Patterned Optoelectronic Tweezers: A New Scheme for Selecting, Moving, and Storing Dielectric Particles and Cells

Light patterning semiconductor nanoparticles by modulating surface charges

Optical patterning of colloidal particles is a scalable and cost-effective approach for creating multiscale functional structures. Existing methods often use high-intensity light sources and customized optical setups, making them less feasible for large-scale microfabrication processes. Here, we report an optical patterning method for semiconductor nanoparticles by light-triggered modulation of their surface charge. Rather than using light as the primary energy source, this method utilizes UV-induced cleavage of surface ligands to modify surface charges, thereby facilitating the self-assembly of nanoparticles on a charged substrate via electrostatic interactions. By using citrate-treated ZnO nanoparticles, uniform ZnO patterns with variable thicknesses can be achieved. These multilayered ZnO patterns are fabricated into a UV detector with an on/off ratio exceeding 104. Our results demonstrate a simple yet effective way to pattern semiconductor nanoparticles, facilitating the large-scale integration of functional nanomaterials into emerging flexible and robotic microdevices.

Recent Advancements in Nanophotonics for Optofluidics

Optofluidics is dedicated to achieving integrated control of particle and fluid motion, particularly on the micrometer scale, by utilizing light to direct fluid flow and particle motion. The field has seen significant growth recently, driven by the concerted efforts of researchers across various scientific disciplines, notably for its successful applications in biomedical science. In this review, we explore a range of optofluidic architectures developed over the past decade, with a primary focus on mechanisms for precise control of micro and nanoscale biological objects and their applications in sensing. Regarding nanoparticle manipulation, we delve into mechanisms based on optical nanotweezers using nanolocalized light fields and light-based hybrid effects with dramatically improved performance and capabilities. In the context of sensing, we emphasize those works that used optofluidics to aggregate molecules or particles to promote sensing and detection. Additionally, we highlight emerging research directions, encompassing both fundamental principles and practical applications in the field.

Steering Micromotors via Reprogrammable Optoelectronic Paths

Steering micromotors is important for using them in practical applications and as model systems for active matter. This functionality often requires magnetic materials in the micromotor, taxis behavior of the micromotor, or the use of specifically designed physical boundaries. Here, we develop an optoelectronic strategy that steers micromotors with programmable light patterns. In this strategy, light illumination turns hydrogenated amorphous silicon conductive, generating local electric field maxima at the edge of the light pattern that attracts micromotors via positive dielectrophoresis. As an example, metallo-dielectric Janus microspheres that self-propelled under alternating current electric fields were steered by static light patterns along customized paths and through complex microstructures. Their long-term directionality was also rectified by ratchet-shaped light patterns. Furthermore, dynamic light patterns that varied in space and time enabled more advanced motion controls such as multiple motion modes, parallel control of multiple micromotors, and the collection and transport of motor swarms. This optoelectronic steering strategy is highly versatile and compatible with a variety of micromotors, and thus it possesses the potential for their programmable control in complex environments.

Programmable, Spatiotemporal Control of Colloidal Motion Waves via Structured Light

Traveling waves in a reaction-diffusion system are essential for long-range communication in living organisms and inspire biomimetic materials of similar capabilities. One recent example is the traveling motion waves among photochemically oscillating, silver (Ag)-containing colloids. Being able to manipulate these colloidal waves holds the key for potential applications. Here, we have discovered that these motion waves can be confined by light patterns and that the chemical clocks of silver particles are moved forward by reducing local light intensity. Using these discoveries as design principles, we have applied structured light technology for the precise and programmable control of colloidal motion waves, including their origins, propagation directions, paths, shapes, annihilation, frequency, and speeds. We have also used the controlled propagation of colloidal waves to guide chemical messages along a predefined path to activate a population of micromotors located far from the signal. Our demonstrated capabilities in manipulating colloidal waves in space and time offer physical insights on their operation and expand their usefulness in the fundamental study of reaction-diffusion processes. Moreover, our findings inspire biomimetic strategies for the directional transport of mass, energy, and information at micro- or even nanoscales.

