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2024

"Material-Efficient Multimaterial Projection Micro-stereolithography Using Droplet-Based Resin Supply"

Jay Tobia, Chen Yang, Jason Kim, Daehoon Han* & Howon Lee*
International Journal of Precision Engineering and Manufacturing-Green Technology (2024)

This paper presents a material-efficient multimaterial projection micro-stereolithography (PμSL), a digital light processing (DLP) additive manufacturing process for printing microstructures. We present a droplet-based resin supply system to address the issue of excessive material waste of the multimaterial PμSL. By depositing droplets of different liquid resins, 3D printing of a microstructure can still be performed without the need for a traditional vat while printing materials can be switched with minimal material consumption. Precise control of small droplet volume is obtained by pressure control of the resin injection nozzles, exact opening times of fluid valves, and appropriate surface coatings in order to portion droplets so that just enough material is brought to the build area. Since PμSL enables micro 3D printing (in-plane resolution of 76 μm), PμSL using droplet-based resin supply module provides multimaterial micro 3D printing with low material consumption. Also reported is that material bleeding, which degrades the printing resolution during multimaterial printing, can be minimized by using a cleaning droplet system. We present 3D printing of highly complex multimaterial 3D microstructures using three different photocurable polymers, demonstrating a material efficiency of 11.4%, which is 500 times higher than that of a previously reported PμSL process using dynamic fluidic control.

2023

"Tunable thermal transport in 4D printed mechanical metamaterials"

Charles Abdol-Hamid Owens, Yueping Wang, Shiva Farzinazar, Chen Yang, Howon Lee*, Jaeho Lee*
Materials & Design , 231 (2023)

Here the authors present an active thermal control system using 4D printed shape memory polymers and demonstrate how distinct deformation mechanisms lead to unique, tunable thermal properties using stretching- and bending-dominated architectures. Infrared thermography measurements with varying temperature and compression settings show that at low strains, radiation drives the effective conductance increase as the view factors among the struts increase with increasing strain, and at higher strains, conduction drives the effective conductance increase as the strut-to-strut contact areas increase. The effective thermal conductance increases from 4.41mW/K to 14.52mW/K and from 3.23mW/K to 10.48mW/K for the Kelvin foam and octet-truss microlattices, respectively, as strain increases from 0% to approximately 70%. As the strain is adjusted, the stretching-dominated octet-truss architecture exhibits abrupt changes in shape and conductance due to buckling. The bending-dominated Kelvin foam architecture allows for gradual geometric changes and precise tuning of thermal conductance. These findings provide a new understanding of thermal transport phenomena in 4D-printed metamaterials, which may be a breakthrough in tunable thermal systems.

"Additively Manufactured Mechanical Metamaterial-Based Pressure Sensor with Tunable Sensing Properties for Stance and Motion Analysis"

Hang-Gyeom Kim, Sugato Hajra, Howon Lee, Namjung Kim*, Hoe Joon Kim*
Advanced Engineering Materials , 2201499 (2023)

Mechanical metamaterials are attracting considerable attention due to theirunique properties not found in natural materials. Advanced geometrical shapessuch as Menger cubes, origami templates, and gyroids offer exciting avenues fordevice engineering. In addition, the recent developments of various additive manufacturing technologies have expanded materials selection and geometrical complexities. Herein, a piezoresistive pressure sensor based on a 3D-printedgyroid structure with a conformal coating of carbon nanotubes (CNTs) is pre-sented. The gyroid structures are printed using fused deposition modeling (FDM)3D printing with thermoplastic polyurethane (TPU), providing mechanical robustness even at low densities. By altering the relative density of the gyroidstructure, Young’s modulus can be tailored, ranging from 0.32 MPa at 30% relative density and 3.61 MPa at 80% relative density. The presented gyroid-based pressure sensor achieves a wide sensing range of up to 1.45 MPa and a high sensitivity of 2.68 MPa-1. The sensor is integrated into a shoe for wearable applications, demonstrating its mechanical robustness and potential for humanstance and motion monitoring.

2022

"Recent Advances in Molecular Programming of Liquid Crystal Elastomers with Additive Manufacturing for 4D Printing"

Yueping Wang, Jongwon An, and Howon Lee*
Molecular Systems Design & Engineering , 7 , 1588 - 1601 (2022)

Liquid crystal elastomers (LCEs) are crosslinked polymers within which liquid crystal molecules are linked with elastomeric polymer chains. Various anisotropic materials properties of LCEs can be created through an orientational control of mesogens during fabrication process. In particular, LCEs exhibit reversible mechanical deformation along the direction of mesogen alignment in response to various external stimuli. Therefore, when properly associated with a suitable additive manufacturing process, LCEs can be fabricated to a 3D object that displays programmed stimuli-responsive deformation. Thus, LCEs offer a potential breakthrough in 4D printing as an alternative material solution that overcomes the limitations of typical 4D printing materials such as shape memory polymers and hydrogels. The orientational order of mesogens in LCEs can be controlled by a wide range of methods, including mechanical stretch, viscous shear flow, magnetic/electric-field, and surface treatment. We review herein the physical principles of the key methods for LCE molecular programming and recent advances in additive manufacturing processes that utilize these principles to enable 4D printing with LCEs. Various applications of additively manufactured LCEs are also highlighted.

