How Katie DiSimone Closed the Loop

Thousands of people today use an open-source diabetes technology called Loop, in large part, because Katie DiSimone saw how it changed her daughter's life. 

When Anna DiSimone was diagnosed with type 1 diabetes at age 12, Katie was determined to find a better way forward than insulin injections and finger pricks. Through an online parents’ group, she discovered Loop — an iPhone app that automated insulin dosing, 24/7.

"Her understanding of the technology took a huge weight off my shoulders," Anna said. "Even my endo would say I had the A1C of a person without diabetes. Every meal, workout, or activity was a learning experience for us. She was always on top of it."

And while Katie wasn't a programmer – she was an engineer for the State of California – she had a keen mind, the skills of a math teacher, and a knack for problem solving. She dove into the Loop code and taught herself how to set it up. 

Once running, the open-source Loop app allowed Anna's iPhone to communicate with a continuous glucose monitor and an insulin pump, closing the "loop" between them. At the time, an FDA-approved automated insulin delivery system was still years away.

After familiarizing herself with the program, she published detailed instructions for others. Her directions made it difficult to go wrong, in part because they emphasized safety and insisted the user proceed carefully to understand the app before using it. 

Nate Racklyeft, a software developer in San Francisco, created Loop after spending half a year using the first open-source AID system, called OpenAPS. Racklyeft wanted a way to discreetly manage his diabetes using his smartphone. In 2015, he programmed Loop to do the job. 

"My work on Loop ended in 2016 with a few dozen users of the app," Racklyeft said. "It was only after I stepped away that Katie enabled hundreds, and eventually thousands, more to use it."

At that time, Kate Farnsworth also had a teenage daughter, Sydney, living with type 1. The two met through an online parents' group, and Kate brought up the idea of starting a Facebook group called “Looped” to simplify what was then a daunting process to set up the app.  

 "I thought we'd reach 100 families max," Farnsworth said. "But the group has grown to about 33,000 people worldwide. I couldn't have done it without her." 

As the Loop app matured, Katie joined a nonprofit called Tidepool and worked with her colleagues to make a more widely accessible version, shepherding the software through the FDA's demanding approval process.  

Unfortunately, in 2021 Katie was diagnosed with an aggressive form of brain cancer called glioblastoma. She received an outpouring of support and responded with this message:

"Thank you, Looped. Those little words that are so incapable of carrying all my appreciation, gratitude, love, surprise, and humility at how you all have lifted me through what has been the biggest challenge of my life. You’ve made this journey something I can bear. And that’s a miracle."

As she received treatment over the next 21 months, her husband Jason said she continued to help, taking calls from people who were just diagnosed or their family members. She'd recommend Loop and help them get started with the app to make managing diabetes easier.

In 2023, shortly after Loop was approved by the FDA, Katie passed away.

Jason wrote an online post to let her community know:

"Like the fighter that you know she was, she went out on her terms. She wanted me to let you know how much she cherished everyone’s friendship, support, and love. You helped to lift her spirits when she was down, motivated her to keep pushing, and made her laugh … with that winning smile."

Here, some of the people who knew Katie well describe how she guided people to a simpler way of managing diabetes.

'She did whatever it took'

After getting involved, Katie became the most familiar face of the Looped Facebook group. People who knew her would share her phone number, and she'd receive calls at all hours with the news of a new type 1 diagnosis. Sometimes the call was made from the hospital within hours after finding out. 

Katie understood how hard those first days could be, Jason said, and she wanted to share information that could help.  

"For everyone that's just recently diagnosed, that's a huge life shift," he said. "If someone texted at 10 p.m. and said their son had just been diagnosed, she'd get on the phone and call them. I can remember her saying, 'Trust me, this is what you want to do. This is so much easier.'" 

From the time she was a child, said her mother, Ellene Gibbs, Katie was all in on anything she put her mind to.

"She did whatever it took," Gibbs said. "She had no faith in the status quo of finger pricks and shots that were prescribed for her daughter after her diagnosis. She had an absolute passion to understand and push the frontiers of diabetes management. She was an awesomely intelligent person – scarily so, sometimes – and had unstoppable tenacity."

As the Looped group grew to thousands of members, Katie would often remind users to read the documentation and search to see if a question had been asked – and answered – possibly multiple times before. 

"She could be fiery and spicy. She was human. But she truly embodied that pay it forward spirit that was started in the We Are Not Waiting movement," said Farnsworth, referencing the open-source diabetes tech community. "She was always willing to jump in and help."

'If you could follow a recipe'

To use Loop, a person has to assemble bits of code into an app before it can run. For anyone outside of a small number of technology pros who were initially using the software, it was almost impossible to understand how to get started and use it effectively. 

“It was really complicated to build Loop in the early days," Farnsworth said. "Katie created instructions so that if you could follow a recipe, you could build Loop, which was no small feat.”  

To help with setup and day-to-day use, Katie published three websites with different goals (instructions, tips, and a blog detailing real-world situations). She also made step-by-step guides using videos.

Her directions were direct and free of extraneous information. Here's an example:

Beyond helping someone past a technical hurdle, Farnsworth said Katie's advice gave users confidence in the system – and their own ability to use it safely.

"There's something fundamentally good about building this yourself and the lifelong skills that you gain," Farnsworth said. "She provided that to people. It brings you tighter together as a community and also strengthens you as a person."

Joe Moran, a software and hardware engineer, was the first person to use Loop with the Omnipod pump and was instrumental to its integration. He developed a close friendship with Katie after meeting at a hackathon in 2016, where people had gathered to build the app.

"Katie was at a table taking pictures, and I thought, 'Who is this super friendly lady?'" Moran said. "So I learned about Loop that morning. I built it that afternoon with a bunch of people and came home running it that same day."

