HAN Research Group - PUBLICATIONS

Selected Publications

"Solution-Processable and Photo-Curable System for Low-Cost and Scalable Transient Electronics", Nature Communications 16 (2025) 9165
"Stretchable and Biodegradable Self-Healing Conductors for Multifunctional Electronics", Science Advances 10 (2024) eadp9818
"Ultra-Stretchable and Biodegradable Elastomers for Soft, Transient Electronics", Nature Communications 14 (2023) 2263
"Zebra-Inspired Stretchable, Biodegradable Radiation Modulator for All-Day Sustainable Energy Havesters", Science Advances 9 (2023) eadf5883
"Tunable and Scalable Fabrication of Block Copolymer-based 3D Polymorphic Artificial Cell Membrane", Nature Communications 13 (2022) 1261

57. Biodegradable Adhesive Systems for Bio-Integrated Applications

W. B. Han†, S. Han†, P. S. Shewale†, H. Cho, L.-H. Kim, S.-W. Hwang*

Advanced Science (2025) e12633

Biodegradable adhesives, unlike their traditional counterparts, are engineered to bond to biological tissues while naturally degraded over time, thereby eliminating the need for removal procedures and reducing the risk of chronic inﬂammation. These unique features are particularly suitable for temporary biomedical applications such as wound closure, internal sealing, or integration with electronics for active/passive functions. The adhesive performance arises from the strategic combination of biodegradable polymers and adhesion mechanisms that dynamically interact with tissue surfaces. This review introduces recent advancements in biodegradable adhesives through a mechanism-based framework, focusing on ﬁve key adhesion strategies: physical interlocking, hydrogen bonding, catechol chemistry, amine-carboxyl coupling, and covalent bonding via diazirine or isocyanate linkages. For each strategy, representative material systems, functional properties, and biomedical implementations that enable strong, conformal adhesion under wet and physiological environments are highlighted, and with a discussion of current challenges and future directions toward intelligent, multifunctional bioadhesives for clinical uses are concluded.

56. Solution-Processable and Photo-Curable System for Low-Cost and Scalable Transient Electronics

W. B. Han†, S. Han†, T. Kim†, T. -M. Jang, G. -J. Ko, J. Joo, T. Kim, K. -S. Lee, Y. -M. Shon, E. Park, V. R. Naganaboina, S.-W. Hwang*

Nature Communications 16 (2025) 9165

The discovery of non-toxic, bioresorbable silicon electronics is a major breakthrough in the fields of transient, dissolvable biomedical implants and environmental monitors, as it opens up the possibility of producing versatile components based on established semiconductor processes. However, given the limited lifespan of such electronics, it is essential to consider economical manufacturing and production strategies that reduce the unit price for commercialization. Here, we introduce a solution-processable and photopatternable approach that is facile, cost-effective, and widely accessible for a monolithic 3D fabrication of soft, stretchable, and transient electronics. Optimized chemical synthesis and rational materials engineering yield biodegradable/biocompatible organic insulators, semiconductors, and conductors that can be layered/assembled in sophisticated configurations without impairing underlying components. Direct solution-casting of the materials enables the fabrication of sensors with various modalities and transistors. In vivo implantation of soft, conformable electrode arrays into the brain and heart of animal models demonstrates spatiotemporal electrophysiological monitoring (electroencephalography and electrocardiography) and therapeutic interventions (epileptic seizure suppression and cardiac pacing), highlighting the broad applicability in diverse bio-integrated electronic systems.

55. A Hierarchically Structured, Stretchable, Anti-Biofouling Encapsulation for Biodegradable Electronics

W. B. Han†, S. Han†, G. -J. Ko, U. Batjargal, W. Jang, V. R. Naganaboina, H. -J. Kim*, S.-W. Hwang*

Advanced Healthcare Materials (2025) e03622

(Featured on the back cover)

Biodegradable polymers have been employed as encapsulants for transient, resorbable implantable devices due to moderate water permeability, mechanical flexibility and biocompatibility, however most of them relatively lack inherent anti-biofouling properties. This limitation can lead to undesired protein adsorption, cell adhesion, and fibrotic encapsulation, compromising device function and biocompatibility, particularly for long-term implantation scenarios. Here, we introduce a soft, stretchable, and anti-biofouling encapsulant engineered by integrating self-assembled organosilicon nanowire networks onto micropatterned biodegradable elastomers. The resulting hierarchical surface architecture imparts superhydrophobicity while preserving mechanical integrity and enhances water barrier performance by up to 420% compared to unmodified films, with stability retained under cyclic strains. Integration into a transient, stretchable optoelectronic device enables prolonged operation in aqueous environments, and in vitro and in vivo evaluations demonstrate suppressed cell adhesion, reduced fibrotic tissue formation, and excellent biocompatibility, highlighting the potential for long-lasting, bioresorbable electronic implants.

54. Bio-inspired strategy for radiation-based thermal management and utilization

H. R. Kim†, W. B. Han† (equal contribution), S. -Y. Heo†, Y. S. Go, S. -W. Hwang*, Y. M. Song*

Advanced Science (2025) e02851

In nature, biological species have evolved unique mechanisms and sophisticated structures for intelligent radiation-based thermal management and utilization over billions of years through natural selection. These adaptive strategies in biological species serve as a significant source of inspiration for the development of advanced thermal engineering materials and systems in modern society, driving innovation in applications such as building heating and cooling, personal thermal management, water acquisition, and next-generation infrared (IR) sensing systems. In this review, advancements in biological and bio-inspired thermal management strategies, including radiative cooling, thermal regulation, and thermal insulation, are comprehensively summarized. Additionally, recent advancements in radiation-based biological and bio-inspired thermal utilization are discussed, focusing on applications such as water harvesting, IR camouflage, and IR detection. Next, various examples of the integration of IR management strategies with electronic and energy systems are introduced, highlighting their potential to enhance efficiency and functionality in thermal management, energy efficiency, and advanced sensing applications. By leveraging these bio-inspired systems, innovative strategies have emerged, encompassing both thermal management and utilization, and enabling efficient heat regulation and energy harvesting across a wide range of technological applications. Finally, a future perspective on the development of radiation-based bio-inspired thermal management and utilization technologies is provided.

53. Stretchable and biodegradable thermally expandable composites with microfluidics for on-demand and programmable destruction of electronics

C.-H. E.om†, W. B. Han† (equal contribution), S. Han†, S. J. Choi, J. Kim, H. Cho, L. -H. Kim, V. R. Naganaboina, G. -J. Ko, T. -M. Jang, S.-W. Hwang*

Advanced Science (2025) e05487

The lifespan of the transient electronic system can be determined in advance (i.e., predefined) or controlled via on-demand and programmable approaches using a diverse range of principles. However, in most cases, dissolution or disappearance requires an aqueous solution and is only possible for the entire system, not for specific or targeted components. Here, a soft, stretchable, thermally expandable system is introduced for precise, localized, on-demand deactivation or destruction of electronic systems. The incorporation of thermal expansion particles into a polymer matrix produces soft, resilient composites that generate substantial thermo-mechanical forces at a predefined temperature, enabling the direct collapse of electronic devices. Integration with multichannel microfluidics and wireless systems creates a vanishing, self-destructive optoelectronic system and bio-safe drug delivery vehicle for frequency-based selective release, demonstrating the broad potential of this approach in the fields of defense/security and biomedical devices as well as other envisioned areas.

52. Wireless, Multifunctional System-integrated Programmable Soft Robot

S. Han†, J. -W. Shin†, J. H. Lee†, B. Li†, G. -J. Ko, T. -M. Jang, A. Dutta, W. B. Han, S. M. Yang, D. -J. Kim, H. Kang, J. H. Lim, C. -H. Eom, S. Choi, H. Cheng*, and S. -W. Hwang*

Nano-Micro Letters 17 (2025) 152

Soft robots have partially or entirely provided versatile opportunities for issues or roles that cannot be addressed by conventional machine robots, although most studies are limited to designs, controls, or physical/mechanical motions. Here, we present a transformable, reconfigurable robotic platform created by the integration of magnetically responsive soft composite matrices with deformable multifunctional electronics. Magnetic compounds engineered to undergo phase transition at a low temperature can readily achieve reversible magnetization and conduct various changes of motions and shapes. Thin and flexible electronic system designed with mechanical dynamics does not interfere with movements of the soft electronic robot, and the performances of wireless circuit, sensors, and devices are independent of a variety of activities, all of which are verified by theoretical studies. Demonstration of navigations and electronic operations in an artificial track highlights the potential of the integrated soft robot for on-demand, environments-responsive movements/metamorphoses, and optoelectrical detection and stimulation. Further improvements to a miniaturized, sophisticated system with material options enable in situ monitoring and treatment in envisioned areas such as biomedical implants.

