Advanced Imaging Strategies Based on Intelligent Micro/Nanomotors

Self-propelled micro-/nanomotors (MNMs), which are defined as micro-/nanodevices capable of converting various energy into autonomous motion, can be used to pick up, transport, and release various cargoes within a liquid medium. They have important potential applications, for example, in drug delivery, biosensors, protein and cell separation, microsurgeries and environment remediation. This review comprehensively introduces the design strategies and structures of self-propelled MNMs along with an outlook for their future development. It starts with the summary of the propulsion mechanisms of self-propelled MNMs of bubble recoiling and self-phoresis induced by the asymmetric release of products or heat. For bubble recoiling propulsion, the continuous momentum change is caused by a jet of bubbles, while for self-phoresis propulsion, the MNMs move in a local electric field, concentration gradient, surface tension gradient, or temperature gradient, etc. After systematically and in-depth understanding these propulsion mechanisms, it has been pointed out that the key to design self-propelled MNMs is to construct an asymmetric field across micro-/ nanoparticles. Following this clue, the structures evolution and simplification methods of self-propelled MNMs are reviewed. Janus structures and multilayer-tubular structures, which are prepared through asymmetric modification process, electrochemical synthesis, template-assisted method, rolled-up nanotech, etc., have been firstly proposed to construct asymmetric fields across micro-/nanoparticles for their propulsion. However, the complicated structure and preparation process hinder the application of MNMs. Anisotropic single-component irregular particles, tubes and bowl-like MNMs, which are obtained by dry spinning method, “growing-bubble”-templated self-assembly, etc., have been subsequently achieved by utilizing their anomalous morphology and the nucleation preference of bubble molecules on different surfaces. This kind of MNMs show somewhat simple structure and can be easily fabricated, but the motion direction is still difficult to control because of the Brownian motion. Isotropic semiconducting MNMs have been recently developed by taking advantage of the limited light penetration depth in the isotropic photoresponsive particles, of which the motion is independent of the rotational Brownian motion. This suggests a remarkable breakthrough in design strategy of MNMs due to the simple isotropic structure of the motor and the controllability in both motion direction and speed by light. Besides the evolution of self-propelled MNMs from the complicated structure to the simplified one, some remarkable progresses have also been made on the motion control, functionalization, etc. For example, the speed and state of MNMs can so far be easily adjusted by the concentration of fuels, the intensity of external fields, etc. The direction can be controlled accurately by magnetic field, electric field, light, etc. Numerous complex tasks can also be performed effectively, such as protein separation, drug delivery, environmental detection and remediation, etc. Lastly, an outlook is also provided on the future development and main challenges of self-propelled MNMs. The future development of MNMs should be focused on improving energy conversion efficiency through optimization of structures, exploring new propulsion mechanisms and endowing MNMs with environmental responses for self-navigation, detection, and specific operations. In this way, MNMs will approach to the practical applications in biomedicine, environment treatment, microengineering, etc.

Reporter genes are useful scientific tools for analyzing promoter activity, transfection efficiency, and cell migration. The current study has validated the use of tyrosinase (involved in melanin production) as a dual reporter gene for magnetic resonance and photoacoustic imaging. MCF-7 cells expressing tyrosinase appear brown due to melanin. Magnetic resonance imaging of tyrosinase-expressing MCF-7 cells in 300 μL plastic tubes displayed a 34 to 40% reduction in T1 compared to normal MCF-7 cells when cells were incubated with 250 μM ferric citrate. Photoacoustic imaging of tyrosinase-expressing MCF-7 cells in 700 μm plastic tubes displayed a 20 to 57-fold increase in photoacoustic signal compared to normal MCF-7 cells. The photoacoustic signal from tyrosinase-expressing MCF-7 cells was significantly greater than blood at 650 nm, suggesting that tyrosinase-expressing cells can be differentiated from the vasculature with in vivo photoacoustic imaging. The imaging results suggest that tyrosinase is a useful reporter gene for both magnetic resonance and photoacoustic imaging.

