leehyunhong
KO

Hyunhong Lee · Healthcare app developer · Digital Healthcare · Flutter · Web · WearablesI design,build and operatehealthcare.

Hyunhong Lee · Flutter & Web Software Engineer
From heart-rate workout apps to fertility and stroke-prevention services, I connect app, web and wearables into one product.

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About

A one-person full-stack engineer in medical and healthcare software — responsible from planning through store launch to day-to-day operation.

I write iOS and Android apps in Flutter, web in Next.js, and Apple Watch / Wear OS native modules in Swift and Kotlin. I have designed, built and shipped three-plus products that handle sensitive health data such as heart rate, health checkups and menstrual cycles.

'Build and leave' is not how I work. Crash monitoring, A/B experiments, release management and privacy requirements after launch are handled consistently by one person. I work directly with product owners, clinicians and researchers to turn requirements into code.

Healthcare products shipped & operated
3

onsim · momiplan · HAIM

Lines of code written
300K+

Dart · Swift · Kotlin · TypeScript

Platforms supported
5

iOS · Android · watchOS · Wear OS · Web

Commits
7,000+

onsim alone

Strengths

How I work

  • 01

    App, web, wearable and server — one engineer

    Flutter apps, Swift/Kotlin native modules, NestJS servers and Next.js admin consoles under a single contract. No cross-team interface overhead.

  • 02

    Health-data regulation, implemented in code

    Explicit consent for sensitive data, AES-256-GCM field encryption, pseudonymised research datasets, access audit logs and tamper-proof admin-permission history.

  • 03

    Heart rate · HealthKit · Health Connect specialist

    Real-time workout coaching that arbitrates four heart-rate sources — Apple Watch, Wear OS, BLE sensors and Samsung Health — through one pipeline.

  • 04

    Launch is where the real work starts

    App Store review handling, Crashlytics monitoring, A/B experiments, six rounds of performance work and Firestore cost reduction — with an operations record to show for it.

  • 05

    Decisions documented

    Requests, decisions and changes are preserved in dated worklogs and handoff docs. When people change, the 'why' stays.

  • 06

    Direct collaboration with clinicians and researchers

    Turning ACSM-based exercise prescription rules and a stroke-risk algorithm delivered as slides and spreadsheets into verifiable code.

Services

What you can hand over to me

Not just screens — data structures, permissions, deployment and operations delivered as a whole product. You can also commission only the parts you need.

Flutter mobile apps

Simultaneous iOS and Android launches. Five social logins, onboarding surveys, push, deep links, 13-language localisation, store listing and review handling included.

  • Flutter
  • Dart
  • Provider
  • Firebase
  • App Store / Play review

Wearables & native integration

Standalone Apple Watch (SwiftUI) and Wear OS (Compose) apps, Live Activities, watch-face complications, BLE heart-rate sensors and Samsung Health SDK partnership integration.

  • watchOS
  • Wear OS
  • HealthKit
  • Health Connect
  • BLE

Digital-healthcare domain design

Exercise-prescription rule engines, risk scoring, PHR (FHIR health-checkup) import, cycle and IVF tracking — medical domain logic built as verifiable code.

  • FHIR
  • PHR
  • ACSM
  • Risk models
  • Time-series data

Backend & admin consoles

NestJS REST APIs, Firebase Functions, Supabase (PostgreSQL), AWS ECS / CDK deployment and admin web apps (Next.js · React) for researchers and operators.

  • NestJS
  • Next.js
  • Supabase
  • AWS
  • GitHub Actions

Privacy & security requirements

Field-level encryption, pseudonymised dual-store design against re-identification, access audit logs, DB-trigger permission history, immediate masking and delayed deletion on withdrawal.

  • AES-256-GCM
  • ChaCha20
  • Pseudonymisation
  • Audit logs
  • PIPA (Korea)

Operations, maintenance & handover

Post-launch crash, performance and cost monitoring, regular updates and documented handover. Refactoring and performance work on existing apps is welcome too.

  • Crashlytics
  • A/B testing
  • Performance
  • Documentation
  • Monthly retainer

Selected Work

Healthcare products shipped and in operation

For each project I owned everything from requirements to store launch and operation. Open a card for architecture, engineering challenges and outcomes.

onsim

Digital Healthcare · Wearable

onsim 온심

Heart-rate-based personalised exercise prescription and real-time zone coaching

onsim started from an idea by a sports-medicine professional who has worked with national-team and professional athletes. Based on a health survey and personal profile it proposes a target heart-rate band (the 'onsim zone') and coaches users to stay inside it during workouts.