Like Katie, Moran operated on little sleep and worked late into the night, and they'd often get on calls to talk through ways to make the app easier to use. 

 "She could get through any problem," he said. "It's not easy stuff. There is a lot of detail. And she could communicate that."

Moran dubbed her an honorary type 1, he said, because she understood it as well as anybody who doesn't have diabetes. 

“She would use a CGM and post her numbers,” he said. “She wore an Omnipod for stretches of time. She felt other people's frustration. She lived the experience by helping other people."

'She thought she could, so she did'

Katie taught herself how to code. When she saw a need that wasn’t addressed in the Loop app, she’d develop an offshoot – or branch – with new features users could adopt before the main program was updated.

In one example, she recognized that women dealt with blood sugar variability through their menstrual cycle – and needed tighter blood sugar targets during pregnancy. So Katie released a branch that provided overrides to help address these needs, which is now a commonly used feature in the app.

"She wasn't waiting for someone else to do it,” said Ann Oxley, who at the time also had a teenage daughter, Lydia, with type 1. Like Farnsworth and Katie, Oxley began helping users in the Looped group, eventually becoming a moderator. “It's like that saying: 'She thought she could, and she did.'”  

She often tested the app to spot where a new user might get tripped up, Moran said.

"She spent endless hours trying out things," Moran said. "She'd press the wrong button on purpose to see what happened and keep you from going wrong."

Moran told her he had to make a series of manual updates to the tracking software Nightscout when he changed his Omnipod. Katie had an idea of how to simplify the process, so she took his phone and set it up.

"She customized a shortcut that does things automatically for me," he said. "It's like a little note in there from her. Every three days when I change my pod, I think of Katie. I still use that thing."

Sharing Anna's story

In addition to writing instructions and tips, Katie often posted details about her daughter Anna's day-to-day experiences with Loop. 

"I was a teenager at the height of us using Loop," Anna said. "So of course we would butt heads when she’d take my phone to look at numbers and patterns. She would remind me about how many people have been able to use that information. I’m also glad to have met some people who have been impacted by her posts. Back then, I didn’t fully understand the magnitude of her reach."

Katie wrote about how to adjust settings to deal with complicated food choices, like carb-rich restaurant takeout. Then she'd make visual graphs of Anna's blood sugar showing how their experiment had fared.

"That was so helpful to people, because they could see it was a real scenario," Farnsworth said. 

But she also chronicled any mistakes they'd make along the way.

"She'd say, 'We tried this and it didn't work. This is our experience,'" Moran said. "And I would recommend this because of that. It made her advice more relatable. That made her really happy – knowing that her daughter's experience would help other people."

Jason DiSimone said their daughter's diagnosis also inspired the way Katie explained to people how to use the app. 

"Anna was 12 at the time, and there are kids who are diagnosed earlier," he said. "Katie figured out that she needed to explain this to a seventh grader – to kids. She was really good at communication."

“There's no way to ever pay her back," Oxley added. "I used to tell her that. What do you give the person who gave you your kid's life back?"

Loop's creator, Racklyeft, recalled the last time he heard Katie speak, during a video call hosted by Tidepool. The group was celebrating Tidepool Loop's approval by the FDA. Katie had been key to that effort, which she saw as the next step in making a version of the app more accessible for users.

"She expressed her gratitude toward Loop for restoring normalcy to her relationship with her daughter," Racklyeft said. "It was so much more important than any potential health benefits of the app. She brought that to so many others after experiencing it for herself."

'People like Katie remind us – there's a lot we can do'

Katie’s husband Jason said that her drive to help Loop users continued almost to the end of her battle with cancer. 

"The week before she passed, she was still in contact with those who were recently diagnosed, sending messages of support and guidance," he said.  

Jason posts a few times a year about Katie, and replies come from around the world.

"They say, ‘I just want to let you know, she really helped me out when my child was diagnosed. I'm in Maine, Canada, overseas – and she reached out at a time that was very difficult in our lives and made it much easier," he said.

When Katie was diagnosed with glioblastoma, she again found a community online where she could learn and share her experiences.

"She chronicled everything, trying to figure out as much information as possible – what the best route to go was for medical advice and treatments,” Jason said. “And so she met a lot of people through that who were diagnosed around the same time or had a family member going through the same thing."

She continued posting to the Looped group as well, helping members and collecting supplies for testing, while providing updates on her treatment.

"Katie had a way of being open and vulnerable and sharing that nobody else I've ever seen has," Farnsworth said. "It's a big part of her story. When faced with this insurmountable thing, cancer, she handled it with such grace and candor."

Katie worked to transition the Loop instructions she created over to a group of dedicated volunteers who continue that work today to support the open-source community.

Once they got the hang of the app, Katie frequently encouraged Loop users to reach out and help others. This became an ingrained part of the group’s culture, which spread what she had learned exponentially. 

"Often with type 1 diabetes, we have to educate other people," Moran said. "But a lot of us don't know the things that we can do, and we need people like Katie to help remind us – there's a lot we can do."

Anna recalled how her mother was constantly looking to fine-tune her Loop app, and how those experiences continue to help.

"There were so many times at night when I would wake up to her sitting next to my bed adjusting settings," she said. "Sometimes even multiple times a night."

A friend was recently diagnosed with type 1, and when he reached out for help, Anna pulled out the graphs and logs her mother created around the time of her diagnosis. 

"I explained the basics of different foods and what they typically do to our numbers," she said. "He’s now successfully managing his diabetes and learning how to live a normal life. I felt proud to be able to step into her shoes a little bit." 

Read more stories about people making an impact on diabetes care:

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• 5 Minutes With Diabetes Educator and Advocate Bridget Wood