51. Long-lasting, flexible and fully bioresorbable AZ31-tungsten batteries for transient, biodegradable electronics

G. -J. Ko†, T. -M. Jang†, D. Shin†, H. Kang†, S. M. Yang, S. Han, R. Kaveti, C. -H. Eom, S. J. Choi, W. B. Han, W. -H. Yeo, A. J. Bandodkar, J. Cho*, and S. -W. Hwang*

Journal of Materials Chemistry A 12 (2024) 32712

(Featured on the inside front cover)

Rapid technological revolution produces a wide range of convenient tools, while, in particular, the production and consumption of batteries lead to various issues including environmental pollution. Although efforts to solve such problems increase interest in green and dissolvable batteries, their short service life is still recognized as a major obstacle due to limited options of materials. Here, we propose materials and system designs for eco-friendly and biodegradable magnesium alloy-tungsten (AZ31-W) batteries that offer long-term stability with enhanced corrosion resistance. Materials and electrochemical inspections confirm the superior electrochemical tolerance and stable, reliable potentials of the AZ31 anode and W cathode. The assembly of an individual cell into a commercially available pouch battery yields a high capacity of ~430 mA h g-1, suitable for high-energy applications. The integration of alginate-based soft, elastic electrolytes with the electrodes enables the achievement of completely eco-resorbable solid-state batteries that maintain performance under diverse physical deformations. The results suggest potential for biomedical and eco-friendly applications where commercial batteries pose risks to the environment or human body.

50. Highly Elastic, Bioresorbable Polymeric Materials for Stretchable, Transient Electronic Systems

J. -W. Shin†, D. -J. Kim†, T. -M. Jang†, W. B. Han† (equal contribution), J. H. Lee, G. -J. Ko, S. M. Yang, K. Rajaram, S. Han, H. Kang, J. H. Lim, C. -H. Eom, A. J. Bandodkar, S. -W. Hwang*

Nano-Micro Letters 16 (2024) 102

Substrates or encapsulants in soft and stretchable formats are key components for transient, bioresorbable electronic systems; however, elastomeric polymers with desired mechanical and biochemical properties are very limited compared to non-transient counterparts. Here, we introduce a bioresorbable elastomer, poly(glycolide-co-ε-caprolactone) (PGCL), that contains excellent material properties including high elongation-at-break (< 1300%), resilience and toughness, and tunable dissolution behaviors. Exploitation of PGCLs as polymer matrices, in combination with conducing polymers, yields stretchable, conductive composites for degradable interconnects, sensors, and actuators, which can reliably function under external strains. Integration of device components with wireless modules demonstrates elastic, transient electronic suture system with on-demand drug delivery for rapid recovery of post-surgical wounds in soft, time-dynamic tissues.

49. Recent advances in soft, implantable electronics for dynamic organs

W. B. Han†, T. -M. Jang†, B. Shin†, V. R. Naganaboina†, W. -H. Yeo, S. -W. Hwang*

Biosensors & Bioelectronics 261 (2024) 116472

Unlike conventional rigid counterparts, soft and stretchable electronics forms crack- or defect-free conformal interfaces with biological tissues, enabling precise and reliable interventions in diagnosis and treatment of human diseases. Intrinsically soft and elastic materials, and device designs of innovative configurations and structures leads to the emergence of such features, particularly, the mechanical compliance provides seamless integration into continuous movements and deformations of dynamic organs such as the bladder and heart, without disrupting natural physiological functions. This review introduces the development of soft, implantable electronics tailored for dynamic organs, covering various materials, mechanical design strategies, and representative applications for the bladder and heart, and concludes with insights into future directions toward clinically relevant tools.

48. Materials and designs for extremely efficient encapsulation of soft, biodegradable electronics

G. -J. Ko†, H. Kang†, W. B. Han† (equal contribution), A. Dutta†, J. -W. Shin, T. -M. Jang, S. Han, J. H. Lim, C. -H. Eom, S. Choi, Y. Ryu, W. -H. Yeo, H. Cheng*, S. -W. Hwang*

Advanced Functional Materials 34 (2024) 2403427

Effective encapsulation is essential for reliable operation of bio-integrated electronics, particularly those containing dissolvable elements, under humid environments for desired periods of time; however, conventional inorganic or organic encapsulants often suffer from tissue-incompatible mechanical rigidity and insufficient water-barrier performance. Here, a mechanically resilient and efficient encapsulation strategy is proposed that can exceed a functional lifetime of state-of-the-art soft encapsulations by several tens of magnitudes. The exceptional protection arises from the high aspect ratio of dissolvable yet impermeable inorganic fillers embedded within biodegradable polymers, which significantly extend the diffusion length of biofluids or water components. Theoretical modeling and experimental analysis elucidate the effects of types, shapes, and concentrations of the fillers on encapsulation performance, as well as mechanical/physical properties. The operation of electronic components under aqueous solutions for prolonged periods demonstrates the practical feasibility of the encapsulation approach for versatile types of soft, biodegradable electronics.

47. Stretchable and biodegradable self-healing conductors for multifunctional electronics

T. -M. Jang†, W. B. Han† (equal contribution), S. Han, A. Dutta, J. H. Lim, T. Kim, B. H. Lim, G. -J. Ko, J. -W. Shin, R. Kaveti, H. Kang, C. -H. Eom, S. J. Choi, A. J. Bandodkar, K. -S. Lee, E. Park, H. Cheng, W. -H. Yeo, S. -W. Hwang*

Science Advances 10 (2024) eadp9818

As the regenerative mechanisms of biological organisms, self-healing provides useful functions for soft electronics or associated systems. However, there have been few examples of soft electronics where all components have self-healing properties while also ensuring compatibility between components to achieve multifunctional and resilient bio-integrated electronics. Here, we introduce a stretchable, biodegradable, self-healing conductor constructed by combination of two layers: (i) synthetic self-healing elastomer and (ii) self-healing conductive composite with additives. Abundant dynamic disulfide and hydrogen bonds of the elastomer and conductive composite enable rapid and complete recovery of electrical conductivity (~1000 siemens per centimeter) and stretchability (~500%) in response to repetitive damages, and chemical interactions of interpenetrated polymer chains of these components facilitate robust adhesion strength, even under extreme mechanical stress. System-level demonstration of soft, self-healing electronics with diagnostic/therapeutic functions for the urinary bladder validates the possibility for versatile, practical uses in biomedical research areas.

46. Recent advances in encapsulation strategies for flexible transient electronics

W. B. Han, S. -W. Hwang, W. -H. Yeo*

Flexible and Printed Electronics 9 (2024) 033001

Transient electronics, designed to dissolve, disintegrate, or degrade in a controlled manner after fulfilling their functions without remaining biologically and environmentally harmful byproducts, have emerged as a transformative paradigm with promising applications in temporary biomedical devices, eco-friendly electronics, and security applications. The success of this device development relies significantly on an effective encapsulation to protect their degradable active materials from environmental factors, such as biofluids and water, and secure reliable device functions throughout a desired lifespan. This review article provides an overview of recent advances in various encapsulation strategies for developing flexible, transient electronics. Details include materials selection, key characteristics, water-barrier capabilities, degradation mechanisms, and relevant applications, categorized into inorganic materials, synthetic/natural polymers, and hybrid composites. In addition, our insights into existing challenges and key perspectives for enhancing encapsulation performance are shared.

45. Water-powered, electronics-free dressings that electrically stimulate wounds for rapid wound closure

K. Rajaram†, M. A. Jakus†, H. Chen, B. Jain, D. G. Kennedy, E. A. Caso, N. Mishra, N. Sharma, B. E. Uzunoğlu, W. B. Han, T. -M. Jang, S. -W. Hwang, G. Theocharidis, B. J. Sumpio, A. Veves, S. K. Sia*, A. J. Bandodkar*

Science Advances 10 (2024) eado7538

Chronic wounds affect ~2% of the U.S. population and increase risks of amputation and mortality. Unfortunately, treatments for such wounds are often expensive, complex, and only moderately effective. Electrotherapy represents a cost-effective treatment; however, its reliance on bulky equipment limits its clinical use. Here, we introduce water-powered, electronics-free dressings (WPEDs) that offer a unique solution to this issue. The WPED performs even under harsh conditions—situations wherein many present treatments fail. It uses a flexible, biocompatible magnesium-silver/silver chloride battery and a pair of stimulation electrodes; upon the addition of water, the battery creates a radial electric field. Experiments in diabetic mice confirm the WPED’s ability to accelerate wound closure and promote healing by increasing epidermal thickness, modulating inflammation, and promoting angiogenesis. Across preclinical wound models, the WPED-treated group heals faster than the control with wound closure rates comparable to treatments requiring expensive biologics and/or complex electronics. The results demonstrate the WPED’s potential as an effective and more practical wound treatment dressing.