"Thermal transport in 3D printed shape memory polymer metamaterials"

Shiva Farzinazar, Yueping Wang, Charles Abdol-Hamid Owens, Chen Yang, Howon Lee*, and Jaeho Lee*
APL Materials , 10 , 081105 (2022)

Shape memory polymers are gaining significant interest as one of the major constituent materials for the emerging field of 4D printing. While 3D-printed metamaterials with shape memory polymers show unique thermomechanical behaviors, their thermal transport properties have received relatively little attention. Here, we show that thermal transport in 3D-printed shape memory polymers strongly depends on the shape, solid volume fraction, and temperature and that thermal radiation plays a critical role. Our infrared thermography measurements reveal thermal transport mechanisms of shape memory polymers in varying shapes from bulk to octet-truss and Kelvin-foam microlattices with volume fractions of 4%–7% and over a temperature range of 30–130 °C. The thermal conductivity of bulk shape memory polymers increases from 0.24 to 0.31 W m−1 K−1 around the glass transition temperature, in which the primary mechanism is the phase-dependent change in thermal conduction. On the contrary, thermal radiation dominates heat transfer in microlattices and its contribution to the Kelvin-foam structure ranges from 68% to 83% and to the octet-truss structure ranges from 59% to 76% over the same temperature range. We attribute this significant role of thermal radiation to the unique combination of a high infrared emissivity and a high surface-to-volume ratio in the shape memory polymer microlattices. Our work also presents an effective medium approach to explain the experimental results and model thermal transport properties with varying shapes, volume fractions, and temperatures. These findings provide new insights into understanding thermal transport mechanisms in 4D-printed shape memory polymers and exploring the design space of thermomechanical metamaterials.

"Experiments and modeling of the thermo-mechanically coupled behavior of VHB"

Keven Alkhoury, Nikola Bosnjak, Yueping Wang, Howon Lee, Siva Nadimpalli, and Shawn A. Chester*
International Journal of Solids and Structures , 242 , 111523 (2022)

Temperature is well known to have a significant effect on the overall mechanical performance of viscoelas-tomers. In this work, we investigate the thermo-mechanically coupled behavior of VHB 4910 using a combinedexperimental and modeling approach. We first characterize the material behavior by performing a set of largedeformation uniaxial experiments at different temperatures. We then model the observed thermo-mechanicalbehavior and calibrate that model to the uniaxial experiments. Lastly, the model is implemented as a usermaterial subroutine in a finite element package for validation purposes. A key finding of this work is that whileincreasing the temperature stiffens the elastic contribution, it concurrently reduces the viscoelastic contributionto the overall behavior of VHB.

2021

"High resolution stereolithography fabrication of perfusable scaffolds to enable long-term meso-scale hepatic culture for disease modeling"

Pierre Sphabmixay, Micha Sam Brickman Raredon, Alex J-S Wang, Howon Lee, Paula T Hammond, Nicholas X Fang, and Linda G Griffith*
Biofabrication , 13 , 045024 (2021)

Microphysiological systems (MPS), comprising human cell cultured in formats that capture features of the three-dimensional (3D) microenvironments of native human organs under microperfusion, are promising tools for biomedical research. Here we report the development of a mesoscale physiological system (MePS) enabling the long-term 3D perfused culture of primary human hepatocytes at scales of over 106 cells per MPS. A central feature of the MePS, which employs a commercially-available multiwell bioreactor for perfusion, is a novel scaffold comprising a dense network of nano- and micro-porous polymer channels, designed to provide appropriate convective and diffusive mass transfer of oxygen and other nutrients while maintaining physiological values of shear stress. The scaffold design is realized by a high resolution stereolithography fabrication process employing a novel resin. This new culture system sustains mesoscopic hepatic tissue-like cultures with greater hepatic functionality (assessed by albumin and urea synthesis, and CYP3A4 activity) and lower inflammation markers compared to comparable cultures on the commercial polystyrene scaffold. To illustrate applications to disease modeling, we established an insulin-resistant phenotype by exposing liver cells to hyperglycemic and hyperinsulinemic media. Future applications of the MePS include the co-culture of hepatocytes with resident immune cells and the integration with multiple organs to model complex liver-associated diseases

"Multimaterial Printig for Cephalopod-Inspired Light-Responsive Artificial Chromatophores"

Daehoon Han†, Yueping Wang† († equal contribution), Chen Yang, and Howon Lee*
ACS Applied Materials & Interfaces , 13 , 12735 (2021)

Cephalopods use chromatophores distributed on their soft skin to change skin color and its pattern. Each chromatophore consists of a central sac containing pigment granules and radial muscles surrounding the sac. The contraction of the radial muscle causes the central sac to expand in area, making the color of the pigment more visible. With the chromatophores actuating individually, cephalopods can create extremely complex skin color patterns, which they utilize for exquisite functions including camouflage and communication. Inspired by this mechanism, we present an artificial chromatophore that can modulate its color pattern in response to light. Multimaterial projection microstereolithography is used to integrate three functional components including a photoactive hydrogel composite with polydopamine nanoparticles (PDA-NPs), acrylic acid hydrogel, and poly(ethylene glycol) diacrylate. In order to generate light-driven actuation of the artificial chromatophore, the photothermal effect of the PDA-NPs, light-responsive deformation of the photoactive hydrogel composite, and the produced mechanical stresses are studied. Mechanical properties and interfacial bonding strengths between different materials are also investigated to ensure structural integrity during actuation. We demonstrate pattern modulation of the light-responsive artificial chromatophores (LACs) with the projection of different light patterns. The LAC may suggest a new concept for various engineering applications such as the camouflage interface, biophotonic device, and flexible display.

