Particle physics is a fundamental physics discipline that aims to explore the mysteries of microscopic constitutes of matter and their interactions. This article reviews the major progress and breakthroughs of particle physics in 2021, including dark matter physics, neutrino and astroparticle physics, muon anomalous magnetic moment, flavor physics, new exotic hadron state and strong interaction physics, Higgs physics, electroweak physics and searches for beyond the standard model new physics. A brief outlook of near future particle physics hotspots and large scale scientific facilities is also presented.

Since the birth of laser, optics and photonics have penetrated into all aspects of people's life. This article reviews the 2021 major progress in the field of optics and photonics and draws up an inventory of fifteen optical research directions, in which the related research may likely have enormous impact on human existence and way of life in the future.

This paper reviews the important progress made in the field of astronomy in 2021, including a series of significant achievements and events, as listed below:probes from various countries arrived on Mars and carried out scientific research; the Insight revealed the interior structure of Mars for the first time; LHAASO observed photons of 1.4 PeV; DAMPE obtained precise energy spectrum covering the energy range 70 GeV to 80 TeV of helium nucleus; IceCube captured neutrino from a tidal disruption event and a particle shower at the Glashow resonance. Astronomers also found the evidence of dark energy existence in the early time of the Universe, released the latest research results of the chang'e-5 samples, found an astronomical optical observing site in Lenghu, which was the best in China, detected the gravitational wave definitely from the neutron-black hole coalescence for the first time, discovered the observational evidence showing some fast radio bursts originated from a magnetar, studied a fast radio burst comprehensively with the help of FAST telescope, proposed a novel method to obtain the mass of a black hole, published the polarized image of black hole M87^*. In addition, James Webb Space Telescope was launched, and Chinese Xihe solar satellite was successfully launched as well.

Space science exploration in 2021 achieved fruitful outcoming. In this article we review the major results based on the data of space science missions, mainly in categories of astrophysics, heliophysics, planetary science and space earth science. Besides, we analyze the outstanding engineering success and science achievement of several solar system probes. These results include but not limited to:the man-made satellite for the first time touched the solar corona, a space hurricane in the Earth polar cap region was detected, two-billion-year-old lunar basalts sampling helped to better understand the lunar evolution history, Martian seismic data revealed the deep interior of Mars, persistent plasma waves were detected in interstellar space, and a new all-sky map of Milky Way was charted. Looking into the future, we also analyze two very important strategic plans released in that year, i.e., European Space Agency's Voyager 2050 and National Academy of Sciences' Astro2020. As to newborn Chinese space science missions, the science data of DAMPE mission was released, and science missions like InsightHXMT and Chang'e 5 were yielding world-class output.

Human activities in the broad area of sci-tech research have continued despite the still rampant global COVID-19 pandemic. In the past 2021, significant progress was made in bioscience and many related disciplines. This review highlights a few breakthroughs that directly addressed both immediate and long term needs of humans in facing those contingent and constant challenges. Specifically, we briefly present to the reader the progress made in the fields of theoretical and practical structure biology, chemical biology, COVID-19 antibody development, application of gene editing techniques, drug treatment for human mental disorders and development in imaging techniques. 2021 will be remembered for those Chinese scientists who made their particular contributions to the frontier research.

The human microbiome refers to the sum of all microorganisms and their genetic materials that live on the human body and are mutual, symbiotic and disease-causing. In recent years, research on the human microbiome has accelerated, and the content of it has become more extensive and in-depth. As its scientific significance becomes more prominent, human microbiome is even regarded as "another organ of the human body". In addition, the research has also provided new perspectives for disease treatment and nutritional intervention, and human microbiome becomes another growth point for pharmaceutical and food industries. This article reviews the progress of human microbiome research in 2021 from aspects of composition and distribution characteristics of human microbiome, role in full life cycle, relationship with disease occurrence and development, influence on nutrition metabolism and drug function, as well as the development of reconstruction therapy.

Numerous achievements have been made in clinical psychology during 2021. This paper reviews the hot research topics in this field, including the influence of COVID-19 pandemic on mental health, obesity and mental health, adolescent depression, diagnosis and intervention of autistic children, psychotherapy of anxiety disorders, and virtual reality therapy in mental health. This paper aims to provide comprehensive understanding of clinical psychology for researchers and promote the development of mental health services.

In this review we address the classification, spatiotemporal distribution, and prevalence characteristics of SARS-CoV-2 variants, followed by a description of key mutations and their biological functions in virus variants. We also summarize the novel understandings on genetic origins and interspecies transmission risk of SARS-CoV-2 between humans and animals in 2021. Next, we describe the development and effectiveness of current vaccines and antiviral drugs in 2021, and the potential virus evolution under the pressure of insufficient immune protection provided by current vaccines. In the end, we put forward targeted suggestions on pandemic control, virus surveillance, and the development of highly effective vaccines and drugs.

Wastewater-based epidemiology (WBE) is a powerful and cost-effective tool for investigating chemicals consumption/pathogens infection and health status of populations, and is rapidly evolving as COVID-19 continues to ravage the world. This paper reviews the significant developments and breakthroughs of WBE in 2021 including collection, pretreatment and analysis of sewage samples, materials stability, correction factors calculation and uncertainty analysis, implementation cases and so on, based on the research findings published in international top academic journals or the most influential achievements. It provides reference for clarifying the development of WBE and promoting the research and application of WBE.