Bacterial infection, which can trigger varieties of diseases and tens of thousands of deaths each year, poses serious threats to human health. Particularly, the new dilemma caused by biofilms is gradually becoming a severe and tough problem in the biomedical field. Thus, the strategies to address these problems are considered an urgent task at present. Micro/nanomotors (MNMs), also named micro/nanoscale robots, are mostly driven by chemical energy or external field, exhibiting strong diffusion and self-propulsion in the liquid media, which has the potential for antibacterial applications. In particular, when MNMs are assembled in swarms, they become robust and efficient for biofilm removal. However, there is a lack of comprehensive review discussing the progress in this aspect. Bearing it in mind and based on our own research experience in this regard, the studies on MNMs driven by different mechanisms orchestrated for antibacterial activity and biofilm removal are timely and concisely summarized and discussed in this work, aiming to show the advantages of MNMs brought to this field. In addition, an outlook was proposed, hoping to provide the fundamental guidance for future development in this area.

Objective To detect the efficiency of the newly developed PLGA/IO MPs in tracking tendon stem cells (TSCs) by magnetic resonance (MR) and photoacoustic (PA) imaging. Methods Both PLAG/IO MPs and TSCs were prepared and acquired according to the previous study, and TSCs were incubated with PLGA/IO MPs for labeling.TSCs were collected for MR and PA imaging, prussian blue staining was performed, and the iron concentration of labeled TSCs was determined using inductively coupled plasma optical emission spectrometry (ICP-OES) at 3, 7, 14, 21 and 28 days after labeling respectively. The rotator cuff injury model was built on the right side of SD rats by surgery and the labeled TSCs were implanted instantly. Dual-modal MR/PA imaging was performed to observe the implanted labeled TSCs at day 3, 7, 14, 21 and 28 after implantation respectively. Results Along with the increase of labeling time, both MR and PA signal of labeled TSCs decreased gradually, and the amount of intracellular Fe loading was gradually decreased. At day 28, the difference of Fe concentration per cell between labeled TSCs and non-labeled TSCs was not significant (1.45 pg Fe/cell vs 1.17 pg Fe/cell, P>0.05). MR and PA imaging allowed a long-term tracking of labeled TSCs for 21 and 7 days respectively in the rat rotator cuff injury model. Conclusions PLGA/IO MPs are able to label TSCs for up to 21 days, and dual-modal MR/PA imaging could be used to track the labeled TSCs in the rat rotator cuff injury model. Key words: Tendon stem cells; Cell tracking; Magnetic resonance imaging; Photoacoustic imaging

A caspase-3-activatable bimodal probe for photoacoustic and magnetic resonance imaging of tumor apoptosis in vivo

Inflammation is a carefully orchestrated response of the immune system to repair injured tissues and clear various damage factors. However, dysregulated inflammation can eventually contribute to the development and progression of various inflammatory diseases. Although anti-inflammatory drugs have demonstrated certain therapeutic efficacy in clinical settings, significant limitations still persist, highlighting the necessity for the development of improved approaches to address complex inflammatory conditions. Micro/nanomotors (MNMs) have shown significant promise for applications in the biomedical field due to their micro/nano-scale sizes and autonomous movement. Unlike traditional nanoparticles, which exhibit passive diffusion in biological fluids, MNMs can convert external energy into a driving force for self-propulsion. This capability not only enhances the tissue penetration depth and retention rates but also facilitates interaction with inflammatory lesions. Recent efforts have suggested that MNMs for inflammatory disease therapy could provide an efficient therapeutic effect. Herein, we mainly introduce the recent advances in inflammatory disease therapy based on MNMs. We conclude by discussing both the obstacles and potential opportunities for MNMs innovations in addressing inflammation.