44. Synthesis of shape-programmable elastomer for a bioresorbable, wireless nerve stimulator

J. H. Lim†, W. B. Han† (equal contribution), T. -M. Jang†, G. -J. Ko, J. -W. Shin, S. Han, H. Kang, C. -H. Eom, S. J. Choi, K. Rajaram, A. J. Bandodkar, S. -W. Hwang*

Biosensors & Bioelectronics 254 (2024) 116222

Materials that have the ability to manipulate shapes in response to stimuli such as heat, light, humidity and magnetism offer a means for versatile, sophisticated functions in soft robotics or biomedical implants, while such a reactive transformation has certain drawbacks including high operating temperatures, inherent rigidity and biological hazard. Herein, we introduce biodegradable, self-adhesive, shape-transformable poly(L-lactide-co-ε-caprolactone) (BSS-PLCL) that can be triggered via thermal stimulation near physiological temperature (~38 °C). Chemical inspections confirm the fundamental properties of the synthetic materials in diverse aspects, and study on mechanical and biochemical characteristics validates exceptional stretchability up to 800 % and tunable dissolution behaviors under biological conditions. The integration of the functional polymer with a bioresorbable electronic system highlights potential for a wide range of biomedical applications.

43. Soft, Long-Lived, Bioresorbable Electronic Surgical Mesh with Wireless Pressure Monitor and On-demand Drug Delivery

K. Rajaram†, J. H. Lee†, J. K. Youn†, T. -M. Jang, W. B. Han, S. M. Yang, J. -W. Shin, G.-J. Ko, D. -J. Kim, S. Han, H. Kang, A. J. Bandodkar, H. -Y. Kim*, S. -W. Hwang*

Advanced Materials 36 (2024) 2307391

(Featured on the inside front cover)

Current research in the area of surgical mesh implants is somewhat limited to traditional designs and synthesis of various mesh materials, whereas meshes with multiple functions may be an effective approach to address long-standing challenges including postoperative complications. Herein, a bioresorbable electronic surgical mesh is presented that offers high mechanical strength over extended timeframes, wireless post-operative pressure monitoring, and on-demand drug delivery for the restoration of tissue structure and function. The study of materials and mesh layouts provides a wide range of tunability of mechanical and biochemical properties. Dissolvable dielectric composite with porous structure in a pyramidal shape enhances sensitivity of a wireless capacitive pressure sensor, and resistive microheaters integrated with inductive coils provide thermo-responsive drug delivery system for an antibacterial agent. In vivo evaluations demonstrate reliable, long-lived operation, and effective treatment for abdominal hernia defects, by clear evidence of suppressed complications such as adhesion formation and infections. A bioresorbable electronic surgical mesh is presented that offers high mechanical strength over extended timeframes, wireless post-operative pressure monitoring, and on-demand drug delivery for the restoration of tissue structure and function. In vivo evaluations demonstrate reliable, long-lived operation, and effective treatment for abdominal hernia defects, by clear evidence of suppressed complications such as adhesion formation and infections.

42. Stretchable and biodegradable composite films for disposable, antibacterial, radiative cooling system

W. B. Han†, H. Kang†, S. -Y. Heo†, Y. Ryu, G. Kim, G. -J. Ko, J. -W. Shin, T. -M. Jang, S. Han, J. H. Lim, C. -H. Eom, W. -H. Yeo, Y. M. Song*, S. -W. Hwang*

Chemical Engineering Journal 483 (2024) 149388

Materials that inhibit bacteria and viruses, while ensuring thermal comfort and physical ease, can play a significant role in the realm of protective textiles and equipment for outdoor healthcare activities, particularly in the context of recent global pandemics. Here, we develop radiative cooling and antibacterial composites consisting of stretchable, biodegradable poly(lactide-co-ε-caprolactone) (PLCL) and magnesium oxide (MgO) particles via a scalable solution casting process. Precise control over the size and composition of the particles within the polymer matrix through theoretical and experimental analyses achieves sub-ambient daytime radiative cooling (△T, ~7℃) under diverse weather conditions. Bactericidal MgO particles through surface treatments realize a remarkable reduction of bacterial cell viability by ~100 % in 8 h. The overall results suggest that materials with thermal and infection control capability as well as disposable characteristics have the potential to reduce plastic waste beyond the spread of infection.

41. Electric eel-inspired, soft electrocytes for solid-state power systems

W. B. Han†, D. -J. Kim†, Y. M. Kim†, G. -J. Ko, J. -W. Shin, T. -M. Jang, S. Han, H. Kang, J. H. Lim, C. -H. Eom, J. H. Lee, S. M. Yang, K. Rajaram, A. J. Bandodkar, H. C. Moon*, S. -W. Hwang*

Advanced Functional Materials 34 (2024) 2309781

As the demand for power systems, including portable ones, is growing at an ever-faster pace, many studies are approaching to discover innovative materials for current battery technology or replace the existing ones with new systems through mimicking living things or nature. Here, a soft, solid-state power storage system featuring electric eel-inspired artificial electric organs capable of converting the chemical potential of an ionic gradient into electricity is introduced. These organs are constructed through the assembly of low and high ion-concentrated zwitterionic gel films with cation- and anion-selective intermembranes, which generate a rechargeable open-circuit voltage of ≈135 mV. Combined use of a chemically synthesized room-temperature ionic liquid and a high-boiling point organic solvent as ion-conducting electrolyte allows electric organs to withstand extreme temperatures ranging from −20 and 100 °C, while the thin and stretchable constituent layers facilitate mechanical flexibility without compromising electrical performance. Scalable integration of electric organs in series and parallel configurations achieves high levels of voltage and current outputs, and employment of origami folding geometry enables on-demand discharge upon self-registered folding, paving the way for portable, high-voltage energy sources in the fields of wearable electronics and soft robotics.

40. Stretchable and biodegradable triboelectric nanogenerator based on elastomeric nanocomposites

H. Kang†, W. B. Han† (equal contribution), S. M. Yang, J. H. Lee, G. -J. Ko, J. -W. Shin, T. -M. Jang, S. Han, D. -J. Kim, J. H. Lim, C. -H. Eom, K. Rajaram, A. J. Bandodkar, S. -W. Hwang*

Chemical Engineering Journal 475 (2023) 146208

Biologically benign, dissolvable materials-based triboelectric nanogenerators hold significant potential as a sustainable power source for bioresorbable, transient electronic systems; however poor options in materials and engineering approaches are major obstacles to the desired electrical, physical, and mechanical properties, particularly when considering operations under restrictive, demanding conditions or environments. Here, we present an elastomeric composites-based triboelectric nanogenerator with a package of completely degradable materials. Assembly of inorganic nanoparticles with high charge affinity/permittivity and micro-pyramid structures with high surface area produces enhanced charge density and power outputs over those of existing elements. Study on mechanical and biochemical characteristics validates the capability of maintaining stable, long-life electrical outputs under cyclic tests and aqueous solutions. Demonstrations of energy harvesting at an artificial knee and self-powered motion sensing at a finger joint suggest the practical feasibility in versatile areas of biomedical and eco-resorbable electronics.

39. Micropatterned Elastomeric Composites for Encapsulation of Transient Electronics

W. B. Han†, G. -J. Ko†, S. M. Yang†, H. Kang, J. H. Lee, J. -W. Shin, T. -M. Jang, S. Han, D. -J. Kim, J. H. Lim, K. Rajaram, A. J. Bandodkar, S. -W. Hwang*

ACS Nano 17 (2023) 14822

Although biodegradable, transientelectronic devicesmust dissolveor decompose via environmental factors, an effective waterproofing or encapsulation system is essential for reliable, durable operationfor a desired period of time. Existing protection approaches use multipleor alternate layers of electrically inactive organic/inorganic elements combined with polymers; however, their high mechanical stiffness isnot suitable for soft, time-dynamic biological tissues/skins/organs. Here, we introduce a stretchable, bioresorbable encapsulant using nanoparticle-incorporated elastomeric composites with modifications of surface morphology. Nature-inspired micropatterns reduce the diffusion area for water molecules, and embedded nanoparticles impede water permeation, which synergistically enhances the water-barrier performance. Empirical and theoretical evaluations validate the encapsulation mechanisms under strains. Demonstration of a soft, degradable shield with an optical component under a biological solution highlights the potential applicability of the proposed encapsulation strategy.

38. Ultra-Stretchable and Biodegradable Elastomers for Soft, Transient Electronics

W. B. Han†, G. -J. Ko†, K. -G. Lee, D. Kim, J. H. Lee, S. M. Yang, D. -J. Kim, J. -W. Shin, T. -M. Jang, S. Han, H. Kang, J. H. Lim, K. Rajaram, S. H. Kim, Y. -D. Park, S. -W. Hwang*

Nature Communications 14 (2023) 2263

(Bronze prize awarded at ‘Bio Engineering & Life Science’ division of 29th Samsung Humantech Paper Award)

As rubber-like elastomers have led to scientific breakthroughs in soft, stretchable characteristics-based wearable, implantable electronic devices or relevant research fields, developments of degradable elastomers with comparable mechanical properties could bring similar technological innovations in transient, bioresorbable electronics or expansion into unexplored areas. Here, we introduce ultra-stretchable, biodegradable elastomers capable of stretching up to similar to 1600% with outstanding properties in toughness, tear-tolerance, and storage stability, all of which are validated by comprehensive mechanical and biochemical studies. The facile formation of thin films enables the integration of almost any type of electronic device with tunable, suitable adhesive strengths. Conductive elastomers tolerant/sensitive to mechanical deformations highlight possibilities for versatile monitoring/sensing components, particularly the strain-tolerant composites retain high levels of conductivities even under tensile strains of similar to 550%. Demonstrations of soft electronic grippers and transient, suture-free cardiac jackets could be the cornerstone for sophisticated, multifunctional biodegradable electronics in the fields of soft robots and biomedical implants.