Parallel Manipulation and Flexible Assembly of Micro-Spiral via Optoelectronic Tweezers

Micro-spiral has a wide range of applications in smart materials, such as drug delivery, deformable materials, and micro-scale electronic devices by utilizing the manipulation of electric fields, magnetic fields, and flow fields. However, it is incredibly challenging to achieve a massively parallel manipulation of the micro-spiral to form a particular microstructure in these conventional methods. Here, a simple method is reported for assembling micro-spirals into various microstructures via optoelectronic tweezers (OETs), which can accurately manipulate the micro-/bio-particles by projecting light patterns. The manipulation force of micro-spiral is analyzed and simulated first by the finite element simulation. When the micro-spiral lies at the bottom of the microfluidic chip, it can be translated or rotated toward the target position by applying control forces simultaneously at multiple locations on the long axis of the micro-spiral. Through the OET manipulation, the length of the micro-spiral chain can reach 806.45 μm. Moreover, the different parallel manipulation modes are achieved by utilizing multiple light spots. The results show that the micro-spirulina can be manipulated by a real-time local light pattern and be flexibly assembled into design microstructures by OETs, such as a T-shape circuit, link lever, and micro-coil pairs of devices. This assembly method using OETs has promising potential in fabricating innovative materials and microdevices for practical engineering applications.

Optoelectronic tweezers: a versatile toolbox for nano-/micro-manipulation

This review covers the fundamentals, recent progress and state-of-the-art applications of optoelectronic tweezers technology, and demonstrates that optoelectronic tweezers technology is a versatile and powerful toolbox for nano-/micro-manipulation.

Versatile acoustic manipulation of micro-objects using mode-switchable oscillating bubbles: transportation, trapping, rotation, and revolution

Controllable on-chip multimodal manipulation of micro-objects in microfluidic devices is urgently required for enhancing the efficiency of potential biomedical applications. However, fixed design and driving models make it difficult to achieve switchable multifunction efficiently in a single device. In this study, a versatile bubble-based acoustofluidic device is proposed for multimodal manipulation of micro-objects in a biocompatible manner. Identical bubbles trapped over the bottom microcavities are made to flexibly switch between four different oscillatory motions by varying the applied frequency to generate corresponding modes of streaming patterns in the microchannel. Such regular modes enable stable transportation, trapping, 3D rotation, and circular revolution of the micro-objects, which were experimentally and numerically verified. The mode-switchable manipulations can be noninvasively applied to particles, cells, and organisms with different sizes, shapes, and quantities and can be controlled by key driving parameters. Moreover, 3D cell reconstruction is developed by applying the out-of-plane rotational mode and analyzed for illustration of cell surface morphology while quantifying reliably basic cell properties. Finally, a simple platform is established to integrate user-friendly function control and reconstruction analysis. The mode-switchable acoustofluidic device features a versatile, controllable, and contactless micro-object manipulation method, which provides an efficient solution for biomedical applications.

Interaction between positive and negative dielectric microparticles/microorganism in optoelectronic tweezers

Optoelectronic tweezers (OET) is a noncontact micromanipulation technology for controlling microparticles and cells. In the OET, it is necessary to configure a medium with different electrical properties to manipulate different particles and to avoid the interaction between two particles. Here, a new method exploiting the interaction between different dielectric properties of micro-objects to achieve the trapping, transport, and release of particles in the OET system was proposed. Besides, the effect of interaction between the micro-objects with positive and negative dielectric properties was simulated by the arbitrary Lagrangian-Eulerian (ALE) method. In addition, compared with conventional OET systems relying on fabrication processes involving the assembly of photoelectric materials, a contactless OET platform with an iPad-based wireless-control interface was established to achieve convenient control. Finally, this platform was used in the interaction of swimming microorganisms (positive-dielectric properties) with microparticles (negative-dielectric properties) at different scales. It showed that one particle could interact with 5 particles simultaneously, indicating that the interaction can be applied to enhance the high-throughput transportation capacities of the OET system and assemble some special microstructures. Owing to the low power, microorganisms were free from adverse influence during the experiment. In the future, the interaction of particles in a simple OET platform is a promising alternative in micro-nano manipulation for controlling drug release from uncontaminated cells in targeted therapy research.

