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1. From colloidal particles to photonic crystals: advances in self-assembly and their emerging applications

   https://doi.org/10.1039/D0CS00706D  

   Cai, Zhongyu; Li, Zhiwei; Ravaine, Serge; He, Mingxin; Song, Yanlin; Yin, Yadong; Zheng, Hanbin; Teng, Jinghua; Zhang, Ao (May 2021, Chemical Society Reviews)

   null (Ed.)

   Over the last three decades, photonic crystals (PhCs) have attracted intense interests thanks to their broad potential applications in optics and photonics. Generally, these structures can be fabricated via either “top-down” lithographic or “bottom-up” self-assembly approaches. The self-assembly approaches have attracted particular attention due to their low cost, simple fabrication processes, relative convenience of scaling up, and the ease of creating complex structures with nanometer precision. The self-assembled colloidal crystals (CCs), which are good candidates for PhCs, have offered unprecedented opportunities for photonics, optics, optoelectronics, sensing, energy harvesting, environmental remediation, pigments, and many other applications. The creation of high-quality CCs and their mass fabrication over large areas are the critical limiting factors for real-world applications. This paper reviews the state-of-the-art techniques in the self-assembly of colloidal particles for the fabrication of large-area high-quality CCs and CCs with unique symmetries. The first part of this review summarizes the types of defects commonly encountered in the fabrication process and their effects on the optical properties of the resultant CCs. Next, the mechanisms of the formation of cracks/defects are discussed, and a range of versatile fabrication methods to create large-area crack/defect-free two-dimensional and three-dimensional CCs are described. Meanwhile, we also shed light on both the advantages and limitations of these advanced approaches developed to fabricate high-quality CCs. The self-assembly routes and achievements in the fabrication of CCs with the ability to open a complete photonic bandgap, such as cubic diamond and pyrochlore structure CCs, are discussed as well. Then emerging applications of large-area high-quality CCs and unique photonic structures enabled by the advanced self-assembly methods are illustrated. At the end of this review, we outlook the future approaches in the fabrication of perfect CCs and highlight their novel real-world applications. 

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2. Origami Biosystems: 3D Assembly Methods for Biomedical Applications

   https://doi.org/10.1002/adbi.201800230  

   Bolaños Quiñones, Vladimir A.; Zhu, Hong; Solovev, Alexander A.; Mei, Yongfeng; Gracias, David H. (September 2018, Advanced Biosystems)

   Abstract Conventional assembly of biosystems has relied on bottom‐up techniques, such as directed aggregation, or top‐down techniques, such as layer‐by‐layer integration, using advanced lithographic and additive manufacturing processes. However, these methods often fail to mimic the complex three dimensional (3D) microstructure of naturally occurring biomachinery, cells, and organisms regarding assembly throughput, precision, material heterogeneity, and resolution. Pop‐up, buckling, and self‐folding methods, reminiscent of paper origami, allow the high‐throughput assembly of static or reconfigurable biosystems of relevance to biosensors, biomicrofluidics, cell and tissue engineering, drug delivery, and minimally invasive surgery. The universal principle in these assembly methods is the engineering of intrinsic or extrinsic forces to cause local or global shape changes via bending, curving, or folding resulting in the final 3D structure. The forces can result from stresses that are engineered either during or applied externally after synthesis or fabrication. The methods facilitate the high‐throughput assembly of biosystems in simultaneously micro or nanopatterned and layered geometries that can be challenging if not impossible to assemble by alternate methods. The authors classify methods based on length scale and biologically relevant applications; examples of significant advances and future challenges are highlighted. 