2020

"4D‐Printed Transformable Tube Array for High‐Throughput 3D Cell Culture and Histology"

Chen Yang, Jeffrey Luo, Marianne Polunas, Nikola Bosnjak, Sy‐Tsong Dean Chuen, Michelle Chadwick, Hatem E. Sabaawy, Shawn A. Chester, Ki‐Bum Lee*, and Howon Lee*
Advanced Materials , 7 , 2004285 (2020)

3D cell cultures are rapidly emerging as a promising tool to model various human physiologies and pathologies by closely recapitulating key characteristics and functions of in vivo microenvironment. While high-throughput 3D culture is readily available using multi-well plates, assessing the internal microstructure of 3D cell cultures still remains extremely slow because of the manual, laborious, and time-consuming histological procedures. Here, a 4D-printed transformable tube array (TTA) using a shape-memory polymer that enables massively parallel histological analysis of 3D cultures is presented. The interconnected TTA can be programmed to be expanded by 3.6 times of its printed dimension to match the size of a multi-well plate, with the ability to restore its original dimension for transferring all cultures to a histology cassette in order. Being compatible with microtome sectioning, the TTA allows for parallel histology processing for the entire samples cultured in a multi-well plate. The test result with human neural progenitor cell spheroids suggests a remarkable reduction in histology processing time by an order of magnitude. High-throughput analysis of 3D cultures enabled by this TTA has great potential to further accelerate innovations in various 3D culture applications such as high-throughput/content screening, drug discovery, disease modeling, and personalized medicine.

"Rapid Processing and Drug Evaluation in Glioblastoma Patient-Derived Organoid Models with 4D Bioprinted Arrays"

Michelle Chadwick†, ChenYang† († equal contribution), Liqiong Liu, Christian Moya Gamboa, Kelly Jara, Howon Lee*, and Hatem E.Sabaawy*
iScience , 23(8) , 101365 (2020)

Glioblastoma is the most common and deadly primary brain malignancy. Despite advances in precision medicine oncology (PMO) allowing the identification of molecular vulnerabilities in glioblastoma, treatment options remain limited, and molecular assays guided by genomic and expression profiling to inform patient enrollment in life-saving trials are lacking. Here, we generate four-dimensional (4D) cell-culture arrays for rapid assessment of drug responses in glioblastoma patient-derived models. The arrays are 3D printed with thermo-responsive shape memory polymer (SMP). Upon heating, the SMP arrays self-transform in time from 3D cell-culture inserts into histological cassettes. We assess the utility of these arrays with glioblastoma cells, gliospheres, and patient derived organoid-like (PDO) models and demonstrate their use with glioblastoma PDOs for assessing drug sensitivity, on-target activity, and synergy in drug combinations. When including genomic and drug testing assays, this platform is poised to offer rapid functional drug assessments for future selection of therapies in PMO.

"Recent Advances in Multi-Material Additive Manufacturing: Methods and Applications"

Daehoon Han and Howon Lee*
Current Opinion in Chemical Engineering , 28 , 158-166 (2020)

Multi-material additive manufacturing (MMAM) enables rapid design and direct fabrication of three-dimensional (3D) objects consisting of multiple materials without the need for complex manufacturing process and expensive tooling. Through concurrent integration of multiple materials having their own unique properties, MMAM allows for facile production of multi-functional devices and systems. Various MMAM techniques have recently been developed and a wide range of new applications have been demonstrated using MMAM. This paper reviews recent advances in MMAM methods and key applications enabled by MMAM in three areas; biomedical engineering, soft robotics, and electronics.

"Self-Limiting Electrospray Deposition for the Surface Modification of Additively Manufactured Parts"

Dylan Kovacevich†, Lin Lei† († equal contribution), Daehoon Han, Christianna Kuznetsova, Steven E. Kooi, Howon Lee, and Jonathan P. Singer*
ACS Applied Materials & Interfaces , 12(18) , 20901 (2020)

Electrospray deposition (ESD) is a spray coating process that utilizes a high voltage to atomize a flowing solution into charged microdroplets. These self-repulsive droplets evaporate as they travel to a target substrate, depositing the solution solids. Our previous research investigated the conditions necessary to minimize charge dissipation and deposit a thickness-limited film that grows in area over time through self-limiting electrospray deposition. Such sprays possess the ability to conformally coat complex three-dimensional (3D) objects without changing the location of the spray needle or orientation of the object. This makes them ideally suited for the postprocessing of materials fabricated through additive manufacturing (AM), opening a paradigm of independent bulk and surface functionality. Having demonstrated 3D coating with film thickness in the range of 1–50 μm on a variety of conductive objects, in this study, we employed model substrates to quantitatively study the technique’s limits with regard to geometry and scale. Specifically, we examined the effectiveness of thickness-limited ESD for coating recessed features with gaps ranging from 50 μm to 1 cm, as well as the ability to coat surfaces hidden from the line-of-sight of the spray needle. This was then extended to the coating of hydrogel structures printed by AM, demonstrating that coating could be conducted even into the body of the structures as a means to create hydrophobic surfaces without affecting the absorption-driven humidity response. Further, these coatings were robust enough to create superhydrophobicity in the entire structure, causing it to resist immersion in water.