Optoelectronic manipulation of bio-droplets containing cells or macromolecules by active ferroelectric platforms

Photovoltaic optoelectronic tweezers are a useful platform with many applications in optical manipulation and nanotechnology. They are based on electrical forces associated with the bulk photovoltaic effect presented by certain ferroelectric crystals, such as Fe doped lithium niobate. This manipulation technique has experienced huge developments in recent years, although its use in biology and biomedicine is still scarce. Recently, a novel strategy has been reported that extends the platform capabilities to the manipulation of polar droplets, such as water and aqueous bio-droplets, promising great potential for biological applications. In this work, we are taking this challenge, addressing the manipulation of cells and macromolecules contained inside the droplets by optoelectronic ferroelectric platforms. On the one hand, experiments of photoelectric induced migration of DNA and sperm droplets have been successfully developed and the corresponding droplet dynamics have been analyzed in depth. From this analysis, parameters of the biomaterial such as its concentration and its electrical charge have been evaluated, showing the sensing capabilities of the platform. In fact, the charge of sperm cells has been demonstrated to be negative, and the relative sperm concentration of the samples determined. On the other hand, experiments on the light-induced merging of two droplets have been carried out. Specifically, sperm droplets are mixed with droplets containing acridine orange, a convenient dye for visualization purposes. The spermatozoa become clearly visible in the final droplet through fluorescence imaging. The results point out the multiple possibilities of application of the optoelectronic ferroelectric platform in biology and biomedicine including the development of "lab on a chip" devices. Hence, these capabilities introduce these platforms as an efficient tool in biotechnology.

Reconfigurable multi-component micromachines driven by optoelectronic tweezers

There is great interest in the development of micromotors which can convert energy to motion in sub-millimeter dimensions. Micromachines take the micromotor concept a step further, comprising complex systems in which multiple components work in concert to effectively realize complex mechanical tasks. Here we introduce light-driven micromotors and micromachines that rely on optoelectronic tweezers (OET). Using a circular micro-gear as a unit component, we demonstrate a range of new functionalities, including a touchless micro-feed-roller that allows the programming of precise three-dimensional particle trajectories, multi-component micro-gear trains that serve as torque- or velocity-amplifiers, and micro-rack-and-pinion systems that serve as microfluidic valves. These sophisticated systems suggest great potential for complex micromachines in the future, for application in microrobotics, micromanipulation, microfluidics, and beyond.

Light-driven micromotors can convert energy to motion in sub-millimeter dimensions. Here, the authors extend this concept and introduce reconfigurable micromachines with multiple components, driven by optoelectronic tweezers, and demonstrate new functionalities.

Integrated Assembly and Photopreservation of Topographical Micropatterns

Micromanipulation techniques that are capable of assembling nano/micromaterials into usable structures such as topographical micropatterns (TMPs) have proliferated rapidly in recent years, holding great promise in building artificial electronic and photonic microstructures. Here, a method is reported for forming TMPs based on optoelectronic tweezers in either "bottom-up" or "top-down" modes, combined with in situ photopolymerization to form permanent structures. This work demonstrates that the assembled/cured TMPs can be harvested and transferred to alternate substrates, and illustrates that how permanent conductive traces and capacitive circuits can be formed, paving the way toward applications in microelectronics. The integrated, optical assembly/preservation method described here is accessible, versatile, and applicable for a wide range of materials and structures, suggesting utility for myriad microassembly and microfabrication applications in the future.

Pulsed Microfluid Force-Based On-Chip Modular Fabrication for Liver Lobule-Like 3D Cellular Models

In vitro three-dimensional (3D) cellular models with native tissue-like architectures and functions have potential as alternatives to human tissues in regenerative medicine and drug discovery. However, it is difficult to replicate liver constructs that mimic in vivo microenvironments using current approaches in tissue engineering because of the vessel-embedded 3D structure and complex cell distribution of the liver. This paper reports a pulsed microflow-based on-chip 3D assembly method to construct 3D liver lobule-like models that replicate the spatial structure and functions of the liver lobule. The heterogeneous cell-laden assembly units with hierarchical cell distribution are fabricated through multistep photopatterning of different cell-laden hydrogels. Through fluid force interaction by pulsed microflow, the hierarchical assembly units are driven to a stack, layer by layer, and thus spatially assemble into 3D cellular models in the closed liquid chamber of the assembly chip. The 3D models with liver lobule-like hexagonal morphology and radial cell distribution allow the dynamic perfusion culture to maintain high cell viability and functional expression during long-term culture in vitro. These results demonstrate that the fabricated 3D liver lobule-like models are promising for drug testing and the study of individual diagnoses and treatments.