37. Wireless, Fully Implantable and Expandable Electronic System for Bidirectional Electrical Neuromodulation of the Urinary Bladder

J. H. Lee†, T. -M. Jang†, J. -W. Shin†, B. H. Lim, K. Rajaram, W. B. Han, G. -J. Ko, S. M. Yang, S. Han, D. -J. Kim, H. Kang, J. H. Lim, K. -S. Lee, E. Park, S. -W. Hwang*

ACS Nano 17 (2023) 8511

Current standard clinical options for patients with detrusor underactivity (DUA) or underactive bladder-the inability to release urine naturally-include the use of medications, voiding techniques, and intermittent catheterization, for which the patient inserts a tube directly into the urethra to eliminate urine. Although those are life-saving techniques, there are still unfavorable side effects, including urinary tract infection (UTI), urethritis, irritation, and discomfort. Here, we report a wireless, fully implantable, and expandable electronic complex that enables elaborate management of abnormal bladder function via seamless integrations with the urinary bladder. Such electronics can not only record multiple physiological parameters simultaneously but also provide direct electrical stimulation based on a feedback control system. Uniform distribution of multiple stimulation electrodes via mesh-type geometry realizes low-impedance characteristics, which improves voiding/urination efficiency at the desired times. In vivo evaluations using live, free-moving animal models demonstrate system-level functionality.

36. Zebra-inspired stretchable, biodegradable radiation modulator for all-day sustainable energy harvesters

W. B. Han†, S. -Y. Heo†, D. Kim†, S. M. Yang, G. -J. Ko, G. J. Lee, D. -J. Kim, K. Rajaram, J. H. Lee, J. -W. Shin, T. -M. Jang, S. Han, H. Kang, J. H. Lim, D. -H. Kim, S. H. Kim, Y. M. Song*, S. -W. Hwang*

Science Advances 9 (2023) eadf5883.

(Online featured in Science Advances, ‘Physical and materials sciences’, February)

Recent advances in passive radiative cooling systems describe a variety of strategies to enhance cooling efficiency, while the integration of such technology with a bioinspired design using biodegradable materials can offer a research opportunity to generate energy in a sustainable manner, favorable for the temperature/climate system of the planet. Here, we introduce stretchable and ecoresorbable radiative cooling/heating systems engineered with zebra stripe-like patterns that enable the generation of a large in-plane temperature gradient for thermo-electric generation. A comprehensive study of materials with theoretical evaluations validates the ability to ac-complish the target performances even under external mechanical strains, while all systems eventually disappear under physiological conditions. Use of the zebra print for selective radiative heating demonstrates an unexpected level of temperature difference compared to use of radiative cooling emitters alone, which enables producing energy through resorbable silicon-based thermoelectric devices. The overall result suggests the potential of scalable, ecofriendly renewable energy systems.

35. Soft, Wireless Electronic Dressing System for Wound Analysis and Biophysical Therapy

S. M. Yang†, H. Kim†, G. -J. Ko†, J. C. Choe, J. H. Lee, K. Rajaram, B. An, W. B. Han, D. -J. Kim, J. -W. Shin, T. -M. Jang, H. Kang, S. Han, K. Lee, S. J. Oh, S. -W. Hwang*

Nano Today 47 (2022) 101685

Advances in wearable technology promise effective strategies to improve the management of patients, particularly systematic wound monitoring tools combined with wireless therapeutic stimulation provide medical and allied health professions with unprecedented insight for inpatients/outpatients beyond traditional nursing environments. Here, we introduce a soft, wireless electronic wound dressing system that can offer both real-time monitoring of wound status, and biophysical therapy for acceleration of wound repairing rates, in a completely integrated form. Synthetic hydrogel-based sensor responds to a biochemical marker (i.e., cathepsin) released from the inflammatory reaction, while electronic sensor arrays collect humidity, pH, and temperature related to the healing process. Miniaturized circuit components serve wireless measurements via Bluetooth interfaces and provide programmed patterns of electrical/optical stimulations that can promote the recovery of damaged tissues. In vivo demonstrations illustrate capabilities of systematic medical care in freely behaving mice, thereby suggesting potential for use in advanced medic-free wound managements.

34. 3D artificial cell membranes as versatile platforms for biological applications

W. B. Han†, D. –H. Kang†, T. S. Kim*

Biochip Journal 16 (2022) 215

Artificial cell membranes, emulating biological membranes, have been used to elucidate the physiological functions of cells and realize biological applications. Advanced methodologies, along with considerations of biological/functional aspects, have enabled the development of various forms/types of artificial cell membranes commonly based on a lipid bilayer. Previous review articles have extensively explored 2-dimensional membranes but lack consideration on 3-dimensional membranes, which exhibit many advantages for biological platforms, such as large surface area, high stability, and ease of observation. Indeed, 3-dimensional membranes can accommodate a higher population of membrane proteins and show a more sensitive response to analytes than planar membranes. This review highlights the developments of artificial cell membranes in terms of structures and fabrication strategies, with a focus on 3-dimensional free-standing lipid membranes, and concludes with remarks on current key issues and challenges to evolve artificial cell membranes to another level.

33. Tunable and scalable fabrication of block copolymer-based 3D polymorphic artificial cell membrane array

D. -H. Kang†, W. B. Han† (equal contribution), H. I. Ryu, N. H. Kim, T. Y. Kim, N. Choi, Y. G. Yu, T. S. Kim*

Nature Communications 13 (2022) 1261

(Highlighted in Nature Communications, ‘Structural biology, biochemistry and biophysics’, March)

Owing to their excellent durability, tunable physical properties, and biofunctionality, block copolymer-based membranes provide a platform for various biotechnological applications. However, conventional approaches for fabricating block copolymer membranes produce only planar or suspended polymersome structures, which limits their utilization. This study is the first to demonstrate that an electric-field-assisted self-assembly technique can allow controllable and scalable fabrication of 3-dimensional block copolymer artificial cell membranes (3DBCPMs) immobilized on predefined locations. Topographically and chemically structured microwell array templates facilitate uniform patterning of block copolymers and serve as reactors for the effective growth of 3DBCPMs. Modulating the concentration of the block copolymer and the amplitude/frequency of the electric field generates 3DBCPMs with diverse shapes, controlled sizes, and high stability (100% survival over 50 days). In vitro protein-membrane assays and mimicking of human intestinal organs highlight the potential of 3DBCPMs for a variety of biological applications such as artificial cells, cell-mimetic biosensors, and bioreactors.

32. Materials and Fabrication Strategies for Biocompatible and Biodegradable Conductive Polymer Composites toward Bio-Integrated Electronic Systems

W. B. Han†, S. M. Yang†, R. Kaveti, S. -W. Hwang*

Advanced Sustainable Systems 6 (2022) 2100075.

Biocompatible and biodegradable polymer composite systems equipped with electrical/mechanical functions have been researched as a tool that can be applicable for bio-integrated electronics to obtain a variety of useful information, through formation of reliable interfaces with soft, non-planar skin and organs in the human body. Although previous review articles have explored various types of organic materials (natural/synthetic polymers), including intrinsically conducting polymers (ICPs) as constituent materials for such composites, there has been a lack of consideration of inorganic materials, which may narrow the scope of material options for diverse electrical/functional properties. Indeed, biocompatible and/or biodegradable inorganic elements exhibit high electrical conductivity, various morphologies for effective percolation networks, cytocompatibility, soft mechanical characteristics compared to ICPs, and many functions such as magnetism, photoluminescence, and electrochemical reactivity. This review introduces recent advances in biologically safe and biodegradable conductive polymer composites, including materials selection, relevant fabrication strategies, and various potential applications, aimed at revealing the potential applicability of inorganic elements in conductive composites, which has not been discussed in other reviews. Finally, it concludes with remarks on key issues and challenges to overcome corrosion and oxidation issues of inorganic metals and to improve the electrical performance of composite systems.

31. Materials, devices, and applications for wearable and implantable electronics

W. B. Han, G. -J. Ko, T. -M. Jang, S. -W. Hwang*

ACS Applied Electronic Materials 3 (2021) 485

Recent advances in the engineering or strategy of materials and device design have established ultrathin, soft, lightweight, and skin-conformable characteristics in wearable/implantable electronic systems, allowing precise, long-term monitoring of biological signals from skin/internal organs while reducing signal artifacts upon daily body motions or other external effects. Such a soft, flexible platform offers an opportunity capable of recording and analyzing diverse physical, chemical, and electrophysiological parameters for clinically useful information in the effective prevention, treatment, and management of illness as well as the preservation of physical and mental well-being. Combination with other peculiar functions such as bioresorbable and self-healing properties can enhance the biosafety/reliability of devices and realize unprecedented applications in the fields of biology and medicine or other areas of interest. This Review summarizes the underlying mechanisms of materials science in terms of a mechanical balance between devices and biological structures, discusses the latest biomedical applications with a focus on technological advances and significance, and concludes with an overview of current challenging points and perspectives for future research directions in wearable/implantable electronics.

