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Publication Open Access 3D printing of elastomeric bioinspired complex adhesive microstructures(Wiley, 2021) Dayan, Cem Balda; Chun, Sungwoo; Krishna Subbaiah, Nagaraj; Drotlef, Dirk Michael; Akolpoğlu, Mükrime Birgül; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Bioinspired elastomeric structural adhesives can provide reversible and controllable adhesion on dry/wet and synthetic/biological surfaces for a broad range of commercial applications. Shape complexity and performance of the existing structural adhesives are limited by the used specific fabrication technique, such as molding. To overcome these limitations by proposing complex 3D microstructured adhesive designs, a 3D elastomeric microstructure fabrication approach is implemented using two-photon-polymerization-based 3D printing. A custom aliphatic urethane-acrylate-based elastomer is used as the 3D printing material. Two designs are demonstrated with two combined biological inspirations to show the advanced capabilities enabled by the proposed fabrication approach and custom elastomer. The first design focuses on springtail- and gecko-inspired hybrid microfiber adhesive, which has the multifunctionalities of side-surface liquid super-repellency, top-surface liquid super-repellency, and strong reversible adhesion features in a single fiber array. The second design primarily centers on octopus- and gecko-inspired hybrid adhesive, which exhibits the benefits of both octopus- and gecko-inspired microstructured adhesives for strong reversible adhesion on both wet and dry surfaces, such as skin. This fabrication approach could be used to produce many other 3D complex elastomeric structural adhesives for future real-world applications.
Publication Metadata only Artificial-goosebump-driven microactuation(Nature Portfolio, 2024) Zhang, Mingchao; Pal, Aniket; Lyu, Xianglong; Wu, Yingdan; Department of Mechanical Engineering; Sitti, Metin; Department of Mechanical Engineering; College of Engineering; School of MedicineMicroactuators provide controllable driving forces for precise positioning, manipulation and operation at the microscale. Development of microactuators using active materials is often hampered by their fabrication complexity and limited motion at small scales. Here we report light-fuelled artificial goosebumps to actuate passive microstructures, inspired by the natural reaction of hair bristling (piloerection) on biological skin. We use light-responsive liquid crystal elastomers as the responsive artificial skin to move three-dimensionally printed passive polymer microstructures. When exposed to a programmable femtosecond laser, the liquid crystal elastomer skin generates localized artificial goosebumps, resulting in precise actuation of the surrounding microstructures. Such microactuation can tilt micro-mirrors for the controlled manipulation of light reflection and disassemble capillary-force-induced self-assembled microstructures globally and locally. We demonstrate the potential application of the proposed microactuation system for information storage. This methodology provides precise, localized and controllable manipulation of microstructures, opening new possibilities for the development of programmable micromachines. Light-induced artificial goosebumps on liquid crystal elastomer skin are used to precisely manipulate passive microstructures, achieving a localized and controllable system for programmable micromachines.Publication Metadata only Defect structure in aliovalently-doped and isovalently-substituted PbTiO3 nano-powders(Institute of Physics (IOP) Publishing, 2010) Erdem, Emre; Jakes, Peter; Parashar, S. K. S.; Ruediger, Andreas; Eichel, Ruediger-A; N/A; Department of Chemistry; Kiraz, Kamil; Somer, Mehmet Suat; Researcher; Faculty Member; Department of Chemistry; N/A; College of Sciences; N/A; 178882The defect structure of Fe3+-, Cu2+-, Mn4+- and Gd3+-doped PbTiO3 nano-powders has been studied by electron paramagnetic resonance (EPR) spectroscopy. Analogous to the situation for 'bulk' ferroelectrics, Fe3+ and Cu2+ act as acceptor-type functional centers that form defect complexes with charge-compensating oxygen vacancies. The corresponding defect dipoles are aligned along the direction of spontaneous polarization, PS, and possess an additional defect polarization, P-D. Upon the transition to the nano-regime, the defect structure is modified such that orientations perpendicular to P-S, (Fe-Ti'-V-O(center dot center dot))(perpendicular