A Versatile Optoelectronic Tweezer System for Micro-Objects Manipulation: Transportation, Patterning, Sorting, Rotating and Storage

Non-contact manipulation technology has a wide range of applications in the manipulation and fabrication of micro/nanomaterials. However, the manipulation devices are often complex, operated only by professionals, and limited by a single manipulation function. Here, we propose a simple versatile optoelectronic tweezer (OET) system that can be easily controlled for manipulating microparticles with different sizes. In this work, we designed and established an optoelectronic tweezer manipulation system. The OET system could be used to manipulate particles with a wide range of sizes from 2 μm to 150 μm. The system could also manipulate micro-objects of different dimensions like 1D spherical polystyrene microspheres, 2D rod-shaped euglena gracilis, and 3D spiral microspirulina. Optical microscopic patterns for trapping, storing, parallel transporting, and patterning microparticles were designed for versatile manipulation. The sorting, rotation, and assembly of single particles in a given region were experimentally demonstrated. In addition, temperatures measured under different objective lenses indicate that the system does not generate excessive heat to damage bioparticles. The non-contact versatile manipulation reduces operating process and contamination. In future work, the simple optoelectronic tweezers system can be used to control non-contaminated cell interaction and micro-nano manipulation.

Permeable hollow 3D tissue-like constructs engineered by on-chip hydrodynamic-driven assembly of multicellular hierarchical micromodules

Engineered three-dimensional (3D) microtissues that recapitulate in vivo tissue morphology and microvessel lumens have shown significant potential in drug screening and regenerative medicine. Although microfluidic-based techniques have been developed for bottom-up assembly of 3D tissue models, the spatial organization of heterogeneous micromodules into tissue-specific 3D constructs with embedded microvessels remains challenging. Inspired by a hydrodynamic-based classic game which stacks rings in water through the flow, a facile strategy is proposed for effective assembly of heterogeneous hierarchical micromodules with a central hole, into permeable hollow 3D tissue-like constructs through hydrodynamic interaction in a versatile microfluidic chip. The micromodules are fabricated by in situ multi-step photo-crosslinking of cell-laden hydrogels with different mechanical properties to give the high fidelity. With the hydrodynamic interaction derived from the discontinuous circulating flow, the micromodules are spatially organized layer-by-layer to form a 3D construct with a microvessel-like lumen. As an example, a ten-layered liver lobule-like construct containing inner radial-like poly(ethylene glycol) diacrylate (PEGDA) structure with hepatocytes and outer hexagonal gelatin methacrylate (GelMA) structure with endothelial cells are assembled in 2 min. During 10 days of co-culture, cells maintain high viability and proliferated along with the composite lobule-like morphology. The 3D construct owns a central lumen, which allows perfusion culture to promote albumin secretion. We anticipate that this microassembly strategy can be used to fabricate vascularized 3D tissues with various physiological morphologies as alternatives for biomedical research applications. STATEMENT OF SIGNIFICANCE: Microfluidic-based assembly is an attractive approach for the fabrication of 3D tissue models using cell-laden hydrogel microstructures with single mechanical stability. However, native tissues are complex 3D structures with indispensable vessels and multiple mechanical properties, which is still challenging to recreate. This study proposed a novel strategy to fabricate tissue-like 3D constructs with embedded lumen through hydrodynamic interaction using multicellular micromodules with hierarchical mechanical properties. The resultant hollow 3D constructs allow perfusion co-culture to enhance cell activity. This strategy relies on a simple and facile microfluidic chip to fabricate various 3D tissue-like constructs with hierarchical mechanical properties and permeable lumen, which can potentially be used as in vitro perfusion models for biomedical research.