30. Biologically safe, degradable self-destruction system for on-demand, programmable transient electronics

J. -W. Shin, J. C. Choe, J. H. Lee, W. B. Han, T. -M. Jang, G. -J. Ko, S. M. Yang, Y. -G. Kim, J. Joo, B. H. Lim, E. Park, S. -W. Hwang*

ACS nano 15 (2021) 19310

The lifetime of transient electronic components can be programmed via the use of encapsulation/passivation layers or of on-demand, stimuli-responsive polymers (heat, light, or chemicals), but yet most research is limited to slow dissolution rate, hazardous constituents, or byproducts, or complicated synthesis of reactants. Here we present a physicochemical destruction system with dissolvable, nontoxic materials as an efficient, multipurpose platform, where chemically produced bubbles rapidly collapse device structures and acidic molecules accelerate dissolution of functional traces. Extensive studies of composites based on biodegradable polymers (gelatin and poly(lactic-co-glycolic acid)) and harmless blowing agents (organic acid and bicarbonate salt) validate the capability for the desired system. Integration with wearable/recyclable electronic components, fast-degradable device layouts, and wireless microfluidic devices highlights potential applicability toward versatile/multifunctional transient systems. In vivo toxicity tests demonstrate biological safety of the proposed system.

29. Advanced Materials and Systems for Biodegradable, Transient Electronics

W. B. Han, J. H. Lee, J. -W. Shin, S. -W. Hwang*

Advanced Materials 32 (2020) 2002211

(Featured on the back cover)

Transient electronics refers to an emerging class of advanced technology, defined by an ability to chemically or physically dissolve, disintegrate, and degrade in actively or passively controlled fashions to leave environmentally and physiologically harmless by-products in environments, particularly in bio-fluids or aqueous solutions. The unusual properties that are opposite to operational modes in conventional electronics for a nearly infinite time frame offer unprecedented opportunities in research areas of eco-friendly electronics, temporary biomedical implants, data-secure hardware systems, and others. This review highlights the developments of transient electronics, including materials, manufacturing strategies, electronic components, and transient kinetics, along with various potential applications.

28. Biodegradable, flexible silicon nanomembranes-based NOx gas sensor system with record-high performance for transient environmental monitors and medical implants

G. -J. Ko, S. D. Han, J. -K. Kim, J. Zhu, W. B. Han, J. Chung, S. M. Yang, H. Chung, D. -H. Kim, S. -W. Hwang*

NPG Asia Materials 12 (2020) 71

A novel transient electronics technology that is capable of completely dissolving or decomposing in certain conditions after a period of operation offers unprecedented opportunities for medical implants, environmental sensors, and other applications. Here, we describe a biodegradable, flexible silicon-based electronic system that detects NO species with a record-breaking sensitivity of 136 Rs (5 ppm, NO2) and 100-fold selectivity for NO species over other substances with a fast response (~30 s) and recovery (~60 s). The exceptional features primarily depend on not only materials, dimensions, and design layouts but also temperatures and electrical operations. Large-scale sensor arrays in a mechanically pliable configuration exhibit negligible deterioration in performance under various modes of applied loads, consistent with mechanics modeling. In vitro evaluations demonstrate the capability and stability of integrated NOx devices in severe wet environments for biomedical applications.

27. Advanced material manufacturing for transient electronics

W. B. Han, G. -J. Ko, J. -W. Shin, S. -W. Hwang*

MRS Bulletin 45 (2020) 113

Transient electronics represents recent technology that can partially or completely degrade, dissolve, or disintegrate under certain conditions in actively and passively controlled ways. They offer applications as eco-friendly alternatives to existing electronic components, implantable biomedical devices, and software/hardware protection systems. The degradable characteristics of materials and circuits, however, lead to various fabrication issues and difficulties in manufacturing complex systems requiring fine and elaborate design layouts and microfabrication procedures under thermally and chemically harsh conditions. Identifying advanced materials and the development of manufacturing processes compatible with established transient materials have been conducted for several years to address these issues. In this article, we focus on recent trends in manufacturing technologies for transient electronic systems, including direct fabrication of electronics on transient substrates using organic–inorganic electronic materials, screen-printing approaches particularly for conductive traces, microfabrication combined with multiple transfer-printing techniques, and large-scale, foundry-compatible technologies.

26. Enhancement of membrane protein reconstitution on 3D free-standing lipid bilayer array in a microfluidic channel

W. B. Han, D. H. Kang, J. H. Na, Y. G. Yu, T. S. Kim*

Biosensors & Bioelectronics 141 (2019) 111404

The bio-sensory organs of living creatures have evolved to have the best sensing performance. They have 3-dimensional protrusions that have large surface areas to accommodate a large number of membrane proteins such as ion channels and G-protein coupled receptors, resulting in high sensitivity and specificity to target molecules. From the perspective of mimicking this system, BLM, which has been used extensively as a platform for a single nanopore-based sensing systems, has some limitations, i.e., some residual solvent, low mechanical stability, small surface area for appropriate stability, and difficulty in high-throughput fabrication. Herein, to eliminate these limitations, a solvent-free, size-controllable, 3-dimensional free-standing lipid bilayer (3DFLB) structure array with high stability (similar to 130 h) and high density (similar to 300,000 cm(-2)) is proposed, and its structural advantages for efficient and rapid protein reconstitution, compared to BLM, is demonstrated by human 5-HT3A receptor assay as well as alpha-hemolysin assay. A continuous process of 3DFLB array fabrication, 5-HT3A reconstitution, and 5-HT detections in a microfluidic channel proves the applicability of the proposed structures as a highly-sensitive sensing platform mimicking bio-sensory organs.

25. Generation of solvent-free 3D lipid structure arrays on high aspect ratio Si microwell substrate

W. B. Han, R. Kwak, J. Y. Kang, T. S. Kim*

Advanced Materials Interfaces 6 (2019) 1801554

Artificial lipid membranes are versatile platforms that are used extensively in biological assays and sensing applications. Particularly, a 2D bilayer lipid membrane (BLM) has been focused on over the last several decades as it can be formed easily on solid supports by various methods. However, 3D lipid structures with structural advantages, such as large surface area that can accommodate a number of proteins and steric conformation that can react with target molecules efficiently, for use as highly sensitive sensors rarely have been studied due to the technical limitations of sealing and stability. Herein, the growth mechanism and condition for the formation of a solvent-free, 3D lipid structure array (approximate to 300 000 ea cm(-2)) tightly bound on a microwell array by a gentle hydration method are investigated. Then, applying an electric field during rehydration step, 3D unilamellar structure array with high uniformity (coefficient of variation (CV): approximate to 4.9%) can be formed. A pore-forming protein assay (alpha-hemolysin) shows that the proposed structures are completely sealed and have biofunctionality that allows them to be used for fluorescent signal measurement, which is essential for biosensing applications. This approach is expected to be a promising first step toward the development of artificial biomimetic sensory organs that have high sensitivity.

24. Tightly sealed 3D lipid structure monolithically generated on transparent SU-8 microwell arrays for biosensor applications

D. H. Kang, W. B. Han, N. Choi, Y. J. Kim, T. S. Kim*

ACS Applied Materials Interfaces 10 (2018) 40401

Artificial lipid membranes are excellent candidates for new biosensing platformsbecause their structures are similar to cell membranes and it is relatively easy to modify thecomposition of the membrane. The freestanding structure is preferable for this purposebecause of the more manageable reconstitution of the membrane protein. Therefore, most ofthe lipid membranes for biosensing are based on two-dimensional structures that are fixed ona solid substrate (unlike floating liposomes) even though they have some disadvantages, suchas low stability, small surface area, and potential retention of solvent in the membrane. In thispaper, three-dimensional freestanding lipid bilayer (3D FLB) arrays were fabricated uniformlyon SU-8 microwells without any toxic solvent. The 3D FLBs have better stability and largersurface area due to their cell-like structure. In order to improve the sealing characteristics ofthe 3D FLBs, the applied frequency of the ac field was controlled during the electroformation.The 3D FLBs were observed through transparent SU-8 microwell arrays using confocalmicroscopy and demonstrated perfect sealing until 5.5 days after the electroformation at morethan 1 kHz. Also, the details of the sealing of a fixed 3D freestanding lipid structure werediscussed for the first time. The unilamellarity and biofunctionality of the 3D FLBs were verifiedby a transport protein (alpha-hemolysin) assay.

