How Stefan van der Fluit Built the World's First Reusable Sweat Sensor and Sold Out for Five Straight Quarters
Building a hardware company means constantly pushing a rock uphill, but Stefan van der Fluit wouldn't choose anything else. The FLOWBIO founder and CEO has created the world's first fully reusable sweat sensor, attracted angel investors from companies like Athletic Brewing and Oura, and proven that a future where people are data-driven about hydration is inevitable, and his company will be the one to build it.
Stefan's journey from Cambridge graduate to ex-Meta employee to four-time founder reveals the mindset required to survive the chaos of startup life while staying committed to principles like conscious capitalism, scientific validation, and building products that create genuine value rather than extracting it.
🎧 Tune in here:
Listen on Spotify, Apple Podcasts, or other podcast platforms.
Conscious Capitalism as Competitive Advantage
Stefan discovered conscious capitalism at Cambridge when a case study about Primark exposed him to how many future business leaders hide behind legal compliance rather than ethical responsibility. When half his class defended child labor because it was legally permitted in certain countries, Stefan realized the business world needed a different operating philosophy.
Conscious capitalism means building business from a place of good and long-term thinking, recognizing that transactions are fundamentally human relationships. Stefan points to Henry Ford creating minimum wage not from pure altruism but from recognizing that factory workers who could afford cars would buy more cars, increasing market share and stock price simultaneously.
For FLOWBIO, this philosophy manifests in how Stefan approaches investor relationships and customer development. Rather than hiding behind platitudes, he leads with authenticity and expects the same from others. The approach attracts like-minded investors and customers who share his values around creating lasting value rather than extracting short-term gains.
The Network Effect of Interesting People
Stefan's angel investor roster reads like a who's who of health and fitness innovation: founders from Athletic Brewing, executives from Oura, and athletes using products from Strava to WHOOP. He attributes this network to one simple hack: at the end of every meeting, he asks who else he should speak with.
When Athletic Brewing founders Bill and John invested, they brought a syndicate contributing another half million dollars. When Petri from Oura invested after a 90-minute conversation, he connected Stefan with Oura's seed investor who later led FLOWBIO's third and fourth rounds.
The strategy works because Stefan rejects the notion that people in the wearables and health tech industry wouldn't be excited by breakthrough hydration technology. If someone isn't excited by quantifying one of the four fundamental human needs, they probably shouldn't be in the industry.
Debunking Industry Assumptions Through Science
FLOWBIO developed the world's first independently validated sweat sensor, with validation from University of Rome funded by World Athletics and a paper publishing in January 2025. The validation mattered because FLOWBIO needed to prove that sweat sodium concentration isn't static but fluctuates constantly based on heat acclimation, training load, and environmental conditions.
Stefan's sweat sodium can fluctuate from 400 milligrams per liter to 1.8 grams per liter depending on conditions. This variability means athletes need real-time data rather than a one-time test. FLOWBIO's reusable sensor provides this resolution, and Precision Hydration test centers now sell FLOWBIO sensors to provide customers with ongoing insights beyond their single-test model.
Stefan emphasizes that hydration isn't simply drinking water but the balance of water to sodium. The most common cause of dehydration in athletic settings is over-hydrating with plain water, which dilutes electrolytes and prevents proper cellular absorption.
Surviving Three Near-Death Experiences
Stefan and his co-founder launched FLOWBIO one week before UK lockdown in 2020, immediately losing access to needed lab space. They adapted by creating demand indicators: speaking with athletes from Ironman world record holders to amateurs, gathering testimonials, and building sweat test kits to prove market interest before manufacturing the sensor.
The company faced three near-death experiences. Stefan put his life savings into the business during one crisis. His co-founder burned out from constant fundraising. There were moments Stefan found himself on LinkedIn looking at jobs, only to realize that returning to corporate work after building industry-changing technology would be more depressing than fighting through another quarter of uncertainty.
This determination carried the team through multiple hardware iterations, from single-use sensors they scrapped entirely to the breakthrough reusable architecture that CTO Dr. Duran Mingles engineered.
200% Growth and Five Quarters Sold Out
FLOWBIO has experienced 200% quarter-over-quarter growth and sold out of inventory for five consecutive quarters. Demand massively outstrips supply, with world tour athletes, Ironman world champion Lucy Charles-Barclay, and Olympic silver medalist Hayden Wild all using the device.
The current fundraise is what Stefan calls a happy fundraise because the hard questions are answered. The product works, it's validated, and marquee athletes use it. The capital will go toward manufacturing capacity rather than proving concepts.
Stefan compares building a company to getting a PhD in life. The chaos of fighting fires daily while maintaining long-term vision requires the same mental framework as being a pro athlete, demanding discipline, recovery, and performance optimization.
The Future of Hydration Tech
FLOWBIO's vision extends beyond selling sensors. Stefan sees hydration AI installed in every health and fitness wearable, bringing water and electrolyte loss metrics to Oura, WHOOP, Apple Watch, Samsung Wear, and Fitbit without hardware redesign.
The company will be at the Tour de France this year with a partner they can't yet name. Stefan's journey from bullied kid to national champion to breakthrough health tech founder illustrates what becomes possible when someone commits everything to building a future they believe is inevitable.
Top Takeaways
Conscious capitalism creates competitive advantage by recognizing that the best way to maximize shareholder value long-term is building business from a place of good and authentic human relationships. Consumers increasingly vote with their wallets for brands that align with their values, making ethical business practices not just morally right but strategically smart.
Network effects compound when you consistently ask who else you should speak with at the end of meetings. Interesting people know other interesting people and will make introductions when you demonstrate genuine value, turning single investor relationships into syndicated rounds and opening doors that cold outreach never could.
Hardware companies must find proxy indicators to prove market demand before products exist. FLOWBIO used sweat test kits and athlete testimonials to validate their concept during the years required to engineer the actual sensor, giving investors confidence before manufacturing began.
Scientific validation differentiates health tech products in an industry criticized for anxiety-inducing features. FLOWBIO's independent validation from University of Rome funded by World Athletics establishes credibility that marketing alone cannot achieve, proving the technology works before scaling.
Debunking industry assumptions can create entirely new categories when you prove conventional wisdom wrong. FLOWBIO demonstrated that sweat sodium fluctuates rather than remaining static, transforming the business case from one-time testing to continuous monitoring and opening a market that didn't previously exist.
Building a company requires treating the CEO role with the same discipline as being a professional athlete. The long-term commitment, daily optimization, and recovery protocols are fundamentally similar, making athletic training an unexpected but valuable preparation for entrepreneurship's demands.
Selling out for five consecutive quarters while experiencing 200% growth validates product-market fit and enables happy fundraising focused on scaling rather than proving the concept. When demand massively outstrips supply, the fundraising conversation shifts from if the product works to how fast you can manufacture it.
Stay Connected
🎧 Tune in here:
Listen on Spotify, Apple Podcasts, or other podcast platforms.
Pssssst. Click above to follow along on Substack 👆
About Jon Levitt and For The Long Run
Jon is a runner, cyclist, and podcast host from Boston, MA, who now lives in Boulder, CO. For The Long Run is aimed at exploring the why behind what keeps runners running long, strong, and motivated.
Follow Jon on Instagram, LinkedIn, and Twitter.