Gravitational sedimentation-based approach for ultra-simple and flexible cell patterning coculture on microfluidic device

Combining patterning coculture technique with microfluidics enables the reconstruction of complex in-vivo system to facilitate in-vitro studies on cell-cell and cell-environment interactions. However, simple and versatile approaches for patterning coculture of cells on microfluidic platforms remain lacking. In this study, a novel gravitational sedimentation-based approach is presented to achieve ultra-simple and flexible cell patterning coculture on a microfluidic platform, where multiple cell types can be patterned simultaneously to form a well-organized cell coculture. In contrast to other approaches, the proposed approach allows the rapid patterning of multiple cell types in microfluidic channels without the use of sheath flow and a prepatterned functional surface. This feature greatly simplifies the experimental setup, operation, and chip fabrication. Moreover, cell patterning can be adjusted by simply modifying the cell-loading tubing direction, thereby enabling great flexibility for the construction of different cell patterns without complicating the chip design and flow control. A series of physical and biological experiments are conducted to validate the proposed approach. This research paves a new way for building physiologically realistic in-vitro coculture models on microfluidic platforms for various applications, such as cell-cell interaction and drug screening.

Nanoscale integration of single cell biologics discovery processes using optofluidic manipulation and monitoring

The new and rapid advancement in the complexity of biologics drug discovery has been driven by a deeper understanding of biological systems combined with innovative new therapeutic modalities, paving the way to breakthrough therapies for previously intractable diseases. These exciting times in biomedical innovation require the development of novel technologies to facilitate the sophisticated, multifaceted, high-paced workflows necessary to support modern large molecule drug discovery. A high-level aspiration is a true integration of "lab-on-a-chip" methods that vastly miniaturize cellulmical experiments could transform the speed, cost, and success of multiple workstreams in biologics development. Several microscale bioprocess technologies have been established that incrementally address these needs, yet each is inflexibly designed for a very specific process thus limiting an integrated holistic application. A more fully integrated nanoscale approach that incorporates manipulation, culture, analytics, and traceable digital record keeping of thousands of single cells in a relevant nanoenvironment would be a transformative technology capable of keeping pace with today's rapid and complex drug discovery demands. The recent advent of optical manipulation of cells using light-induced electrokinetics with micro- and nanoscale cell culture is poised to revolutionize both fundamental and applied biological research. In this review, we summarize the current state of the art for optical manipulation techniques and discuss emerging biological applications of this technology. In particular, we focus on promising prospects for drug discovery workflows, including antibody discovery, bioassay development, antibody engineering, and cell line development, which are enabled by the automation and industrialization of an integrated optoelectronic single-cell manipulation and culture platform. Continued development of such platforms will be well positioned to overcome many of the challenges currently associated with fragmented, low-throughput bioprocess workflows in biopharma and life science research.

Size-scaling effects for microparticles and cells manipulated by optoelectronic tweezers

In this work, we investigated the use of optoelectronic tweezers (OET) to manipulate objects that are larger than those commonly positioned with standard optical tweezers. We studied the forces that could be produced on differently sized polystyrene microbeads and MCF-7 breast cancer cells with light-induced dielectrophoresis (DEP). It was found that the DEP force imposed on the bead/cell did not increase linearly with the volume of the bead/cell, primarily because of the non-uniform distribution of the electric field above the OET bottom plate. Although this size-scaling work focuses on microparticles and cells, we propose that the physical mechanism elucidated in this research will be insightful for other micro-objects, biological samples, and micro-actuators undergoing OET manipulation.

The optoelectronic microrobot: A versatile toolbox for micromanipulation

Microrobotics extends the reach of human-controlled machines to submillimeter dimensions. We introduce a microrobot that relies on optoelectronic tweezers (OET) that is straightforward to manufacture, can take nearly any desirable shape or form, and can be programmed to carry out sophisticated, multiaxis operations. One particularly useful program is a serial combination of "load," "transport," and "deliver," which can be applied to manipulate a wide range of micrometer-dimension payloads. Importantly, microrobots programmed in this manner are much gentler on fragile mammalian cells than conventional OET techniques. The microrobotic system described here was demonstrated to be useful for single-cell isolation, clonal expansion, RNA sequencing, manipulation within enclosed systems, controlling cell-cell interactions, and isolating precious microtissues from heterogeneous mixtures. We propose that the optoelectronic microrobotic system, which can be implemented using a microscope and consumer-grade optical projector, will be useful for a wide range of applications in the life sciences and beyond.