MOF-based micro/nanomotors (MOFtors): Recent progress and challenges

Life is a highly ordered combination, and the basic biological processes of cells and tissues are essentially controlled by the structural order of biomolecular assembly, in which the conformational characteristics of biomolecule arrangement, orientation, helix, and folding are closely related to the physiological functions of biological tissues. In the skin, muscle, and nerve tissues of living animals, for instance, fibrous proteins, collagen, nerve fibers, and DNA frequently exhibit molecular spatial conformation properties such as particular alignment or helical structure, and such tissues have distinct optical polarization responses. The fundamental structural foundation for tissues to carry out certain activities is provided by molecular conformational characteristics. Early illness diagnosis will be aided by the accurate detection and efficient revelation of molecular conformational characteristics and their changes. The microscopic organization, structure, orientation, chirality, and other structural details of living things or materials can be obtained by using polarization imaging. The analysis of the imaging depth and polarization data is challenging, despite its widespread usage in the fields of material detection and biological imaging. Photoacoustic imaging preserves both the great contrast of optical imaging and the deep penetration of ultrasonic imaging by using light as an excitation source and ultrasound as the carrier for information transmission. While keeping the benefits of non-invasiveness, it is capable of high-resolution imaging, deep penetration, and functional imaging. A polarized photoacoustic imaging technology has recently been developed to complement polarization optical imaging and allow the collection of three-dimensional polarization data from deeper layers of the medium. This provides a straightforward and efficient method of measuring the polarimetry of tissues, suggesting substantial promise for both biological imaging and substance detection. The evolution of polarized photoacoustic imaging technology is outlined in this paper. First, the technical underpinnings of polarized photoacoustic imaging are described. Then, from the two applications of biological tissue imaging and nanomaterial detection, the related research progress of polarized photoacoustic microscopic imaging, polarized photoacoustic computational tomography, and polarized photoacoustic nanoparticles' molecular imaging is presented. We briefly explain the depolarization that results from particle size, density, and organization as polarized light travels through tissue. In an anisotropic medium, the change in the mid-incident polarization state of such a sample is caused by tissue birefringence and scattering because of the inherent birefringence effect of molecules, whereas in the isotropic medium, depolarization is largely determined by the density and size of the scatter. The potential applications of polarized photoacoustic imaging are then discussed.

Integrating magnetic resonance (MR) and photoacoustic (PA) contrast agents into porous nanomaterials is a favorable way for screening of potential theranostic nanomedicines. Hollow carbon nanospheres (HCSs) dotted with GdPO4 and γ-Fe2O3 (Gd-Fe) nanoparticles are therefore prepared and studied in this work. The resultant Gd-Fe/HCSs possess a size of ∼100 nm with a cavity of ∼80 nm and a shell thickness of ∼10 nm, where the magnetic Gd-Fe nanoparticles are dotted. Owing to the synergistic effects, the Gd-Fe/HCSs give 2.5 times enhanced PA signals as compared with HCSs as well as the inherited MR imaging properties from Gd-Fe nanoparticles. In vivo MR and PA imaging of the liver in mice are consequently evaluated and validated. Furthermore, taking the tunable particle size, hollow cavity, shell thickness, and dotted amounts of nanoparticles into consideration, our studies here provide a useful structural model for the synergistic integration of MR and PA imaging in HCSs.

Optoacoustic Imaging: An Emerging Modality for the Gastrointestinal Tract

To date, imaging-guided multimodality therapy is important to improve the accuracy of the diagnosis of renal fibrosis, and nanoplatforms for imaging-guided multimodality diagnosis are gaining more and more attention. There are many limitations and deficiencies in clinical use for early-stage diagnosis of renal fibrosis, and multimodal imaging can contribute more thoroughly and provide in-detail information for effective clinical diagnosis. Melanin is an endogenous biomaterial, and we developed an ultrasmall particle size melanin nanoprobe (MNP-PEG-Mn) based on photoacoustic (PA) and magnetic resonance (MR) dual-modal imaging. MNP-PEG-Mn nanoprobe, with the average diameter about 2.7 nm, can be passively targeted for accumulation in the kidney, and it has excellent free radical scavenging and antioxidant abilities without further exacerbating renal fibrosis. Using the normal group signal as a control, the dual-modal imaging results showed that the MR imaging (MAI) and PA imaging (PAI) signals reached the strongest at 6 h when MNP-PEG-Mn entered the 7 day renal fibrosis group via the left vein of the tail end of the mice; however, the strength of the dual-modal imaging signal and the gradient of signal change were significantly weaker in the 28 day renal fibrosis group than in the 7 day renal fibrosis group and normal group. The phenomenon preliminarily indicates that as a PAI/MRI dual-modality contrast medium candidate, MNP-PEG-Mn has outstanding ability in clinical application potential.