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3. Application and patterning of atomic and small-molecule resists for area-selective deposition on silicon

   Raffaelle, Patrick R (September 2024, University of Rochester)

   The fabrication of 2D devices with micro/nano-scale features often rely on assembly from a top-down perspective, where the design emphasis is on the removal of material to generate surface features. Common “top-down” approaches to fabrication often include “pattern and subtract” techniques which require energy-intensive processing and result in a high volume of material waste of substances such as photoresists, etchant, and developers. In addition to high energy and material dissipation, traditional “top-down” approaches have also struggled to adapt to the continuous downsizing of critical dimensions of 3D device components. Thus, instead of generating devices from a “top-down” perspective, there has been a push over the last two decades to instead leverage the intrinsic differences in chemical behavior between surface species, such that feature deposition selectively begins at the surface and grows vertically in an additive fashion via reaction from the “bottom-up”. Here, I will evaluate the ability of different small molecule and atomic layers to enable selective deposition on a silicon substrate. Specifically, I will be investigating a carbenylated organic molecule, a perfluorinated amine, and atomic halogen species on their ability to inhibit deposition atomic layer deposition (ALD) of a metal oxide. When paired with a hydrolyzed surface (which promotes metal oxide growth), these inhibiting species may be used to form complementary resist systems which can enable area-selective ALD (AS-ALD) on a surface. Another primary consideration in “bottom-up” approaches to feature fabrication is the ability to pattern these small molecule and atomic surface layers such that they form a template for selective growth. To this end, I will explore using ultrafast laser patterning and contact transfer printing to selectively deposit or alter these surface layers to generate complementary surface domains that can serve as a foundation for a AS-ALD platform. 

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4. Middle-out methods for spatiotemporal tissue engineering of organoids

   https://doi.org/10.1038/s44222-023-00039-3  

   Blatchley, Michael R.; Anseth, Kristi S. (May 2023, Nature Reviews Bioengineering)

   Organoids recapitulate many aspects of the complex three-dimensional (3D) organization found within native tissues and even display tissue and organ-level functionality. Traditional approaches to organoid culture have largely employed a top-down tissue engineering strategy, whereby cells are encapsulated in a 3D matrix, such as Matrigel, alongside well-defined biochemical cues that direct morphogenesis. However, the lack of spatiotemporal control over niche properties renders cellular processes largely stochastic. Therefore, bottom-up tissue engineering approaches have evolved to address some of these limitations and focus on strategies to assemble tissue building blocks with defined multi-scale spatial organization. However, bottom-up design reduces the capacity for self-organization that underpins organoid morphogenesis. Here, we introduce an emerging framework, which we term middle-out strategies, that relies on existing design principles and combines top-down design of defined synthetic matrices that support proliferation and self-organization with bottom-up modular engineered intervention to limit the degrees of freedom in the dynamic process of organoid morphogenesis. We posit that this strategy will provide key advances to guide the growth of organoids with precise geometries, structures and function, thereby facilitating an unprecedented level of biomimicry to accelerate the utility of organoids to more translationally relevant applications. 

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5. Self‐Assembled Microactuators Using Chiral Liquid Crystal Elastomers

   https://doi.org/10.1002/smll.202302774  

   Lee, Yoo Jin; Abdelrahman, Mustafa K.; Kalairaj, Manivannan Sivaperuman; Ware, Taylor H. (June 2023, Small)

   Abstract Materials that undergo reversible changes in form typically require top‐down processing to program the microstructure of the material. As a result, it is difficult to program microscale, 3D shape‐morphing materials that undergo non‐uniaxial deformations. Here, a simple bottom‐up fabrication approach to prepare bending microactuators is described. Spontaneous self‐assembly of liquid crystal (LC) monomers with controlled chirality within 3D micromold results in a change in molecular orientation across thickness of the microstructure. As a result, heating induces bending in these microactuators. The concentration of chiral dopant is varied to adjust the chirality of the monomer mixture. Liquid crystal elastomer (LCE) microactuators doped with 0.05 wt% of chiral dopant produce needle‐shaped actuators that bend from flat to an angle of 27.2 ± 11.3° at 180 °C. Higher concentrations of chiral dopant lead to actuators with reduced bending, and lower concentrations of chiral dopant lead to actuators with poorly controlled bending. Asymmetric molecular alignment inside 3D structure is confirmed by sectioning actuators. Arrays of microactuators that all bend in the same direction can be fabricated if symmetry of geometry of the microstructure is broken. It is envisioned that the new platform to synthesize microstructures can further be applied in soft robotics and biomedical devices. 

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