1 Introduction
2 Digital Twin
2.1 Definition and Characteristics
2.2 Architecture and Enabling Technologies
3 Methodology
4 Human Digital Twin - Definitions, Categorization, and Emerging Challenges
4.1 Definition and Categorization
4.2 Emerging Challenges
5 Discussion and Implications
5.1 Definition of Human Digital Twins
5.2 Challenges of Human Digital Twins
5.3 Design Considerations
5.4 Limitations
5.5 Implications and Future Work
6 Conclusion
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