"Immobilization of Laccase on a Graphene Interface: Direct Electron Transfer and Molecular Dynamics Study"

Taeyoung Yoon, Inchul Baek, Seonwoo Lee, Hyunsung Choi, Seongho Yoon, Howon Lee, Sunwoong Kim, and Sungso Na
Applied Surface Science , 521 , 146378 (2020)

Direct electron transfer (DET) in biocatalysts and the interactions of biocatalysts at electrode interfaces are critical issues for the development of electrochemical devices. In comparison to high-performance complex electrodes, graphene-based electrodes have attracted significant attention based on their superior electrical conductivity, material properties, and low cost. However, the immobilization of laccase (LAC), an oxygen-reducing enzyme with high catalytic activity that is applied to cathodes, and interfaces formed between LAC and graphene have rarely been explored. In this study, electrochemical experiments employing cyclic voltammetry and electrochemical impedance spectroscopy were performed, and it was determined that graphene exhibits a maximum of a 1.57-fold increase in terms of its oxygen reduction rate compared to Au and carbon nanotubes. Additionally, DET rate revealed that graphene behaves more efficiently on immobilized LAC. Furthermore, absorbed morphologies were visualized, and computational methods were applied to verify binding sites, orientations, structures, and binding affinities in atomic scale. The axial ligands at T1 Cu sites were mutated using different hydrophobic amino acids, and the effects of mutation on interactions at interfaces were compared. Based on our experimental and theoretical results, LAC immobilization on graphene appears to be stronger than that on a charged surface without critical structural changes.

"4D Printing of a Bioinspired Microneedle Array with Backward-Facing Barbs for Enhanced Tissue Adhesiona"

Daehoon Han†, Riddish S. Morde†, Stefano Mariani† († equal contribution), Antonino A. La Mattina, Emanuele Vignali, Chen Yang, Giuseppe Barillaro*, and Howon Lee*
Advanced Functional Materials , 3 , 1909197 (2020)

Microneedle (MN), a miniaturized needle with a length-scale of hundreds of micrometers, has received a great deal of attention because of its minimally invasive, pain-free, and easy-to-use nature. However, a major challenge for controlled long-term drug delivery or biosensing using MN is its low tissue adhesion. Although microscopic structures with high tissue adhesion are found from living creatures in nature (e.g., microhooks of parasites, barbed stingers of honeybees, quills of porcupines), creating MNs with such complex microscopic features is still challenging with traditional fabrication methods. Here, a MN with bioinspired backward-facing curved barbs for enhanced tissue adhesion, manufactured by a digital light processing 3D printing technique, is presented. Backward-facing barbs on a MN are created by desolvation-induced deformation utilizing cross-linking density gradient in a photocurable polymer. Barb thickness and bending curvature are controlled by printing parameters and material composition. It is demonstrated that tissue adhesion of a backward-facing barbed MN is 18 times stronger than that of barbless MN. Also demonstrated is sustained drug release with barbed MNs in tissue. Improved tissue adhesion of the bioinspired MN allows for more stable and robust performance for drug delivery, biofluid collection, and biosensing.

2019

"Rapid Multi-Material 3D Printing with Projection Micro-Stereolithography Using Dynamic Fluidic Control"

Daehoon Han, Chen Yang, Nicholas X. Fang, and Howon Lee*
Additive Manufacturing , 27 , 606 (2019)

Mask projection stereolithography is a digital light processing-based additive manufacturing technique that has various advantages, such as high-resolution, scanning-free parallel process, wide material sets available, and support-structure-free three-dimensional (3D) printing. However, multi-material 3D printing with mask projection stereolithography has been challenging due to difficulties of exchanging a liquid-state material in a vat. In this work, we report a rapid multi-material projection micro-stereolithography using dynamic fluidic control of multiple liquid photopolymers within an integrated fluidic cell. Highly complex multi-material 3D micro-structures are rapidly fabricated through an active material exchange process. Material flow rate in the fluidic cell, material exchange efficiency, and the effects of energy dosage on curing depth are studied for various photopolymers. In addition, the degree of cross-contamination between different materials in a 3D printed multi-material structure is evaluated to assess the quality of multi-material printing. The pressure-tight and leak-free fluidic cell enables active and fast switch between liquid photopolymers, even including micro-/nano-particle suspensions, which could potentially lead to facile 3D printing of multi-material metallic/ceramic structures or heterogeneous biomaterials. In addition, a multi-responsive hydrogel micro-structure is printed using a thermo-responsive hydrogel and an electroactive hydrogel, showing various modes of swelling actuation in response to multiple external stimuli. This new ability to rapidly and heterogeneously integrate multiple functional materials in three-dimension at micro-scale has potential to accelerate advances in many emerging areas including 3D metamaterials, tissue engineering, and soft robotics.

"4D Printing Reconfigurable, Deployable and Mechanically Tunable Metamaterials"

Chen Yang, Manish Boorugu, Andrew Dopp, Jie Ren, Raymond Martin, Daehoon Han, Wonjoon Choi, and Howon Lee*
Materials Horizons , 6 , 1244 (2019)

The exotic properties of mechanical metamaterials emerge from the topology of micro-structural elements. Once manufactured, however, the metamaterials have fixed properties without the ability to adapt and adjust. Here, we present geometrically reconfigurable, functionally deployable, and mechanically tunable lightweight metamaterials created through four-dimensional (4D) printing. Using digital micro 3D printing with a shape memory polymer, dramatic and reversible changes in the stiffness, geometry, and functions of the metamaterials are achieved.

"Spatial Uncertainty Modeling for Surface Roughness of Additively Manufactured Microstructures via Image Segmentation"

Namjung Kim*, Chen Yang, Howon Lee*, and Narayana R. Aluru
Applied Sciences , 9 , 1093 (2019)

Despite recent advances in additive manufacturing (AM) that shifts the paradigm of modern manufacturing by its fast, flexible, and affordable manufacturing method, the achievement of high-dimensional accuracy in AM to ensure product consistency and reliability is still an unmet challenge. This study suggests a general method to establish a mathematical spatial uncertainty model based on the measured geometry of AM microstructures. Spatial uncertainty is specified as the deviation between the planned and the actual AM geometries of a model structure, high-aspect-ratio struts. The detailed steps of quantifying spatial uncertainties in the AM geometry are as follows: (1) image segmentation to extract the sidewall profiles of AM geometry; (2) variability-based sampling; (3) Gaussian process modeling for spatial uncertainty. The modeled spatial uncertainty is superimposed in the CAD geometry and finite element analysis is performed to quantify its effect on the mechanical behavior of AM struts with different printing angles under compressive loading conditions. The results indicate that the stiffness of AM struts with spatial uncertainty is reduced to 70% of the stiffness of CAD geometry and the maximum von Mises stress under compressive loading is significantly increased by the spatial uncertainties. The proposed modeling framework enables the high fidelity of computer-based predictive tools by seamlessly incorporating spatial uncertainties from digital images of AM parts into a traditional finite element model. It can also be applied to parts produced by other manufacturing processes as well as other AM techniques.

