Houston founder shares how he's using tech to make digital media more effective and equitable

houston innovators podcast episode 173

This month, Mark Walker is celebrating his company's one year anniversary of going public — only the ninth Black-founded business to accomplish this feat on a U.S. stock exchange. Photo courtesy

After working in both sides of the advertising world, Mark Walker thought he could reimagine a platform that would be more efficient and equitable.

Walker co-founded his company, Direct Digital Holdings, an adtech platform, after serving in several roles — from an early hire at Houston digital media startup Questia to business development director at NRG Energy and COO of EBONY Media. He shares on the Houston Innovators Platform how he took this experience in tech, advertising, and media to create his company's platform.

"NRG Energy gave me a top-down view of the value chain, and Ebony gave me a bottoms-up view of the value chain of how media is purchased," Walker says on the show. "At Direct Digital Holdings, we help companies buy and sell media — and we leverage technology to do it. It's really the culmination of both of those experiences."

With over 30,000 publishers on its platform, Direct Digital makes it easier for its core customers — middle market companies looking to buy into the digital media ecosystem — to tap into these opportunities without the tech know-how they might otherwise need. Walker explains that at EBONY, he saw how small to midsize publications — especially the multicultural ones — were being left out on the ad selling side of the equation. The Direct Digital platform bridges the gap on each end.

Founded in 2018 in partnership with Keith Smith, who went through similar professional experiences, Direct Digital went public exactly one year ago after growing the company through strategic M&A activity. Walker says the decision to IPO made the most sense for his company — though it wasn't an easy process. Direct Digital is only the ninth company founded by a Black entrepreneur to go public on a US stock exchange.

"If you think the process is hard — it actually is," Walker says on the journey to IPO. "We were a privately held company, and we knew we had a good growth trajectory and we looked a couple different options. We decided to go public in a very traditional way."

Walker explains there were some risks involved, but the co-founders ultimately decided to shy away from adding in investors who might not have the same ideas for the company's future.

Direct Digital has been a Houston company from the star — despite the city not being home to a booming adtech ecosystem. Instead, Houston — with its collection of Fortune 500 companies and rich diversity — has allowed the business to stand out.

"If you look at and reflect on how our company has been built — from our board of directors to our leadership and management team — we're a majority minority organization all the way across the board," Walker says. "Diversity is very important to us. It's the lifeblood of our business — especially because we're serving publishers in those communities in big way. And moreso, we think you get the best product, thoughts, and ideas from a diverse workforce, and Houston fits right into that mold for us."

Walker shares more about his company's future, advice on IPO, and what all he's watching in adtech — from AI to streaming — on the podcast. Listen to the interview below — or wherever you stream your podcasts — and subscribe for weekly episodes.


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Rice University professor earns $550k NSF award for wearable imaging tech​

science supported

Another Houston scientist has won one of the highly competitive National Science Foundation (NSF) CAREER Awards.

Lei Li, an assistant professor of electrical and computer engineering at Rice University, has received a $550,000, five-year grant to develop wearable, hospital-grade medical imaging technology capable of visualizing deep tissue function in real-time, according to the NSF. The CAREER grants are given to "early career faculty members who demonstrate the potential to serve as academic models and leaders in research and education."

“This is about giving people access to powerful diagnostic tools that were once confined to hospitals,” Li said in a news release from Rice. “If we can make imaging affordable, wearable and continuous, we can catch disease earlier and treat it more effectively.”

Li’s research focuses on photoacoustic imaging, which merges light and sound to produce high-resolution images of structures deep inside the body. It relies on pulses of laser light that are absorbed by tissue, leading to a rapid temperature rise. During this process, the heat causes the tissue to expand by a fraction, generating ultrasound waves that travel back to the surface and are detected and converted into an image. The process is known to yield more detailed images without dyes or contrast agents used in some traditional ultrasounds.

However, current photoacoustic systems tend to use a variety of sensors, making them bulky, expensive and impractical. Li and his team are taking a different approach.

Instead of using hundreds of separate sensors, Li and his researchers are developing a method that allows a single sensor to capture the same information via a specially designed encoder. The encoder assigns a unique spatiotemporal signature to each incoming sound wave. A reconstruction algorithm then interprets and decodes the signals.

These advances have the potential to lower the size, cost and power consumption of imaging systems. The researchers believe the device could be used in telemedicine, remote diagnostics and real-time disease monitoring. Li’s lab will also collaborate with clinicians to explore how the miniaturized technology could help monitor cancer treatment and other conditions.

“Reducing the number of detection channels from hundreds to one could shrink these devices from bench-top systems into compact, energy-efficient wearables,” Li said in the release. “That opens the door to continuous health monitoring in daily life—not just in hospitals.”

Amanda Marciel, the William Marsh Rice Trustee Chair of chemical and biomolecular engineering and an assistant professor at Rice, received an NSF CAREER Award last year. Read more here.

Houston Spaceport launches $12M expansion for leading space tech co.

to the moon

Houston will get one step closer to the moon, as the Houston Spaceport at Ellington Airport (EFD) has announced an expansion of the lease for Intuitive Machines, the Houston space tech leader dedicated to furthering lunar exploration.

