Entrepreneurs are always on the lookout for the next big problem to solve, and in recent years, a handful of the richest ones have decided to tackle an age-old frustration: death. 

PayPal co-founder and investor Peter Thiel, 47, has long been interested in longevity research, and he's personally invested in the extension of his own life. Thiel told Bloomberg that, as part of his efforts to reach age 120, he takes human growth hormone (HGH) pills, follows the Paleo Diet, and tries to limit his sugar intake as much as possible.

Thiel said that HGH lessens the likelihood of arthritis, but increases his risk for cancer. And when asked if he's worried about that, he said, "I'm hopeful that we'll get cancer cured in the next decade." 

For more on Thiel and other multimillionaires who are bullish on anti-aging research, read on. 

Peter Thiel

"I think understanding aging in and seeing if there ways to slow it or even reverse it remains one of the great scientific projects of all time," Thiel said at TechCrunch Disrupt this year. Thiel has given more than $6 million to researcher Aubrey de Grey who founded Methuselah Foundation, a non-profit organization dedicated to extending the human lifespan, according to The Telegraph. De Grey has famously said he believes that the first person who will make it to 1,000 years old is already alive. 

Larry Ellison

"Death makes me very angry," Oracle Co-founder Larry Ellison once told his biographer Mike Wilson. "Death has never made any sense to me. How can a person be there and then just vanish, just not be there?” Ellison, now 70 years old, created the The Ellison Medical Foundation in 1997 to support aging research. The nonprofit  has awarded nearly $430 million in grants since then, according to the San Francisco Business Times. However the foundation stopped making new grants in 2013 due to financial strain, the paper reported. 

Peter Diamandis

Peter Diamandis, 53, founder and chairman of the nonprofit X PRIZE Foundation, believes that the answers to medical questions on aging lie in genomics and stem cell sciences. His new venture Human Longevity Inc., which has $70 million in private backing, focuses on both. 

"Over the next decade, Human Longevity Inc. has the objective of sequencing 1 million individuals at a minimum, but in addition to their sequence we will also be collecting phenotypic data, microbiome data, imaging data and metabalomic data," Diamandis explained in a discussion on Reddit. "In the arena of stem cells, we will begin harnessing stem cells as the regenerative engine of the body."

Cancer is the first age-related disease Human Longevity will focus on, according to Reuters.

Dmitry Itskov

New Media Stars founder and Russian multimillionaire Dmitry Itskov is taking an entirely different approach to the problem of aging. Since our bodies will inevitably fail us, Itskov proposes ditching them for a durable avatar. Itskov, who is in his early 30s, founded the 2045 Initiative, named for the year he hopes the project is completed, reports the New York Times. In the end, he hopes to upload a digital copy of your brain into a lifelike, low-cost avatar, totally immune from the ill effects of aging.

Sergey Brin

In 2013 Google introduced Calico, a company that was founded by Arthur Levinson, former CEO of Genentech, and aims to research the biology that affects lifespan. Google co-founder Sergey Brin, 41, is known to support projects "on the cusp of viability" as he's told The Guardian, and Google CEO Larry Page credited Brin for helping to bring Calico to life. Brin has a somewhat more defined stake in anti-aging research compared to many, as he discovered in 2008 that he has a flawed gene, which gives him a 50 percent chance of developing Parkinson’s disease by age 70, according to Bloomberg

John Sperling

The late John Sperling, founder of the University of Phoenix, hated the idea of saying goodbye to the ones that he loved. That's why, in order to buy more time with his dog, Missy, he funded pet-cloning research at Texas A&M University, according to a Bloomberg obituary. Sperling was also interested in extending human longevity. He poured millions into therapeutic cloning, stem cell medicine, and genetic engineering, hoping the research would help "to alleviate human suffering and the fear of death," Wired reported. Sperling died in August at the age of 93.