"Modeling of fiber-reinforced polymeric gels"

NikolaBosnjak, ShuolunWang, DaehoonHan, HowonLee, and Shawn A.Chester*
Mechanics Research Communications , 96 , 7 (2019)

When a polymer network is exposed to a suitable solvent, the migration of solvent molecules into the network will cause volumetric deformation, known as swelling, but more importantly forms a mixture that is known as a polymeric gel. Despite numerous potential applications, many aspects of the coupled diffusion-deformation behavior in polymeric gels have not yet been thoroughly investigated. Here, we focus our attention on the coupled deformation-diffusion response of fiber-reinforced polymeric gels. The presence of embedded fibers in a swellable polymer matrix leads to anisotropy in the overall behavior. In order to capture this response, we have developed a constitutive model for fiber-reinforced polymeric gels, that explicitly takes into account anisotropy in both the mechanical and diffusive behavior. The constitutive model is implemented as user element subroutine (UEL) in the commercial finite element software package Abaqus/Standard. Numerical simulations are performed to show the behavior of the model, and qualitative comparisons are made to experiments of a soft robotic gripper.

"Temperature-Responsive Thermal Metamaterials Enabled by Modular Design of Thermally Tunable Unit Cells"

Sunggu Kang, Jonghwan Cha, Kyeongbeom Seo, Sejun Kim, Youngsun Cha, HowonLee, Jingsung Park and Wonjoon Choi*
International Journal of Mass and Heat Transfer , 130 , 469 (2019)

Integrated circuits or miniaturized portable electronics require adaptive thermal control under certain temperatures. Thermal metamaterials (TMs), which artificially manipulate the heat passing through mediums have shown innovative thermal functions at a continuum scale. However, they cannot implement tunable thermal functions at local spots depending on the operating temperatures. Herein, we introduce temperature-responsive TMs enabled by modular design of thermally tunable unit cells. As ambient temperature changes, tunable thermal shifters can dynamically turn on/off their intrinsic functions to guide anisotropic heat transfer through the transition of thermal conductivities from the inner phase change nanocomposites (PCNCs), and their modular design realizes temperature-responsive thermal shields having switchable functions. The layered structures of stainless steel and the PCNC of n-octadecane embedding carbon nanotubes and copper powder are fabricated as tunable thermal shifters. Their 4 × 4 modular structure confirms the feasibility of temperature-responsive TMs, verified by the disappearance and appearance of thermally shielded regimes at low- and high-temperature ranges. The potential use of the developed concept was demonstrated as tunable interfaces between thermal dissipation and insulation for protecting temperature-sensitive components. This work can offer new capabilities for conventional passive TMs, such as local thermal adaptation, active thermal control interface, and thermal disturbance mitigation.

"Improving Surface Roughness of Additively Manufactured Parts Using a Photopolymerization Model and Multi-objective Particle Swarm Optimization"

Namjung Kim†, Ishan Bhalerao† († equal contribution), Daehoon Han, Chen Yang, and Howon Lee*
Applied Sciences , 9 , 151 (2019)

Although additive manufacturing (AM) offers great potential to revolutionize modern manufacturing, its layer-by-layer process results in a staircase-like rough surface profile of the printed part, which degrades dimensional accuracy and often leads to a significant reduction in mechanical performance. In this paper, we present a systematic approach to improve the surface profile of AM parts using a computational model and a multi-objective optimization technique. A photopolymerization model for a micro 3D printing process, projection micro-stereolithography (PμSL), is implemented by using a commercial finite element solver (COMSOL Multiphysics software). First, the effect of various process parameters on the surface roughness of the printed part is analyzed using Taguchi’s method. Second, a metaheuristic optimization algorithm, called multi-objective particle swarm optimization, is employed to suggest the optimal PμSL process parameters (photo-initiator and photo-absorber concentrations, layer thickness, and curing time) that minimize two objectives; printing time and surface roughness. The result shows that the proposed optimization framework increases 18% of surface quality of the angled strut even at the fastest printing speed, and also reduces 50% of printing time while keeping the surface quality equal for the vertical strut, compared to the samples produced with non-optimized parameters. The systematic approach developed in this study significantly increase the efficiency of optimizing the printing parameters compared to the heuristic approach. It also helps to achieve 3D printed parts with high surface quality in various printing angles while minimizing printing time.

2018

"Soft Robotic Manipulation and Locomotion with a 3D Printed Electroactive Hydrogel"

Daehoon Han, Cindy Farino, Chen Yang, Tracy Scott, Daniel Browe, Wonjoon Choi, Joseph W. Freeman, and Howon Lee*
ACS Applied Materials & Interfaces , 10 , 17512 (2018)

Electroactive hydrogels (EAH) that exhibit large deformation in response to an electric field have received great attention as a potential actuating material for soft robots and artificial muscle. However, their application has been limited due to the use of traditional two-dimensional (2D) fabrication methods. Here we present soft robotic manipulation and locomotion with 3D printed EAH microstructures. Through 3D design and precise dimensional control enabled by a digital light processing (DLP) based micro 3D printing technique, complex 3D actuations of EAH are achieved. We demonstrate soft robotic actuations including gripping and transporting an object and a bidirectional locomotion.

"Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography"

Daehoon Han, Zhaocheng Lu, Shawn A. Chester, and Howon Lee*
Scientific Reports , 8 , 1963 (2018)

Stimuli-responsive hydrogels exhibiting physical or chemical changes in response to environmental conditions have attracted growing attention for the past few decades. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm. This fast, high resolution, and scalable 3D printing method for stimuli-responsive hydrogels may enable many new applications in diverse areas, including flexible sensors and actuators, bio-medical devices, and tissue engineering.

"Rapid Pulsed Light Sintering of Silver Nanowires on Woven Polyester for Personal Thermal Management with Enhanced Performance, Durability, and Cost-Effectiveness"

Hyun-jun Hwang, Harish Devaraj, Chen Yang, Jongwei Gao, Chih-hung Chang, Howon Lee, and Rajiv Malhotra*
Scientific Reports , 8 , 17159 (2018)

Fabric-based personal heating patches have small geometric profiles and can be attached to selected areas of garments for personal thermal management to enable significant energy savings in built environments. Scalable fabrication of such patches with high thermal performance at low applied voltage, high durability and low materials cost is critical to the widespread implementation of these energy savings. This work investigates a scalable Intense Pulsed Light (IPL) sintering process for fabricating silver nanowire on woven polyester heating patches. Just 300 microseconds of IPL sintering results in 30% lesser electrical resistance, 70% higher thermal performance, greater durability (under bending up to 2 mm radius of curvature, washing, humidity and high temperature), with only 50% the added nanowire mass compared to state-of-the-art. Computational modeling combining electromagnetic and thermal simulations is performed to uncover the nanoscale temperature gradients during IPL sintering, and the underlying reason for greater durability of the nanowire-fabric after sintering. This large-area, high speed, and ambient-condition IPL sintering process represents an attractive strategy for scalably fabricating personal heating fabric-patches with greater thermal performance, higher durability and reduced costs.

"Layer-by-Layer Assembled Carbon Nanotube Polyethyleneimine Coating Inside Copper-Sintered Heat Pipes for Enhanced Thermal Performance"

Seunghyeon Lee, Jaemin Lee, Hayoung Hwang, Taehan Yeo, Howon Lee, and Wonjoon Choi*
Carbon , 140 , 521 (2018)

Biporous structures at the nano–microscale are promising candidates for controlling phase change heat transfer, through their enhanced capillary wicking and fluid transportation. However, existing methods for fabricating biporous structures involve complex process which is not suitable for small-scale thermal devices such as heat pipes, owing to their confined and non-flat inner structures. Herein, we report the biporous structures inside copper-sintered heat pipes, enabled by layer-by-layer (LbL) assembled multi-walled carbon nanotube (MWCNT)-polyethyleneimine (PEI) coating for enhanced thermal performance. The repetitive filling and removing of the oppositely charged solutions with MWCNT-PEI and carboxylic-functionalized MWCNTs assembled the nanoporous MWCNT-PEI coatings (10, 20, and 40 bilayers) on the microporous copper-sintered inner surfaces. The fiber-like MWCNT networks structurally manipulated morphology and thickness of biporous structures, while the hydrophilic PEI shells chemically optimized wettability. A reduced thermal resistance (∼14.3%) was observed for MWCNT-PEI coating in 10 bilayers, due to the enhanced capillary wicking, interfacial contact areas, and bubble dynamics, whereas the 40 bilayers did not exhibit improved thermal performance owing to the redundant nanoporous layers causing reduced volume of microporous structures and increased thermal resistance. The LbL-assembled MWCNT-PEI coatings would act as functional layers to improve the performance of miniaturized and thin-film-based thermal devices.

2017

"Tunable Multifunctional Thermal Metamaterials: Manipulation of Local Heat Flux via Assembly of Unit-Cell Thermal Shifters"

Gwanwoo Park, Sunggu Kang, Howon Lee, and Wonjoon Choi*
Scientific Reports , 7 , 41000 (2017)

Thermal metamaterials, designed by transformation thermodynamics are artificial structures that can actively control heat flux at a continuum scale. However, fabrication of them is very challenging because it requires a continuous change of thermal properties in materials, for one specific function. Herein, we introduce tunable thermal metamaterials that use the assembly of unit-cell thermal shifters for a remarkable enhancement in multifunctionality as well as manufacturability. Similar to the digitization of a two-dimensional image, designed thermal metamaterials by transformation thermodynamics are disassembled as unit-cells thermal shifters in tiny areas, representing discretized heat flux lines in local spots. The programmed-reassembly of thermal shifters inspired by LEGO enable the four significant functions of thermal metamaterials—shield, concentrator, diffuser, and rotator—in both simulation and experimental verification using finite element method and fabricated structures made from copper and PDMS. This work paves the way for overcoming the structural and functional limitations of thermal metamaterials.

