# Most-cited Engineering papers of 2021, with free PDFs

The 25 most-cited Engineering research papers published in 2021, ranked by citations in OpenAlex, with their original abstracts and free PDFs where available.

1. [NeRF](https://nerdi.pages.dev/paper/W4200150166) · 6,718 citations
2. [ORB-SLAM3: An Accurate Open-Source Library for Visual, Visual–Inertial, and Multimap SLAM](https://nerdi.pages.dev/paper/W3043971245) · 4,268 citations · Free PDF
3. [The 30 m annual land cover dataset and its dynamics in China from 1990 to 2019](https://nerdi.pages.dev/paper/W3188524028) · 3,839 citations · Free PDF
4. [Intelligent Reflecting Surface-Aided Wireless Communications: A Tutorial](https://nerdi.pages.dev/paper/W3129486236) · 3,011 citations · Free PDF
5. [Industry 4.0 and Industry 5.0—Inception, conception and perception](https://nerdi.pages.dev/paper/W3205172848) · 2,560 citations
6. [Metal additive manufacturing in aerospace: A review](https://nerdi.pages.dev/paper/W3186352373) · 2,272 citations
7. [A Review on Biosensors and Recent Development of Nanostructured Materials-Enabled Biosensors](https://nerdi.pages.dev/paper/W3126951392) · 2,065 citations · Free PDF
8. [PCT: Point cloud transformer](https://nerdi.pages.dev/paper/W3111535274) · 1,918 citations · Free PDF
9. [Future global urban water scarcity and potential solutions](https://nerdi.pages.dev/paper/W3192714106) · 1,879 citations · Free PDF
10. [Reconfigurable Intelligent Surfaces: Principles and Opportunities](https://nerdi.pages.dev/paper/W3159402647) · 1,863 citations · Free PDF
11. [Material-structure-performance integrated laser-metal additive manufacturing](https://nerdi.pages.dev/paper/W3165805510) · 1,728 citations
12. [The Road Towards 6G: A Comprehensive Survey](https://nerdi.pages.dev/paper/W3126686235) · 1,715 citations · Free PDF
13. [Deep Learning Enabled Semantic Communication Systems](https://nerdi.pages.dev/paper/W3036851434) · 1,583 citations · Free PDF
14. [New approaches and procedures for cancer treatment: Current perspectives](https://nerdi.pages.dev/paper/W3190191592) · 1,577 citations
15. [6G Wireless Systems: Vision, Requirements, Challenges, Insights, and Opportunities](https://nerdi.pages.dev/paper/W3048081097) · 1,556 citations · Free PDF
16. [State of the Art and Prospects for Halide Perovskite Nanocrystals](https://nerdi.pages.dev/paper/W3171574644) · 1,484 citations
17. [Single‐Junction Organic Photovoltaic Cell with 19% Efficiency](https://nerdi.pages.dev/paper/W3197814413) · 1,479 citations
18. [6G Internet of Things: A Comprehensive Survey](https://nerdi.pages.dev/paper/W3188554269) · 1,454 citations · Free PDF
19. [Algorithm Unrolling: Interpretable, Efficient Deep Learning for Signal and Image Processing](https://nerdi.pages.dev/paper/W2995038946) · 1,431 citations · Free PDF
20. [A Brief History of OLEDs—Emitter Development and Industry Milestones](https://nerdi.pages.dev/paper/W3121439431) · 1,416 citations · Free PDF
21. [A Review on Electric Vehicles: Technologies and Challenges](https://nerdi.pages.dev/paper/W3136124754) · 1,349 citations · Free PDF
22. [Electrochemical Impedance Spectroscopy (EIS): Principles, Construction, and Biosensing Applications](https://nerdi.pages.dev/paper/W3202325891) · 1,346 citations · Free PDF
23. [Technologies and perspectives for achieving carbon neutrality](https://nerdi.pages.dev/paper/W3208259726) · 1,332 citations
24. [Augmented reality and virtual reality displays: emerging technologies and future perspectives](https://nerdi.pages.dev/paper/W3209992943) · 1,292 citations · Free PDF
25. [Engineering Design Methods: Strategies for Product Design](https://nerdi.pages.dev/paper/W1492968835) · 1,244 citations

Canonical page: https://nerdi.pages.dev/papers/engineering/most-cited-2021