to)(center dot) and (Cu-Ti ''-V-O(center dot center dot))(perpendicular to)(x) also become realized. Moreover, the binding energy for the defect complexes is lowered such that instead 'free' Fe-Ti' and V-O(center dot center dot)-centers are formed. As a consequence, the concentration of mobile V-O(center dot center dot) that enhances the ionic conductivity through drift diffusion is increased for the nano-powders. Finally, in the nano-regime the ferroelectric 'hardening' is expected to be considerably decreased as compared to the 'bulk' compounds. In contrast to the acceptor-type dopants, the donor-type Gd3+ dopant is incorporated as an 'isolated' functional center, where charge compensation by means of lead vacancies is performed in distant coordination spheres.Publication Metadata only Designing covalent organic framework-based light-driven microswimmers toward therapeutic applications(Wiley-V C H Verlag Gmbh, 2023) Sridhar, Varun; Yildiz, Erdost; Rodriguez-Camargo, Andres; Lyu, Xianglong; Yao, Liang; Wrede, Paul; Aghakhani, Amirreza; Akolpoglu, Birgul M.; Podjaski, Filip; Lotsch, Bettina V.; Department of Mechanical Engineering; Sitti, Metin; Department of Mechanical Engineering; College of Engineering; School of MedicineWhile micromachines with tailored functionalities enable therapeutic applications in biological environments, their controlled motion and targeted drug delivery in biological media require sophisticated designs for practical applications. Covalent organic frameworks (COFs), a new generation of crystalline and nanoporous polymers, offer new perspectives for light-driven microswimmers in heterogeneous biological environments including intraocular fluids, thus setting the stage for biomedical applications such as retinal drug delivery. Two different types of COFs, uniformly spherical TABP-PDA-COF sub-micrometer particles and texturally nanoporous, micrometer-sized TpAzo-COF particles are described and compared as light-driven microrobots. They can be used as highly efficient visible-light-driven drug carriers in aqueous ionic and cellular media. Their absorption ranging down to red light enables phototaxis even in deeper and viscous biological media, while the organic nature of COFs ensures their biocompatibility. Their inherently porous structures with approximate to 2.6 and approximate to 3.4 nm pores, and large surface areas allow for targeted and efficient drug loading even for insoluble drugs, which can be released on demand. Additionally, indocyanine green (ICG) dye loading in the pores enables photoacoustic imaging, optical coherence tomography, and hyperthermia in operando conditions. This real-time visualization of the drug-loaded COF microswimmers enables unique insights into the action of photoactive porous drug carriers for therapeutic applications.Publication Metadata only Development of color tunable aqueous cds-cysteine quantum dots with improved efficiency and investigation of cytotoxicity(Amer Scientific Publishers, 2010) N/A; N/A; Department of Chemistry; Department of Chemistry; Öztürk, Sinan S.; Selçukbiricik, Fatih; Acar, Havva Funda Yağcı; Master Student; N/A; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 178902Cysteine capped aqueous CdS quantum dots with improved luminescence and excellent colloidal-luminescence stability were developed in a simple one pot aqueous method from safer precursors at low temperatures. Investigation of size and luminescence as a function of cysteine amount, pH and temperature revealed an optimum value for all these variables to maximize the quantum yield. Cysteine:Cd ratio of 2, reaction pH of 9.5 and synthesis at room temperature-30 degrees C emerged as the best conditions for the highest QY of 19%. Yet, QY can be improved up to 55% if QDs are cleaned from excess cysteine and ions and redispersed in pH 7 medium. Size of the QDs, therefore the color of luminescence, can be tuned by the reaction temperature in this simple process. Higher temperatures provide larger particles. Cell uptake and cell viability studies in a wide range of doses and different incubation times with MCF-7 and HeLa cell lines revealed cell dependent differences. MCF-7 cells uptake more ODs but are much more viable than HeLa cells. At low doses such as 0.025 mg QD/ml all cells are viable. At 24 h incubation times MCF-7 cells demonstrate viability above 75% up to 0.15 mg QD/ml. On the other hand HeLa cells loose viability with increasing dose.Publication Metadata only Development of highly stable and luminescent aqueous