23. Adhesion of sputter-deposited Cu/Ti film on plasma-treated polymer substrate

Y. Oh, E. J. Kim, Y. D. Kim, K. S. Choi, W. B. Han, H. S. Kim, C. S. Yoon*

Thin Solid Films 600 (2016) 90

The effect of Ar plasma pretreatment on the adhesion of the sputter-deposited Cu/Ti film, which was used as a seed layer for subsequent electrodeposition of thick Cu film, on an Ajinomoto build-up film (ABF) was evaluated as a function of the plasma power. The Ar plasma pretreatment of the ABF (above 1.2 kW) surface resulted in three-fold increase of the peel-off strength (0.70 kN/m) compared to the untreated sample (0.23 kN/m). The Ar plasma treatment produced a nanoscale worm-like surface roughness on the ABF surface which was responsible for the improved adhesion of the Cu/Ti film. Examination of the fractured surfaces revealed that when the substrate was plasma-treated above 1.2 kW, the fracture occurred in the substrate rather than by delamination of the Cu/Ti film. In fact, the fracture of the ABF substrate, which consists of the SiO2 filler embedded in a polymer resin matrix, proceeded mainly by decohesion of the SiO2 microspheres from the polymer matrix. Hence, to further improve the adhesion of the Cu/Ti film, it is advisable to consider strengthening the interface between the SiO2 filler and the resin matrix through surface modification of the SiO2 microspheres.

22. Magnetocaloric refrigerant with wide operating temperature range based on Mn5-xGe3(Co,Fe)x composite

Y. D. Kim, K. H. Kang, J. H. Kim, K. S. Choi, W. B. Han, H. S. Kim, Y. Oh, C. S. Yoon*

J. Alloy. Compd. 644 (2015) 464

The Curie temperature of Mn5Ge3 was raised or lowered by alloying the compound with Fe or Co; hence, the temperature at which maximum magnetocaloric effect manifested was easily manipulated by forming Mn5-xGe3(Co,Fe)x alloys to ultimately develop a composite magnetic refrigerant consisting of multiple Mn5Ge3-based alloys. The composite refrigerant was designed to produce a table-like ΔSM-T curve and to maximize the RC value near room temperature. Linear combination of the ΔSM-T curves for the constituent materials showed that a composite composed of physical mixture of four different Mn5Ge3-based compounds: Mn5Ge3, Mn5.1Ge2.9, Mn4.75Co0.25Ge3 and Mn4.75Fe0.25Ge3, generated an optimal ΔSM-T curve with a table-like shape. The composite refrigerant with refrigeration capacity of 52 J kg-1, peak entropy change at 300 K, and operating temperature range of 45 K under ΔH = 10 kOe was suitable for room-temperature magnetic refrigeration. Moreover, by changing the relative fraction of each constituent material in the composite, it was shown that the operating temperature range could be fine-tuned as needed. The calculated ΔSM-T curve for the composite refrigerant by linear combination reasonably well matched the experimental ΔSM-T curve estimated from the M-H curves of the composite refrigerant, suggesting that the simple linear combination of the experimental ΔSM-T curves can be used to predict the performance of the composite magnetic refrigerants whose MCE was originated from the second order magnetic transition. In conclusion, being rare-earth free and lacking thermal or magnetic hysteresis, the Mn5Ge3-based composite can be a competitive candidate magnetic refrigerant material operating at room temperature in terms of both economy and reliability compared to the giant MCE materials such as Gd-based intermetallic compounds or Heusler alloys.

21. Magnetocaloric effect of Mn5+xGe3-x alloys

Y. D. Kim, E. J. Kim, K. S. Choi, W. B. Han, H. S. Kim, C. S. Yoon*

J. Alloy. Compd. 620 (2015) 164

Magnetic properties and magnetocaloric effect (MCE) of off-stoichiometric Mn5+xGe3-x (x = -0.1 and 0.3) were studied. Depletion of Mn (x = -0.1) decreased the magnetic moment of the compound due to the reduction of the ferromagnetic Mn-Mn interactions. Mn5.1Ge2.9 appeared to be near the optimal composition since Mn5.1Ge2.9 exhibited increase in the magnetic moment compared to the stoichiometric compound and further Mn-enrichment beyond x = 0.1 led to the formation of the weakly magnetic secondary phase. Mn5.1Ge2.9 produced the maximum entropy change of 2.9 J kg-1 K-1 at 296 K and the refrigeration capacity of 44.5 J kg-1 with an applied field of 10 kOe, making Mn5.1Ge2.9 a possible candidate magnetic refrigerant with reasonably high magnetic entropy change and negligible hysteresis.

20. Molecular dynamics simulation of interlayer water embedded in phospholipid bilayer

W. B. Han, S. J. Kim, H. H. An, H. S. Kim, Y. D. Kim, C. S. Yoon*

Mater. Sci. Eng. C 36 (2014) 49

1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayer with a thin layer of water molecules inserted in the hydrophobic region was simulated at 300 K to observe the pore structure formation during escape of the water molecules from the hydrophobic region. The transformation of the water slab into a cylindrical droplet in the hydrophobic region, which locally deformed the lipid monolayer, was prerequisite to the pore formation. If the thickness of the interlayer water was increased beyond a critical value, the local deformation was suppressed as such deformation would rupture the lipid bilayer. Hence, it was demonstrated that the pore structure formation or local permeability of the lipid membrane is closely related to the rigidity of the lipid membrane.

19. Stabilization of solid-supported phospholipid multilayer against water by gramicidin addition

W. B. Han, Y. D. Kim, H. H. An, H. S. Kim, C. S. Yoon*

J. Phys. Chem. B 118 (2014) 3035

It was demonstrated that hydrophobicity of solid supported planar dipalmitoyl phosphatidylcholine (DPPC) phospholipid multilayer can be greatly increased by incorporating a transmembrane protein, gramicidin, into the DPPC membrane. The contact angle of deionized water droplet on the gramicidin-modified DPPC membrane increased from 0° (complete wetting) without gramicidin to 55° after adding 15 mol % gramicidin. The increased hydrophobicity of the gramicidin-modified DPPC membrane allowed the membrane to remain stable at the air/water interface as well as underwater. The Au nanoparticles deposited on the gramicidin-modified DPPC membrane reproduced the characteristic surface plasmon resonance peak after being kept underwater or in phosphate-buffered saline solution for 5 days, attesting to the membrane stability in an aqueous environment. The enhanced underwater stability of the lipid multilayer substantially broadens the potential application of the lipid multilayer which includes biosensing, enzymatic fuel cell, surface enhanced Raman spectroscopy substrate.

18. Preparation of SERS active Ag nanoparticles encapsulated by phospholipids

H. H. An, W. B. Han, Y. D. Kim, H. S. Kim, Y. Oh, C. S. Yoon*

J. Raman Spectrosc. 45 (2014) 292

A cost-effective way of fabricating lipid-coated surface-enhanced Raman spectroscopy (SERS) substrate having reproducible high SERS activity was proposed. Ag nanoparticle embedded in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) membranes was produced by direct deposition of a 5-nm-thick layer of Ag onto the solid-supported phospholipid membrane, and subsequent dissolution of the Ag nanoparticle-embedded membrane in iso-octane allowed easy one-pot fabrication of DOPC- or DOTAP-coated Ag nanoparticles. In particular, DOTAP produced nearly monodisperse lipid-encapsulated Ag nanoparticles (9 nm in diameter) exhibiting reproducible high SERS activity (detecting up to 10 nM of rhodamine 6G and 0.5 μM of glutathione). In addition, the process was modified to incorporate variety of Raman active molecules (rhodamine 6G, malachite green, 4-aminothiopheonol, 4-mercaptopyridine) into the particle-encapsulating lipid bilayer. The DOTAP/Raman dye-coated Ag nanoparticles also generated high SERS activity to enable potential application of the DOTAP/Raman dye-coated Ag nanoparticles feasible in different areas.

17. Room-temperature magnetocaloric effect of Ni-Co-Mn-Al Heusler alloys

Y. D. Kim, E. J. Kim, K. S. Choi, W. B. Han, H. S. Kim, Y. Shon, C. S. Yoon*

J. Alloy. Compd. 616 (2014) 66

Magnetocaloric effect of the Ni-Co-Mn-Al quaternary alloys was evaluated with the aim of fine-tuning the composition such that the temperature at which the maximum entropy change (|ΔSM|pk) occurs coincided with room temperature. Out of the three alloys tested (Ni41.5Co8.5Mn32Al18, Ni41Co9Mn32.5Al17.5 and Ni41Co9Mn32Al18), Ni41Co9Mn32Al18 was optimal with |ΔSM|pk of 2.1 J kg-1 K-1 at 10 kOe observed at 300 K as designed and the refrigeration capacity (RC) was 52 J kg-1. Both values do compare favorably with other Heusler-type alloys, considering the relatively low material cost of the Ni-Mn-Al alloy. In addition, Ni41.5Co8.5Mn32Al18 and Ni41Co9Mn32Al18 alloys were chemically joined (using an epoxy) to attain |ΔSM|pk and RC values that nearly corresponded to the average values of the original alloys, providing a method for further tailoring the magnetic properties of the refrigerant in addition to the refinement of the composition.

