dynamic, wet biological tissue is important in injury treatment yet difficult to achieve. ... more Gluing dynamic, wet biological tissue is important in injury treatment yet difficult to achieve. Polymeric adhesives are inconvenient to handle due to rapid cross-linking and can raise biocompatibility concerns. Inorganic nanoparticles adhere weakly to wet surfaces. Herein, an aqueous suspension of guanidiniumfunctionalized chitin nanoparticles as a biomedical adhesive with biocompatible, hemostatic, and antibacterial properties is developed. It glues porcine skin up to 3000-fold more strongly (30 kPa) than inorganic nanoparticles at the same concentration and adheres at neutral pH, which is unachievable with mussel-inspired adhesives alone. The glue exhibits an instant adhesion (2 min) to fully wet surfaces, and the glued assembly endures one-week underwater immersion. The suspension is lowly viscous and stable, hence sprayable and convenient to store. A nanomechanic study reveals that guanidinium moieties are chaotropic, creating strong, multifaceted noncovalent bonds with proteins: salt bridges comprising ionic attraction and bidentate hydrogen bonding with acidic moieties, cation−π interactions with aromatic moieties, and hydrophobic interactions. The adhesion mechanism provides a blueprint for advanced tissue adhesives.

Loading a photocatalytic TiO 2 to organic carriers has been desired for volumetric TiO 2 incorpor... more Loading a photocatalytic TiO 2 to organic carriers has been desired for volumetric TiO 2 incorporation, facile retrieval, and sustainable utilization. Traditionally, suspended TiO 2 nanoparticles or its thin film on twodimensional substrate are popularly fabricated for pollutants decomposition without carriers; due to poor thermomechanical properties of the organic carriers. Herein, a combination of the chitin nanofiber carrier and atomic layer deposition proves relevance for formation of anatase TiO 2 thin layer so that photocatalytic decomposition in three-dimensional surface. Moreover, chitin nanofiber is capable of holding the TiO 2 nanoparticles for multiple cycles of photocatalysis. Those types of TiO 2 show characteristic degradation performance for gaseous (acetaldehyde) and aqueous pollutants (4-chlorophenol and rhodamine B). After catalytic reaction, chitin/TiO 2 is retrievable owing to carrier's robustness even in water without TiO 2 aggregation and loss. This work suggests that chitin-based photocatalyst is applicable to numerous pollutants through chitin's relatively high chemical resistance and stably wedged TiO 2 during photocatalytic reaction.

Carbohydrate Polymers, 2020

The major disadvantage of electrospun nanofibrous mats is their poor mechanical properties, which... more The major disadvantage of electrospun nanofibrous mats is their poor mechanical properties, which result from interfibrillar slips, porous structures, and the isotropic conformation of functional groups in fibers. In this work, we develop a tough electrospun mat without cost of both the stiffness and extensibility by combining two mutually exclusive polymers, i.e., generally “ductile” poly(vinyl alcohol) (PVA) and “stiff” α-chitin. The toughness of PVA/α-chitin is considerably higher (∼20 times) compared to PVA via intermolecular-fitted design and stoichiometric balance between hydrogen bonding donors and acceptors. Moreover, consistently oriented functional groups that are perpendicular to nanofibers improve mechanical properties. As a result, stiffness and extensibility are simultaneously increased by ∼19.3 and ∼3.8 times, respectively compared to PVA. The thermal stability with a 2.80-fold larger melting enthalpy of 823.95 ± 7.05 J g−1 than PVA. The great thermomechanical performance provides an insight for molecular design in electrospun nanofibers with chitin polymorphs.

Composites Science and Technology, 2019

A biobased and biocompatible thermoplastic elastomer (TPE) with superior 3D printability was demo... more A biobased and biocompatible thermoplastic elastomer (TPE) with superior 3D printability was demonstrated with great potential for customized manufacturing technologies and fabrication of biointegrated devices. The inherent structural and stereochemical disadvantages of biobased monomers, such as 2,5-furandicarboxylic acid, in comparison with today used petroleum based monomers like terephthalic acid generally lead to lower mechanical performance for the biobased replacement polymers. This is additionally enhanced by poor interfacial adhesion and fusion commonly encountered during customized manufacturing technologies like 3D printing. Herein, we demonstrate
that in-situ polymerization in the presence of trace amounts of nanocellulose can homogeneously distribute the nanofiller leading to dramatically strengthened thermally 3D-printable bio-furan-based TPE. This TPE exhibited a tensile strength of 67 MPa which is 1.5–7-fold higher than the values reported for silicone and thermoplastic urethane,
which are widely used in biomedical applications. In addition, the TPE had an impressive extensibility of 860% and negligible in vivo cytotoxicity; such properties have not been reported to date for bio-based or petrochemical TPEs. While a petrochemical 3D printed TPE counterpart retained only half of the tensile strength compared to the hot-pressed analogue, the 3D-printed biobased TPE in-situ modified with nanocellulose maintained 70–80% of its strength under the same experimental conditions. This is explained by inter-diffusion between interfaces facilitated by the nanocellulose and the furan rings. Using the ergonomic shape of a wrist as a 3D-printable design, we successfully
manufactured a wearable thermal therapeutic device from the nanocellulose modified biobased TPE, giving promise for wide variety of future applications.