CdS quantum dots with the poly(acrylic acid)/mercaptoacetic acid binary coating system(Amer Scientific Publishers, 2009) Lieberwirth, I.; Department of Chemistry; N/A; Department of Chemical and Biological Engineering; Acar, Havva Funda Yağcı; Çelebi, Serdar; Serttunalı, Nazlı İpek; Faculty Member; Master Student; Undergraduate Student; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; Graduate School of Sciences and Engineering; College of Engineering; 178902; N/A; N/AHighly stable and luminescent CdS quantum dots (QD) were prepared in aqueous solutions via in situ capping of the crystals with the poly(acrylic acid) (PAA) and mercaptoacetic acid (MAA) binary mixtures. The effect of reaction temperature and coating composition on the particle size, colloidal stability and luminescence were investigated and discussed in detail. CdS QDs coated with either PAA or MAA were also prepared and compared in terms of properties. CdS-MAA QDs were highly luminescent but increasing reaction temperature caused an increase in the crystal size and a significant decrease in the quantum yield (QY). Although less luminescent and bigger than CdS-MAA, CdS-PAA QDs maintained the room temperature size and QY at higher reaction temperatures. CdS-MAA QDs lacked long term colloidal stability whereas CdS-PAA QDs showed excellent stability over a year. Use of PAA/MAA mixture as a coating for CdS nanoparticles during the synthesis provided excellent stability, high QY and ability to tune the size and the color of the emission. Combination of all of these properties can be achieved only with the mixed coating. CdS coated with PAA/MAA at 40/60 ratio displayed the highest QY (50% of Rhodamine B) among the other compositions.Publication Metadata only Hydrogel muscles powering reconfigurable micro-metastructures with wide-spectrum programmability(Nature Portfolio, 2023) Zhang, Mingchao; Pal, Aniket; Zheng, Zhiqiang; Gardi, Gaurav; Yildiz, Erdost; Department of Mechanical Engineering; Sitti, Metin; Department of Mechanical Engineering; College of Engineering; School of MedicineStimuli-responsive geometric transformations endow metamaterials with dynamic properties and functionalities. However, using existing transformation mechanisms to program a single geometry to transform into diverse final configurations remains challenging, imposing crucial design restrictions on achieving versatile functionalities. Here, we present a programmable strategy for wide-spectrum reconfigurable micro-metastructures using linearly responsive transparent hydrogels as artificial muscles. Actuated by the hydrogel, the transformation of micro-metastructures arises from the collaborative buckling of their building blocks. Rationally designing the three-dimensional printing parameters and geometry features of the metastructures enables their locally isotropic or anisotropic deformation, allowing controllable wide-spectrum pattern transformation with programmable chirality and optical anisotropy. This reconfiguration mechanism can be applied to various materials with a wide range of mechanical properties. Our strategy enables a thermally reconfigurable printed metalattice with pixel-by-pixel mapping of different printing powers and angles for displaying or hiding complex information, providing opportunities for encryption, miniature robotics, photonics and phononics applications. It is difficult to program a single stimuli-responsive geometry to transform into diverse final configurations in a systematic manner. Here, linearly responsive transparent hydrogels are developed to create micro-metastructures with wide-spectrum thermal reconfigurability.Publication Open Access Liquid crystal eastomer actuated reconfigurable microscale kirigami metastructures(Wiley, 2021) Zhang, Mingchao; Shahsavan, Hamed; Guo, Yubing; Pena-Francesch, Abdon; Zhang, Yingying; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Programmable actuation of metastructures with predesigned geometrical configurations has recently drawn significant attention in many applications, such as smart structures, medical devices, soft robotics, prosthetics, and wearable devices. Despite remarkable progress in this field, achieving wireless miniaturized reconfigurable metastructures remains a challenge due to the difficult nature of the fabrication and actuation processes at the micrometer scale. Herein, microscale thermo-responsive reconfigurable metasurfaces using stimuli-responsive liquid crystal elastomers (LCEs) is fabricated as an artificial muscle for reconfiguring