Inspired by electromagnetic metamaterials, mechanical metamaterials have been a focus in various fields. In recent years, benefited from the rapid development of 3D printing and automated machine learning designing methods, the mechanical metamaterials study has achieved a series of excellent results, enabling metamaterials to have unconventional and counterintuitive mechanical properties that are rare or even non-existent in nature. Unlike common materials, the properties of mechanical metamaterials are mainly derived from artificial structures rather than chemical components, which permits a huge designing space. This article reviews some relevant research highlights of mechanical metamaterials of 2021 in terms of unusual mechanical properties, intelligent design methods and additive manufacturing-based fabrication technology, in which the fusion of mechanical metamaterials and machine learning, modular design and 3D printing is focused and discussed.

Structural color has attracted considerable scientific interest and industrial exploration in a wide range of technologies, such as next-generation display, healthy coating, chromaticity sensor, and anti-counterfeiting label. Printing as a facile way to pattern structural color is essential to achieve these advanced functions. In 2021, structural color printing made great breakthroughs in mechanisms, techniques, and functions. This article reviews the research hotspots and representative achievements in structural color printing including full-color printing with transparent ink, high-performance printing with colloidal photonic crystals, high-resolution printing with media materials, dynamic printing with responsive materials, etc.

In 2021, reconfigurable intelligent surface technology made a series of breakthroughs. As a possible emerging paradigm in future wireless network research, reconfigurable intelligent surface is expected to be a way to flexibly control electromagnetic waves through programming so as to build an intelligent wireless environment. In this paper, key hotspots of reconfigurable intelligent surface technology in 2021 are summarized. By reviewing the integration of intelligent reconfigurable surface and advanced communication technologies, including multiple input multiple output technology, non-orthogonal multiple access technology, mobile edge computing technology and UAV communication technology, the future development of reconfigurable intelligent surface technology is discusssed.

In 2021, the concept of metaverse sparked a new stage of rapid development of the extended reality industry. Over the past year, the extended reality (XR) industry has made significant breakthroughs in hardware, software, contents and applications. XR as an important entrance to metaverse is facing new opportunity and challenge. Corresponding suggestions and countermeasures are put forward through the analysis of the main problems in the development process of China's XR industry, such as increasing innovative R & D support for cost reduction and mass production of bottom core parts, breaking the barrier to transform technical achievement of system software and development tool, consolidating new network infrastructure, and enhancing content production capacity.

In recent years, construction of smart cities has gradually shifted from the information technological project-driven model to a human-centric development model. In 2021, the research on smart cities went in-depth, and planning and construction of smart cities were efficient and coordinated. The integration development of digital scenes and urban physical spaces became increasingly obvious. This study reviews the hot topics on construction and research of smart cities in 2021 from six aspects, including achieving the goals of carbon emissions and carbon neutral driven by smart city construction, spatial governance modernization promotion by digital governance, digital economy development and new drivers of high-quality development, quality life empowering by smart community development, new-style infrastructure promotion by application of intelligent technologies, and urban scenes construction driven by intelligent technology innovation.

Urban renewal, resilient city construction, protection and restoration of water ecosystems are important issues in the construction of ecological civilization in the new era of China. Waterfront space is an important spatial element for the coordinated development of urban and rural life, production and ecology, which also has a great impact on the quality, environment and ecology of the city. At the same time, the ecological restoration of the waterfront space is closely related to the "double-carbon" goals and promotes each other. This article summarizes the construction of water-suitable cities and reviews the research results of waterfront space among ecological restoration, planning and construction, operation, and management, new technology application and technical specification guide in 2021. On this basis, it proposes that the future research of urban planning and construction should further strengthen the waterfront space value enhancement and the methodology construction of ecological restoration. In addition, interdisciplinary integration and application of new technologies are still important in the future research, and introduction and practice of international advanced concepts should also be emphasized.

With the development of artificial intelligence and enhancement of airborne task platform performance, the unmanned aerial vehicle (UAV) technology and its application have presented a new development situation. This paper summarizes and analyzes the technological hotspots of UAVs in 2021 with respect to UAV regulations, autonomous control technologies, antiUAV technologies and application fields for UAVs. Artificial intelligence and visual navigation technology not only enable UAVs to acquire more powerful functions but also largely extend and deepen the comprehensive application field. Especially, the human-robot will be getting closer and closer with the constant improvement of human-robot interaction technology. In summary, UAVs will be more miniaturized, clustered, intelligent and cross-domain. The deep integration with artificial intelligence and cross-platform practical applications of UAVs are still hotspots for the present and future.

Facing the era of knowledge network, innovation design presents new features such as green and low-carbon, network intelligence, open integration, co-creation and sharing. In 2021, the innovative design research made important progress in theoretical methods, technical means, and application expansion. From the perspectives of bio-inspired design, data-driven design, sustainable design, artificial-intelligence-based design, and COVID-oriented design, the annual research hotspots and representative results of innovation design of 2021 are reviewed, and the future research hotspots of innovative design are prospected.