16. Deposition temperature dependence of titanium oxide thin films grown by remote-plasma atomic layer deposition

J. S. Lee, S. J. Lee, W. B. Han, H. Y. Jeon, J. G. Park, W. C. Jang, C. S. Yoon, H. T. Jeon*

Phys. Status Solidi A 210 (2013) 276

Titanium dioxide (TiO2) thin films were deposited by remote-plasma atomic layer deposition (RPALD). The process window was determined in the range from 150 to 300 °C for atomic layer deposition of TiO2 thin film. The crystal structure and grain size of the TiO2 thin films deposited by RPALD was controlled via the variations of the deposition temperature and post-deposition thermal annealing. The as-deposited TiO2 thin film grown at 150 °C was amorphous whereas the TiO2 thin films grown above 200 °C were polycrystalline, consisting of anatase phase. As the deposition temperature increased, the grain size of the anatase phase progressively decreased. Meanwhile, when annealed at 900 °C, the amorphous TiO2 thin film deposited at 150 °C crystallized into anatase structure. The film deposited at 200 °C retained the anatase structure up to 900 °C while incurring minimal grain growth. However, the post-annealed TiO2 thin films deposited at 250 and 300 °C partially transformed to the rutile structure, resulting in a mixture of anatase and rutile phases. It is speculated that the relatively large grain size of the films deposited below 200 °C likely suppressed the anatase → rutile transformation during annealing as the reduction of total fraction of grains boundaries, which acted as primary nucleation sites for the rutile transition, delayed the anatase → rutile transformation.

15. Phase transitions and magnetocaloric effect of Ni1.7Co0.3Mn1+xAl1-x Heusler alloys

Y. D. Kim, W. B. Han, H. S. Kim, H. H. An, C. S. Yoon*

J. Alloy. Compd. 557 (2013) 265

Structural and magnetic phase transitions of Ni1.7Co0.3Mn1+xAl1-x (x = 0.22-0.3) alloys were characterized to evaluate the magnetocaloric effect (MCE) of the alloys. The Curie temperature remained at ~360 K regardless of the composition whereas the Al fraction determined the temperature and extent of the martensitic transition. The alloy with x = 0.26 had the highest peak magnetic entropy change (|ΔSM|pk) of 2.1 J kg-1 K-1 and refrigerant capacity (RC) of 49 J kg-1 with ΔH = 10 kOe, originating from the martensitic transformation. The |ΔSM|pk and RC values were comparable to those of other Ni-Mn-X alloys. The alloy with x = 0.3 may also serve as a magnetic refrigerant near room temperature in spite of its relatively low |ΔSM|pk and RC values. Our result suggests that it should be possible to fine-tune the composition of the quaternary NiCoMnAl alloy to attain optimal magnetocaloric effect near room temperature.

14. Structure and magnetic properties of low-temperature annealed Ni-Mn-Al alloys

Y. D. Kim, S. J. Lee, W. B. Han, H. S. Kim, H. H. An, C. S. Yoon*

J. Appl. Phys. 113 (2013) 17B102

Off-stoichiometric Ni2Mn2-xAlx (x = 0.5-1.2) and Ni2+y(Mn1.3Al0.7)1-y/2, (y = 0.04-0.12) alloys were prepared to locate the composition range which is likely to form the ferromagnetic ordered L21 phase through low-temperature thermal annealing. In the Ni2Mn2-xAlx (x = 0.5-1.2) series, the alloy partially transformed to the ferromagnetic phase in the narrow composition range of x = 0.8-1.1. For the Ni-enriched composition series, Ni2+y(Mn1.3Al0.7)1-y/2, (y = 0.04-0.12), magnetic ordering was not observed and the magnetic susceptibility proportionally decreased with the Ni concentration. It was concluded that moving away from stoichiometric composition would likely increase the disordering of Mn and Al site occupation and is not conducive to achieving ferromagnetism in Ni2MnAl.

13. Coalescence and Polygonization of Au Nanoparticles Embedded in Liquid-Crystalline Lipid Membrane

S. J. Lee, H. H. An, W. B. Han, H. S. Kim, Y. D. Kim, C. S. Yoon*

J. Nanosci. Nanotechnol. 13 (2013) 6150

Coarsening behavior of Au nanoparticles (AuNPs) embedded in a liquid crystalline lipid (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP) membrane was investigated by heat treating the AuNP-embedded DOTAP membrane at 80 °C with 15% and 80% relative humidity (RH). The coarsening rate was 〈D〉 ~t 0.6 regardless of the humidity; however, the spatial distribution and the coarsening mechanism differed depending on the humidity. In addition, extended treatment at 15% RH resulted in formation of large polygonized AuNPs from the lipid segregation.

12. Effect of in-situ application of ultrasonic waves during formation of silver nanoparticles embedded in phospholipid membrane

Y. D. Kim, H. H. An, W. B. Han, H. S. Kim, S. J. Kim, C. S. Yoon*

J. Appl. Phys. 114 (2013) 144702

Effect of in-situ application of ultrasonic waves (up to 1 MHz) on the Ag nanoparticles spontaneously produced inside the 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) membrane by thermal evaporation of Ag was studied by placing the membrane on a ultrasonic transducer during the metal deposition. Application of the ultrasonic vibration promoted spatial ordering of the deposited nanoparticles due to the induced phase transition from Lα to HII for DOPE. Arising from the agitation effect, particle size refinement, which depended on the amplitude of the ultrasonic vibration, was observed. It was also shown that a stiff gel state 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane can be made locally permeable to incident Ag atoms by introducing DOPE molecules into the DPPC membrane as the Ag nanoparticles preferentially nucleated in the DOPE-rich region. Application of ultrasonic vibration with increasingly higher amplitude or frequency made the Ag nanoparticles uniformly distributed in the DPPC, suggesting that the permeability of the DPPC membrane can be temporarily increased without permanently damaging the membrane by addition of liquid crystalline lipids and subsequent application of ultrasonic waves.

11. Deposition of metal nanoparticles on phospholipid multilayer membranes modified by gramicidin

W. B. Han, Y. D. Kim, H. H. An, H. S. Kim, C. S. Yoon*

Langmuir 29 (2013) 13251

A planar dipalmitoyl phosphatidylcholine (DPPC) multilayer phospholipid membrane was structurally modified by introducing a transmembrane protein, gramicidin (up to 25 mol %), to study its effect on the metal nanoparticles deposited on the membrane. Without gramicidin, when 3-nm-thick Ag, Sn, Al, and Au were deposited, the nanoparticles hardly nucleated on the DPPC membrane in rigid gel state (except for Au); however, the gramicidin addition dramatically enhanced the DPPC membrane surface’s affinity for metal atoms so that a dense array of metal (Ag, Sn, and Au) or metal-oxide (Al-oxide) nanoparticles was produced on the membrane surface. The particle sizes ranged from 3 to 15 nm depending on the metal and gramicidin concentration, whereas the particle density was strongly dictated by the gramicidin concentration. The proposed method provides a convenient, generally applicable synthesis route for preparing different metal or metal-oxide nanoparticles on a relatively robust biocompatible membrane.

10. Surface-enhanced Raman scattering substrate based on silver nanoparticle-deposited phospholipid multilayer

H. H. An, Y. D. Kim, W. B. Han, H. S. Kim, S. Lee, S. C. Yi, D. H. Kim, C. S. Yoon*

Appl. Surf. Sci. 287 (2013) 369

Surface enhanced Raman scattering (SERS) substrate was fabricated by directly depositing Ag onto a multilayer lipid membrane consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. The Ag nanoparticles were spontaneously formed on the POPC membrane surface during deposition and using 4-mercaptopyridine as a probe molecule, significant SERS enhancement was observed on the Ag nanoparticle-deposited POPC membrane over the Ag nanoparticles without POPC. It was conjectured that the presence of the POPC molecules assisted in forming a monolayer coverage of the analyte molecules on the nanoparticle surface, hence bringing the adsorbate molecules in closer proximity with the metal surface.

9. Effect of plasma etching on photoluminescence of SnOxSn nanoparticles deposited on DOPC lipid membrane

H. H. An, S. J. Lee, S. H. Baek, W. B. Han, Y. H. Kim, C. S. Yoon, S. H. Suh*

J. Coll. Interf. Sci. 368 (2012) 257

The photoluminescence characteristic of the SnOx/Sn nanoparticles deposited on a solid supported liquid–crystalline phospholipid (1,2-dioleoyl-sn-glycero-3-phosphocholine) membrane was probed after plasma etching the nanoparticle monolayer. It was shown that the plasma etching of the nanoparticle surface greatly altered the particle morphology and enhanced the PL effect, especially when the particle size was below 10 nm in spite of strong presence of surrounding carbon. The enhancement mainly stemmed from the growth of a new PL peak due to the additional defect states produced on the nanoparticle surface by the plasma etching. It was also shown that hydrating the SnOx/Sn nanoparticles similarly improved the PL response of the nanoparticles as the hydration produced an additional oxygen-rich oxide layer on the particle surface.