the 2D microscale kirigami structures. Such structures are fabricated via two-photon polymerization with sub-micrometer precision. Through rationally designed experiments guided by simulations, the optimal formulation of the LCE artificial muscle is explored and the relationship between shape transformation behaviors and geometrical parameters of the kirigami structures is build. As a proof of concept demonstration, the constructs for temperature-dependent switching and information encryption is applied. Such reconfigurable kirigami metastructures have significant potential for boosting the fundamental small-scale metastructure research and the design and fabrication of wireless functional devices, wearables, and soft robots at the microscale as well.Publication Open Access Liquid crystal structure of supercooled liquid gallium and eutectic gallium-indium(Wiley, 2021) Yunusa, Muhammad; Adaka, Alex; Aghakhani, Amirreza; Shahsavan, Hamed; Guo, Yubing; Alapan, Yunus; Jakli, Antal; Department of Mechanical Engineering; Sitti, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; School of Medicine; 297104Understanding the origin of structural ordering in supercooled liquid gallium (Ga) has been a great scientific quest in the past decades. Here, reflective polarized optical microscopy on Ga sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection at 120 degrees C that increases on cooling and persists down to room temperature or below. The polymer rubbing usually aligns the director of thermotropic liquid crystals (LCs) parallel to the rubbing direction. On the other hand, when Ga is sandwiched between substrates that align conventional LC molecules normal to the surface, the reflection is isotropic, but mechanical shear force induces anisotropic reflection that relaxes in seconds. Such alignment effects and shear-induced realignment are typical to conventional thermotropic LCs and indicate a LC structure of liquid Ga. Specifically, Ga textures obtained by atomic force and scanning electron microscopy reveal the existence of a lamellar structure corresponding to a smectic LC phase, while the nanometer-thin lamellar structure is transparent under transmission polarized optical microscopy. Such spatial molecular arrangements may be attributed to dimer molecular entities in the supercooled liquid Ga. The LC structure observation of electrically conductive liquid Ga can provide new opportunities in materials science and LC applications.Publication Metadata only Liquid metal actuators: a comparative analysis of surface tension controlled actuation(Wiley-V C H Verlag Gmbh, 2023) Liao, Jiahe; Majidi, Carmel; Department of Mechanical Engineering; Sitti, Metin; Department of Mechanical Engineering; College of Engineering; School of MedicineLiquid metals, with their unique combination of electrical and mechanical properties, offer great opportunities for actuation based on surface tension modulation. Thanks to the scaling laws of surface tension, which can be electrochemically controlled at low voltages, liquid metal actuators stand out from other soft actuators for their remarkable characteristics such as high contractile strain rates and higher work densities at smaller length scales. This review summarizes the principles of liquid metal actuators and discusses their performance as well as theoretical pathways toward higher performances. The objective is to provide a comparative analysis of the ongoing development of liquid metal actuators. The design principles of the liquid metal actuators are analyzed, including low-level elemental principles (kinematics and electrochemistry), mid-level structural principles (reversibility, integrity, and scalability), and high-level functionalities. A wide range of practical use cases of liquid metal actuators from robotic locomotion and object manipulation to logic and computation is reviewed. From an energy perspective, strategies are compared for coupling the liquid metal actuators with an energy source toward fully untethered robots. The review concludes by offering a roadmap of future research directions of liquid metal actuators. This review summarizes the operation and design principles of surface tension-controlled actuation by liquid metals and discusses their performance and functionalities. Theoretical pathways toward higher performances, thanks to the unique scaling law of surface tension, are analyzed and compared to other popular soft actuators. The review concludes by offering a roadmap for future research directions.