Leon Holub, Developer in Frankfurt, Hesse, Germany
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Leon Holub

Verified Expert  in Engineering

Software Developer

Frankfurt, Hesse, Germany
Toptal Member Since
October 19, 2022

Leon is a dedicated software developer with a passion for data and security. He has more than ten years of experience with freelance projects and helping various companies develop complex applications before founding his own health-tech startup. Leon is currently looking for exciting freelance projects on a part-time basis.


Kotlin, Swift, iOS, Android, Python, Django, Jupyter Notebook, SciPy...
Python, Django, PostgreSQL, Fitbit API, Pandas, Jupyter Notebook, Docker...
Python, Django, Microservices, Continuous Delivery (CD), Docker, Kubernetes...




Preferred Environment

Linux, MacOS, PyCharm, Python, GitLab

The most amazing...

...experience I've had is founding PATIENT.ONE, a startup focused on helping patients with rheumatoid arthritis that was inspired by my personal history.

Work Experience

Founder and CEO

2020 - 2022
  • Developed an app to gather Bluetooth Low Energy (BLE) sensor data from patients in a clinical study. Managed the study from the study application to evaluation working with clinicians and regulators.
  • Oversaw the European General Data Protection Regulation (GDPR) and Medical Device Regulation (MDR) compliance while working with regulators, lawyers, and experts in data protection, clinical trials, and security.
  • Managed financial planning of the startup and fundraising process, as well as clinical trials.
  • Procured devices and product prototypes while collaborating closely with manufacturers.
Technologies: Kotlin, Swift, iOS, Android, Python, Django, Jupyter Notebook, SciPy, Elasticsearch, Bluetooth LE, Wearables, Cloud, Microservices, GDPR, Medical Devices, Good Clinical Practice (GCP), Software Design, CTO, REST APIs, APIs

Lead and Full-stack Developer

2018 - 2022
  • Developed a web application to transfer data from Fitbit users to a research application for use in a research project.
  • Created data analysis and reporting functions according to researchers' needs. Aggregated and prepared data for further use in research projects.
  • Advised on study design and data protection. Collaborated with IT and scientific researchers.
Technologies: Python, Django, PostgreSQL, Fitbit API, Pandas, Jupyter Notebook, Docker, GitLab CI/CD, Software Design, Web Development, REST APIs, APIs

Back-end Developer

2017 - 2018
  • Developed microservice back ends for various applications.
  • Refactored existing monolithic applications into a microservice architecture.
  • Integrated external services, such as Google Maps API and Elasticsearch, into service back ends.
Technologies: Python, Django, Microservices, Continuous Delivery (CD), Docker, Kubernetes, PostgreSQL, Software Design, Web Development, REST APIs, APIs

Lead Software Development Engineer | CTO

2014 - 2017
  • Worked as a lead developer for an eCommerce startup, leading decisions on the Magento platform development.
  • Extended an existing eCommerce platform to integrate various new custom features, design relaunches, and integrations to in-store apps and manufacturers.
  • Developed an in-store tool for the made-to-measure products of the startup, reducing error rates drastically in the measure-and-order process.
Technologies: PHP, Magento, eCommerce, Data Analytics, JavaScript, AngularJS, MySQL, iOS, Software Design, Web Development, CTO, REST APIs, APIs

Freelance Software Developer | Consultant

2011 - 2016
  • Developed various websites and web applications for multiple clients.
  • Worked with small and medium-sized companies on new and relaunched websites and online shops.
  • Advised clients on an online strategy and technology decisions.
Technologies: PHP, HTML, Java, Magento, Drupal, WordPress, CSS, MySQL, Software Design, Web Development, REST APIs, APIs

Data Aggregation App for Scientific Research Projects

A Django-based app that transfers data from Fitbit users to present researchers with advanced analytics on users' sleep patterns. As a lead developer, I created the application architecture. Also, I worked with researchers, heads of departments, and IT administration to design the application and assisted in approving GDPR compliance for scientific studies and launching the application in a research project.

Mobile App for Data Collection in Clinical Research

An iOS-based mobile app with a connection to BLE sensors and internal iPhone sensors to collect data in a clinical research project. As a lead developer, I collaborated with research experts and designers on creating the app layout. Also, I implemented it and completed all necessary documentation for GDPR and MDR compliance, mediating between experts and regulators.
2015 - 2022

Progress Toward Bachelor's Degree (Paused) in Business Informatics

The University of Hagen - Hagen, Germany

2012 - 2015

Coursework Toward Bachelor's Degree in Computer Science

Technical University Darmstadt - Darmstadt, Germany


Auditgarant Certificate of Completion

Johner Institute


REST APIs, Fitbit API, SciPy, Pandas


Celery, PyCharm, GitLab, GitLab CI/CD, ELK (Elastic Stack)


Django, AngularJS


Microservices, Good Clinical Practice (GCP), Continuous Delivery (CD)


Python, Kotlin, Swift, PHP, JavaScript, HTML, Java, CSS, Rust


Elasticsearch, PostgreSQL, MySQL, MongoDB, Neo4j, Redis, Databases


Linux, Bluetooth LE, MacOS, iOS, Android, Jupyter Notebook, Docker, Kubernetes, Magento, Drupal, WordPress


Software Design, Web Development, APIs, Wearables, GDPR, Software Project Management, Cloud, eCommerce, Data Analytics, Medical Devices, Risk Management, ISO 14971, IEC 62304, IEC 62366-1, IT Security, iPhone, iOS Mobile Device Management (MDM), Clinical Trials, MDR, Computer Science, Programming, Software Architecture, Business Administration, Economics, CTO

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