On July 15, the City of Houston announced passage of Amendment 1, which would add three acres of commercial space for Intuitive Machines at the spaceport and a $12 million infrastructure expansion. Approved by the city council and Mayor John Whitmire, the expansion will include new production, testing and support facilities. The amendment extends the current lease for Intuitive Machines from 20 years to 25 years.

"I want to shout out to Intuitive Machines about everything they’re doing at the Houston Spaceport. It’s exciting to see them expand. We’re starting to reach a critical mass out there — more and more aerospace companies want to be at the Spaceport because that’s where innovation is happening,” said Fred Flinkinger, who represents District E on the Houston City Council. “It’s a great sign of momentum, and we’re proud to have them here in Houston."

Intuitive Machines was the first commercial tenant for the Houston Spaceport when it moved into the facility in August 2016. Founded by Stephen Altemus, Kam Ghaffarian, and Tim Crain in 2013, the company holds three contracts with the National Aeronautics and Space Administration (NASA) to deliver payloads to the lunar surface. In 2023, the company opened its doors in Houston with a 105,572-square-foot Lunar Production and Operations Center that contains research and development labs, clean rooms, mission control centers, and a spacecraft assembly floor.


Intuitive Machines landed Odysseus on the moon in February 2024, the first privately owned soft lunar landing ever and the first soft landing since 1972.

The Houston Spaceport is owned and operated by the City of Houston and Houston Airports, who have an eye of keeping the city a prime name in space exploration. As "Houston" was the first word spoken on the moon when Apollo 11 landed in 1969, lunar exploration in particular has a soft place in the heart of the metropolis formerly known as Space City.

“This agreement reinforces Houston’s leadership in space innovation,” said Jim Szczesniak, director of aviation for Houston Airports. “We’re building infrastructure and supporting the next era of lunar and deep space exploration, right here at Houston Spaceport. This partnership represents the forward-thinking development that fuels job creation and drives long-term economic growth.”

Houston hardtech accelerator names 8 scientists to 2025 cohort

ready, set, activate

National hardtech-focused organization Activate has named its 2025 cohort of scientists, which includes new members to Activate Houston.

The Houston hub was introduced last year, and joins others in Boston, New York, and Berkley, California—where Activate is headquartered. The organization also offers a virtual and remote cohort, known as Activate Anywhere. Collectively, the 2025 Activate Fellowship consists of 47 scientists and engineers from nine U.S. states.

This year's cohort comprises subject matter experts across various fields, including quantum, robotics, biology, agriculture, energy and direct air capture.

Activate aims to support scientists at "the outset of their entrepreneurial journey." It partners with U.S.-based funders and research institutions to support its fellows in developing high-impact technology. The fellows receive a living stipend, connections from Activate's robust network of mentors and access to a curriculum specific to the program for two years.

“Science entrepreneurship is the origin story of tomorrow’s industries,” Cyrus Wadia, CEO of Activate, said in an announcement. “The U.S. has long been a world center for science leadership and technological advancement. When it comes to solving the world’s biggest challenges, hard-tech innovation is how we unlock the best solutions. From infrastructure to energy to agriculture, these Activate Fellows are the bold thinkers who are building the next generation of science-focused companies to lead us into the future.”

The Houston fellows selected for the 2025 class include:

  • Jonathan Bessette, founder and CEO of KIRA, which uses its adaptive electrodialysis system to treat diverse water sources and reduce CO2 emissions
  • Victoria Coll Araoz, co-founder and chief science officer of Florida-based SEMION, an agricultural technology company developing pest control strategies by restoring crops' natural defenses
  • Eugene Chung, co-founder and CEO of Lift Biolabs, a biomanufacturing company developing low-cost, nanobubble-based purification reagents. Chung is completing his Ph.D. in bioengineering at Rice University.
  • Isaac Ju, co-founder of EarthFlow AI, which has developed an AI-powered platform for subsurface modeling, enabling the rapid scaling of carbon storage, geothermal energy and lithium extraction
  • Junho Lee, principal geotechnical engineer of Houston-based Deep Anchor Solutions, a startup developing innovative anchoring systems for floating renewables and offshore infrastructure
  • Sotiria (Iria) Mostrou, principal inventor at Houston-based Biosimo Chemicals, a chemical engineering startup that develops and operates processes to produce bio-based platform chemicals
  • Becca Segel, CEO and founder of Pittsburgh-based FlowCellutions, which prevents power outages for critical infrastructure such as hospitals, data centers and the grid through predictive battery diagnostics
  • Joshua Yang, CEO and co‑founder of Cambridge, Massachusetts-based Brightlight Photonics, which develops chip-scale titanium: sapphire lasers to bring cost-effective, lab-grade performance to quantum technologies, diagnostics and advanced manufacturing

The program, led locally by Houston Managing Director Jeremy Pitts, has supported 296 Activate fellows since the organization was founded in 2015. Members have gone on to raise roughly $4 billion in follow-on funding, according to Activate's website.

Activate officially named its Houston office in the Ion last year.

Charlie Childs, co-founder and CEO of Intero Biosystems, which won both the top-place finish and the largest total investment at this year's Rice Business Plan Competition, was named to the Activate Anywhere cohort. Read more about the Boston, New York, Berkley and Activate Anywhere cohorts here.

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