8. Effect of Temperature and Humidity on Coarsening Behavior of Au Nanoparticles Embedded in Liquid Crystalline Lipid Membrane

S. J. Lee, H. H. An, W. B. Han, H. S. Kim, C. S. Yoon*

Langmuir 28 (2012) 10980

Coarsening behavior of the Au nanoparticles produced by thermal evaporation of Au onto a liquid crystalline lipid (1,2-dioleoyl-3-trimethylammonium-propane, DOTAP) membrane was investigated by subjecting the nanoparticle-embedded DOTAP membrane to two different annealing conditions (at 100 °C under no humidity and at 20 °C and 80% relative humidity). Although the coarsening rate was relatively slow because of the low temperature (from 5.6 nm in the as-deposited state to ~7 nm after 30 h), it was identified that at 100 °C without humidity the Au nanoparticles resulted in shape refinement whereas the high humidity at 20 °C induced self-organization of the nanoparticles into a monolayer. It was also found that annealing in both cases tended to segregate the lipid molecules from the nanoparticle array and forced the nanoparticles into a tighter area. In the case of the high-humidity sample, the lipid segregation eventually led to extensive coalescence of the Au nanoparticles.

7. Effect of crystal structure and grain size on photo-catalytic activities of remote-plasma atomic layer deposited titanium oxide thin film

J. S. Lee, S. J. Lee, W. B. Han, H. Y. Jeon, J. G. Park, H. J. Kim, C. S. Yoon, H. T. Jeon*

J. Solid State Sci. Techn. 1 (2012) Q63

The photo-catalytic activities of TiO2 thin films grown by remote plasma atomic layer deposition (RPALD) were investigated as a function of grain size and crystal structure. As the deposition temperature increased, it was shown that grain size of the anatase phase in the as-deposited TiO2 thin films progressively decreased. When the TiO2 films grown by RPALD were annealed at 900°C, the anatase TiO2 phase with relatively small grains provided large number of nucleation sites for formation of more stable rutile phase. On the other hand, the phase transition to rutile was retarded due to the reduction in grain boundaries in the case of the anatase TiO2 films with large grains. Thus, the anatase TiO2 phase with large grains has excellent thermal stability against the phase transition to rutile. Because the large grain boundary volume is a primary cause of degradation of photo-catalytic activity, the photo-catalytic activity of the TiO2 thin film was maximized when the anatase phase with large grains was formed in the TiO2 thin film. Therefore, RPALD can be effectively utilized to control the grain size of the as-deposited anatase TiO2 thin film with high thermal stability for potential application of photo-catalysts.

6. Coarsening of Au nanoparticles embedded in solid-supported lipid membrane at 80℃ under different humidity

S. J. Lee, H. H. An, H. S. Kim, W. B. Han, C. S. Yoon*

Colloids Surf., A 409 (2012) 138

Au nanoparticles (AuNPs) embedded in a lipid membrane were produced by direct deposition of Au on 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) membranes in liquid crystalline state. The AuNPs embedded in DOTAP and DOPC membranes coarsened quickly at 80 °C in a humid environment (50% relative humidity) after 10 h (5.6 ± 1.5 → 8.7 ± 1.7 for DOTAP and 5.9 ± 1.8 → 10.4 ± 3.3 for DOPC). On the other hand, in a dry environment in which the lipid mobility was retarded the coarsening of the AuNPs was nearly completely suppressed at 80 °C. Meanwhile, the shape of the AuNPs annealed in a dry environment tended to be spherical whereas the final shape was dictated by the lipid head chemistry in the humid environment. Although polydispersity of the prepared AuNPs does not match the AuNPs fabricated using some of state-of-art techniques currently available, the annealing the AuNPs at a moderate temperature can be used as a means of tailoring the size and shape of the AuNPs distributed in a monolayer over a large area (2 cm × 2 cm).

5. Structure of solid-supported lipid membrane probed by noble metal nanoparticle deposition

H. H. An, S. J. Lee, H. S. Kim, W. B. Han, C. S. Yoon*

Biochim. Biophys. Acta 1818 (2012) 2884

Direct deposition of a noble metal layer onto a solid-supported membrane was proposed as an indirect microscopy tool to visually observe different lipid phases that may develop in the lipid membrane. The method relied on the different permeability of the lipid membrane towards the incident atoms during deposition. Liquid state or structural defects such as phase boundaries, step ledges in a multi-lamellar stack, and pores permitted the metal atoms to penetrate and nucleate inside the membrane whereas rigid gel state was relatively impermeable to the incident atoms, thus enabling visualization of liquid phase or structural defects inside the gel state. Based on the proposed method, we demonstrated the phase states resulting from thermotropic transitions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), dioleoylphosphatidylethanolamine (DOPE)/DPPC mixture, and 1,2-dioleoyl-3-trimethylammonium propane (DOTAP). Although the proposed method does not allow in-situ observation of equilibrium states, the method should be an excellent complementary tool for visualizing the lipid phases as the method can resolve fine structural details (up to tens of nanometer scale) as seen in the DPPC membrane while providing macroscopic images (up to several micrometers).

4. Effect of precipitates on hot ductility behavior of steel containing Ti and Nb

W. B. Han, J. H. Lee, H. S. Kim, H. H. An, S. J. Lee, S. W. Kim, S. J. Seo, C. S. Yoon*

Korean J. Met. Mater. 50 (2012) 285

Hot ductility behavior of precipitation-hardened low-carbon iron alloys containing 0.02 wt% Ti and 0.05 wt% Nb was characterized by a hot tensile stress test. Carbon (0.05, 0.1, 0.25 wt%) and boron (0.002 wt%) contents were varied to study the effect of precipitates on the high-temperature embrittlement of the alloys in the temperature range of 600~800℃. Ductility loss was observed at 700℃ for the tested alloys. The cause of the ductility loss was mainly attributed to the carbides and ferrite films formed at the grain boundaries during deformation. Although the carbon content tended to raise the total fraction of Nb (C, N), the precipitates were formed mostly in the grain interior as the precipitation temperature was raised above the deformation temperature by the high carbon content. Hence, carbon in excess suppressed the hot ductility loss. Meanwhile, boron addition improved the hot ductility of the alloys. The improvement is likely due to the boron atoms capturing carbon atoms and thus retarding the carbide formation.

3. Facile method of fabricating Sn nanoparticle monolayer using solid-supported liquid-crystalline phospholipid membrane

H. H. An, J. H. Kim, S. J. Lee, W. B. Han, J. H. Lee, H. Kim, S. H. Suh, I. T. Yoon, Y. Shon, C. S. Yoon*

Appl. Surf. Sci. 257 (2011) 8702

1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) phospholipid membrane was used to fabricate a tightly packed 2-dimensional array of Sn nanoparticles through direct deposition of Sn on the DOPC membrane. Major advantage of the proposed method was that the vertical ordering of the nanoparticles extended to a centimeter (2 cm × 2 cm substrate) scale in the as-prepared state. It was also shown that the particle size and morphology were altered depending on processing conditions. Experimental evidences indicated that the Sn nanoparticle surface, which spontaneously oxidized during deposition, was encapsulated by the DOPC molecules. After removing the encapsulating lipid layer, the oxide-covered Sn nanoparticles exhibited strong photoluminescence. It was also demonstrated that the Sn particle morphology and ordering are related to the lipid membrane structure and chemistry. The proposed method can be easily extended to other metals that are susceptible to oxidation to produce various metal oxide nanoparticles.

2. Magnetocaloric effect of Fe64Mn15-xCoxSi10B11 amorphous alloys

J. H. Lee, S. J. Lee, W. B. Han, H. H. An, C. S. Yoon*

J. Alloy. Compd. 509 (2011) 7764

The magnetocaloric effect (MCE) of melt-spun Fe64Mn15-xCoxSi10B11 amorphous alloys with x = 0, 0.2, 0.5, 0.7, and 1.0 was evaluated with the aim of maximizing the magnetic entropy change close to the room temperature. The peak magnetic entropy change, for Fe64Mn15Si10B11 at 309 K at H = 15 kOe was limited to 0.82 J/kg K. The magnetic properties and ensuing MCE were very sensitive to the Co content that it is conjectured that with a smaller amount of Co addition the can be optimized while maintaining the Curie temperature near room temperature. It was also shown that the Fe64Mn15-xCoxSi10B11 amorphous alloys obeyed the master curve behavior as the magnetic entropy change curve collapsed on to a single universal curve.

1. Polypyrrole-modified graphitized carbon black as a catalyst support for methanol oxidation

S. S. Jeon, W. B. Han, H. H. An, S. S. Im, C. S. Yoon*

Appl. Catal. A-gen 409-410 (2011) 156

Surface-modified graphitized carbon black (GCB) was produced through carbonization of polypyrrole (Ppy) on the GCB surface to be used as a catalyst support for Pt nanoparticles in electro-oxidation of methanol. The Ppy modification led to the enhancement of the catalytic activity for the Pt catalysts. The improvement is largely attributed to uniform dispersion of the Pt nanoparticles as the Ppy modification appears to reduce the agglomeration of the Pt nanoparticles. The chronoamperometry and the accelerated cycling tests indicated that the Ppy modification did not improve the long-term stability of the GCB support for Pt.

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