Stem cell imaging in vivo is critical to elucidate the homing, distribution, survival, and repair mechanisms and to evaluate the therapeutic effects of engrafted stem cells. Unfortunately, unimodal imaging of stem cells does not simultaneously satisfy all current requirements owing to their intrinsic limitations. Obviously, bimodal or multimodal imaging of stem cells is a promising strategy for circumventing this issue. This study aimed to design and synthesize a novel dual-modal polyethylene glycol-modified magnetic nanoparticle (Fe3+-PEG-MNP) based on natural biomaterials including melanin and Fe ions for photoacoustic (PA) and magnetic resonance (MR) imaging of stem cells in vivo. The Fe3+-PEG-MNPs were characterized and their PA/MR imaging capability and cytotoxicity were evaluated. Bone marrow mesenchymal stem cells (BM-MSCs) labeled with Fe3+-PEG-MNPs were subjected to PA and MR imaging in vitro and in vivo. Consequently, Fe3+-PEG-MNPs displayed many superior properties, including ultra-small particle size, higher stability, water solubility, easy labeling of cells, lower cytotoxicity, high biosafety, excellent capability of PA/MR imaging, high sensitivity and long-term monitoring in vitro and in vivo. In particular, PA and MR signals of labeled BM-MSCs were maintained for at least 35 and 28 d, respectively, in vivo. Therefore, Fe3+-PEG-MNPs are ideal dual-modal PA/MR nanoparticles for non-invasive and effective monitoring of engrafted stem cells in vivo.

Titania-Based Micro/Nanomotors: Design Principles, Biomimetic Collective Behavior, and Applications

Micro/nanomotors (MNMs) are highly versatile small-scale devices capable of converting external energy inputs into active motion. Among the various energy sources, light stands out due to its abundance and ability to provide spatiotemporal control. However, the effectiveness of light-driven motion in complex environments, such as biological tissues or turbid water, is often limited by light scattering and reduced penetration. To overcome these challenges, recent innovations have integrated light-based actuation with other external stimuli-such as magnetic, acoustic, and electrical fields-broadening the functional range and control of MNMs. This review highlights the cutting-edge developments in dual-energy powered MNMs, emphasizing examples where light is paired with secondary energy sources for enhanced propulsion and task performance. Furthermore, insights are offered into the fabrication techniques, biomedical applications, and the future directions of such hybrid MNMs, while addressing the remaining challenges in this rapidly evolving field.

Targeted therapies specific to the BRAF-MEK-ERK signaling pathway have shown great promise in the treatment of malignant melanoma in the last few years, with these drugs now commonly used in clinic. Melanoma cells treated using these agents are known to exhibit increased levels of melanin pigment and tyrosinase activity. In this study we assessed the potential of non-invasive imaging approaches (photoacoustic imaging (PAI) and magnetic resonance imaging (MRI)) to detect melanin induction in SKMEL28 human melanoma cells, following inhibition of Hsp90 and BRAF signaling using 17-AAG and vemurafenib, respectively. We confirmed, using western blot and spectrophotometry, that Hsp90 or BRAF inhibitor-induced melanoma cell differentiation resulted in an upregulation of tyrosinase and melanin expression levels, in comparison to control cells. This post-treatment increase in cellular pigmentation induced a significant increase in PAI signals that are spectrally identifiable and shortening of the MRI relaxation times T1 and {{boldsymbol{T}}}_{{bf{2}}}^{{boldsymbol{ast }}}. This proof-of-concept study demonstrates the potential of MRI and PAI for detecting the downstream cellular changes induced by Hsp90 and BRAF-MEK-targeted therapies in melanoma cells with potential significance for in vivo imaging.