2016

"Highly Sensitive, Direct Real-Time Detection of Silver Nanowires by Using a Quartz Crystal Microbalance"

Kuewhan Jang, Chanho Park, Juneseok You, Jaeyeong Choi, Hyunjun Park, Jinsung Park, Howon Lee, Chang-Hwan Choi, and Sungsoo Na*
Nanotechnology , 27 , 475506 (2016)

For several decades, silver nanomaterials (AgNMs) have been used in various research areas and commercial products. Among the many AgNMs, silver nanowires (AgNWs) are one of the mostly widely used nanomaterials due to their high electrical and thermal conductivity. However, recent studies have investigated the toxicity of AgNWs. For this reason, it is necessary to develop a successful detection method of AgNWs for protecting human health. In this study, label-free, highly sensitive, direct, and real-time detection of AgNWs is performed for the first time. The detection mechanism is based on the resonance frequency shift upon the mass change from the hybridization between the probe DNA on the electrode and the linker DNA attached on AgNWs. The frequency shift is measured by using a quartz crystal microbalance. We are able to detect 1 ng ml−1 of AgNWs in deionized water in real-time. Moreover, our detection method can selectively detect AgNWs among other types of one-dimensional nanomaterials and can also be applied to detection in drinking water.

"Multimaterial 4D Printing with Tailorable Shape Memory Polymers"

Q. Ge*, A. H. Sakhaei, Howon Lee, C. K. Dunn, Nicholas. X. Fang*, and M. L. Dunn*
Scientific Reports , 6 , 31110 (2016)

We present a new 4D printing approach that can create high resolution (up to a few microns), multimaterial shape memory polymer (SMP) architectures. The approach is based on high resolution projection microstereolithography (PμSL) and uses a family of photo-curable methacrylate based copolymer networks. We designed the constituents and compositions to exhibit desired thermomechanical behavior (including rubbery modulus, glass transition temperature and failure strain which is more than 300% and larger than any existing printable materials) to enable controlled shape memory behavior. We used a high resolution, high contrast digital micro display to ensure high resolution of photo-curing methacrylate based SMPs that requires higher exposure energy than more common acrylate based polymers. An automated material exchange process enables the manufacture of 3D composite architectures from multiple photo-curable SMPs. In order to understand the behavior of the 3D composite microarchitectures, we carry out high fidelity computational simulations of their complex nonlinear, time-dependent behavior and study important design considerations including local deformation, shape fixity and free recovery rate. Simulations are in good agreement with experiments for a series of single and multimaterial components and can be used to facilitate the design of SMP 3D structures.

"Ultra-sensitive detection of zinc oxide nanowires using a quartz crystal microbalance and phosphoric acid DNA"

K. Jang, J. You, C. Park, H. Park, J. Choi, C.-H. Choi, J. Park, Howon Lee, S. Na
Nanotechnology , 27 , 365501 (2016)

Recent advancements of nanomaterials have inspired numerous scientific and industrial applications. Zinc oxide nanowires (ZnO NWs) is one of the most important nanomaterials due to their extraordinary properties. However, studies performed over the past decade have reported toxicity of ZnO NWs. Therefore, there has been increasing demand for effective detection of ZnO NWs. In this study, we propose a method for the detection of ZnO NW using a quartz crystal microbalance (QCM) and DNA probes. The detection method is based on the covalent interaction between ZnO NWs and the phosphoric acid group of single-stranded DNA (i.e., linker DNA), and DNA hybridization between the linker DNA and the probe DNA strand on the QCM electrode. Rapid, high sensitivity, in situ detection of ZnO NWs was demonstrated for the first time. The limit of detection was 10−4 μg ml−1 in deionized water, which represents a sensitivity that is 100000 times higher than the toxic ZnO NW concentration level. Moreover,the selectivity of the ZnO NW detection method was demonstrated by comparison with other types of nanowires and the method was able to detect ZnO NWs in tap water sensitively even after stored for 14 d in a refrigerator. The performance of our proposed method was sufficient to achieve detection of ZnO NW in the ‘real-world’ environment.

"Polytope Sector-based Synthesis and Analysis of Microstructural Architectures with Tunable Thermal Conductivity and Expansion"

J. B. Hopkins*, Y. Song, Howon Lee, N. X. Fang, and C. M. Spadaccini
Journal of Mechanical Design , 138 , 051401 (2016)

The aim of this paper is to (1) introduce an approach, called polytope sector-based synthesis (PSS), for synthesizing 2D or 3D microstructural architectures that exhibit a desired bulk-property directionality (e.g., isotropic, cubic, orthotropic, etc.), and (2) provide general analytical methods that can be used to rapidly optimize the geometric parameters of these architectures such that they achieve a desired combination of bulk thermal conductivity and thermal expansion properties. Although the methods introduced can be applied to general beam-based microstructural architectures, we demonstrate their utility in the context of an architecture that can be tuned to achieve a large range of extreme thermal expansion coefficients—positive, zero, and negative. The material-property-combination region that can be achieved by this architecture is determined within an Ashby-material-property plot of thermal expansion versus thermal conductivity using the analytical methods introduced. These methods are verified using finite-element analysis (FEA) and both 2D and 3D versions of the design have been fabricated using projection microstereolithography.

2015

"High Sensitive, Direct, and Label-Free Technique for Hg2+ Detection by Using Kelvin Probe Force Microscopy"

C. Park, K. Jang, S. Lee, J. You, S. Lee, H. Ha, K. Yun, J. Kim, Howon Lee, J. Park, S. Na*
Nanotechnology , 26 , 305501 (2015)

For several decades, various nanomaterials have been used in a wide range of industrial fields, research areas, and commercial products. Among many nanomaterials, nano-sized mercury materials are one of the most widely used nanomaterials in real life. However, due to the high toxicity of Hg2+, it is imperative to develop an effective and practical detection method for Hg2+ to protect human health and environment. In this study, a highly sensitive, label-free method of detecting Hg2+ that requires only a single drop of solution was developed. The detection mechanism is based on the different surface potential arising from Hg2+ binding to mismatched thymine–thymine sequences, creating a very stable base pair. The surface potential is measured with Kelvin probe force microscopy (KPFM) to a molecular resolution. The developed method is capable of detecting 2 fmol of Hg2+, which is 500 times more sensitive than previously reported techniques. Moreover, our method can selectively detect Hg2+ and can also be applied to tap water and river water. This KPFM-based Hg2+ detection method can be used as an early detection technique for practical applications.