  • Flutter
  • Swift · SwiftUI
  • Kotlin · Compose
  • NestJS
  • Firebase
  • HealthKit
  • Health Connect
  • BLE
Client
HB Connect
Period
Mar 2024 — present (backend since Jul 2023)
Role
Lead for app, wearables and admin console; sole backend owner since Aug 2026
Platforms
iOS · watchOS · Android · Wear OS · Web (Admin)

3,000+

Cumulative users

845 files

Dart

210K+ lines

Hand-written code

App Store Google Play
View details
momiplan

Women's Health · Fertility

momiplan 마미플랜

Cycle tracking, IVF records and heart-rate workout coaching for women preparing for pregnancy

momiplan is a companion service for women undergoing fertility treatment or preparing for pregnancy. A 34-step onboarding survey — treatment stage, clinic, time trying, supplements, sleep issues — builds an understanding of each user, and menstrual cycles and IVF procedures are recorded in detail.

  • Flutter
  • Firebase
  • Cloud Functions
  • BLE
  • Apple · Google Sign-In
  • Material 3
Client
Healthcare startup (B2B2C · undisclosed)
Period
Jan 2026 — Jul 2026
Role
Sole developer — Flutter app, Firebase backend, admin functions
Platforms
iOS · Android · Web (Admin)

110 files

Dart

15K lines

Code

34 steps

Onboarding

View details
HAIM

Clinical Research · Preventive Care

HAIM HAIM

Stroke-risk prediction and personalised health-management research app with a researcher console

HAIM is a research healthcare app that predicts stroke risk for study participants aged 60 and over and supports 12 weeks of personalised health management. The research team's risk algorithm (ver. 2), delivered as slides and spreadsheets, was ported to TypeScript and verified, and health-checkup FHIR files from Korea's 'My Health Record' are imported to derive risk factors.

  • Flutter
  • NestJS
  • Supabase
  • Firestore
  • AWS ECS
  • FHIR
  • Health Connect
  • React
Client
Kangwon National University × HB Connect
Period
Jul 2026 — Sep 2026
Role
Sole developer with full ownership of app, server and admin web
Platforms
Android · Web (Admin)

~40K lines

Total code

44%

Test ratio

282

Commits

View details

Process

How a project runs

From the first email to store launch and handover — what you receive at each stage is clear up front.

  1. 01Free · within a week

    Discovery

    Over email, we align on goals, users, platforms, timeline and budget, and flag regulatory issues (medical data, privacy) up front.

    • Requirements summary
    • Regulatory & risk checklist
    • Rough estimate range
  2. 021–2 weeks

    Design & estimate

    Screen flows, data model, server and third-party integration scope, then a fixed estimate and milestone schedule.

    • Screen flows · data model
    • API & integration contracts
    • Milestone plan · contract
  3. 03Per project

    Build with weekly updates

    Weekly test builds (TestFlight · internal testing track) and worklogs. Change requests are recorded and scheduled.

    • Weekly test builds
    • Worklog · decision record
    • Test code
  4. 041–3 weeks

    Launch & store review

    Store listings, privacy policy, review rejections and the deployment pipeline — handled end to end.

    • App Store · Play launch
    • CI/CD pipeline
    • Operations guide
  5. 05Monthly

    Operate & hand over

    Crash, performance and cost monitoring with regular releases. System docs and handover docs when moving to an in-house team.

    • Monitoring reports
    • System documentation set
    • Maintenance contract (optional)

Project

Fixed-scope project

Scope and milestones agreed up front at a fixed price. The most common choice from MVP to full launch.

Retainer

Monthly development & operations

Feature work and operations for a live product on a monthly basis, with priorities set together each month.

Consulting

Technical advisory & code review

Advice and reviews for existing teams on health-data integration, wearables, performance and cost.

Contact

Have a project in mind? Let's talk.

An early idea is fine. Send me the goal, platforms, timeline and budget range and I will reply within one to two business days, followed by a free written review with an estimate and schedule.

Email works best. Live calls in English are difficult for me, but I communicate comfortably in writing. Email, chat, shared docs and weekly written updates all work well, and I reply within one to two business days.

Email me
Location
South Korea · Remote-friendly
Status
Open to new projects
Response
Within 1–2 business days