2014

"Ultralight and Ultra-Stiff Mechanical Metamaterials"

X. Zheng*, Howon Lee, T. Weisgraber, M. Shusteff, J. Deotte, E. Duoss, J. Kuntz, M. Biener, Q. Ge, J. Jackson, S. O. Kucheyev, N. X. Fang*, and C. Spadaccini*
Science , 344 , 1373-1377 (2014)

The mechanical properties of ordinary materials degrade substantially with reduced density because their structural elements bend under applied load. We report a class of microarchitected materials that maintain a nearly constant stiffness per unit mass density, even at ultralow density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with polymers, metals, or ceramics as constituent materials, is made possible by projection microstereolithography (an additive micromanufacturing technique) combined with nanoscale coating and postprocessing. We found that these materials exhibit ultrastiff properties across more than three orders of magnitude in density, regardless of the constituent material.

2012

"Prescribed Pattern Transformation in Swelling Gel Tubes by Elastic Instability"

Howon Lee, Jiaping Zhang, Hanqing Jiang, and Nicholas X. Fang*
Physical Review Letters , 108 , 214304 (2012)

We present a study on swelling-induced circumferential buckling of tubular shaped gels. Inhomogeneous stress develops as the gel swells under mechanical constraints, which gives rise to spontaneous buckling instability without an external force. Full control over the postbuckling pattern is experimentally demonstrated. A simple analytical model is developed using elastic energy to predict stability and postbuckling patterns upon swelling. Analysis reveals that the height to diameter ratio is the most critical design parameter to determine the buckling pattern, which agrees well with experimental and numerical results.

"Design and Optimization of an LED Projection Micro-Stereolithography Three-Dimensional Manufacturing System"

X.R. Zheng, J. Deotte, M. Allonso, G. Farquar, T. Weisgraber, S. Gemberling, Howon Lee, Nicholas X. Fang, and C.M. Spadaccini*
Review of Scientific Instruments , 83 , 125001 (2012)

The rapid manufacture of complex three-dimensional micro-scale components has eluded researchers for decades. Several additive manufacturing options have been limited by either speed or the ability to fabricate true three-dimensional structures. Projection micro-stereolithography (PμSL) is a low cost, high throughput additive fabrication technique capable of generating three-dimensional microstructures in a bottom-up, layer by layer fashion. The PμSL system is reliable and capable of manufacturing a variety of highly complex, three-dimensional structures from micro- to meso-scales with micro-scale architecture and submicron precision. Our PμSL system utilizes a reconfigurable digital mask and a 395 nm light-emitting diode (LED) array to polymerize a liquid monomer in a layer-by-layer manufacturing process. This paper discusses the critical process parameters that influence polymerization depth and structure quality. Experimental characterization and performance of the LED-based PμSL system for fabricating highly complex three-dimensional structures for a large range of applications is presented.

"Micro 3D Printing Using a Digital Projector and its Application in the Study of Soft Materials Mechanics"

Howon Lee and Nicholas X. Fang*
Journal of Visualized Experiments , 69 , e4457 (2012)

We demonstrate controlled pattern transformation of swelling gel tubes by elastic instability. A simple projection micro stereo-lithography setup is built using an off-the-shelf digital data projector to fabricate three-dimensional polymeric structures in a layer-by-layer fashion. Swelling hydrogel tubes under mechanical constraint display various circumferential buckling modes depending on dimension.

2010

"Solvent-Driven Polymeric Micro Beam Device"

Chunguang Xia, Howon Lee, and Nicholas X. Fang*
Journal of Micromechanics and Microengineering , 20 , 085030 (2010)

The response of current hydrogel devices mainly depends on the diffusion of stimuli. However, diffusion is a slow transport mechanism compared to advection, which therefore limits the response speed of hydrogel devices. To overcome this limitation, we introduce a capillary network and elastic instability mechanism. Particularly, an open surface capillary delivers and distributes solvent, thus triggering the swelling and bending of curved polymeric beams. To demonstrate this concept, we fabricate these polymeric microstructures using projection micro-stereolithography (PµSL). Combined with instability criteria analysis based on static beam theory, this device is designed to exhibit two-way snap-through behavior. Our analysis provides the minimum dimensionless stiffness β for the beam device to snap during solvent actuation. Here, β is a well-defined dimensionless parameter in our analysis that indicates whether the device can provide sufficient axial force to trigger the snap-through of the beam. The actuation displacement can be as high as 45% of the length of the beam. We observe a maximum midpoint speed of 3.1 cm s−1 for a beam 2 mm long—20 times higher than that for a beam without an elastic instability mechanism. This device can be used in artificial muscle and as the key component for fluidic-to-mechanical signal transduction in active micro-fluidic circuits.

"First Jump of Microgel; Actuation Speed Enhancement by Elastic Instability"

Howon Lee, Chunguang Xia, and Nicholas X. Fang*
Soft Matter , 6 , 4342 (2010)

Swelling-induced snap-buckling in a 3D micro hydrogel device, inspired by the insect-trapping action of Venus flytrap, makes it possible to generate astonishingly fast actuation. We demonstrate that elastic energy is effectively stored and quickly released from the device by incorporating elastic instability. Utilizing its rapid actuation speed, the device can even jump by itself upon wetting.

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