A group of chimpanzees once used in laboratory experiments for hepatitis B vaccines are receiving medical care at a sanctuary in Liberia, where their caretakers hope to give them a second chance at a more natural life.

The West African chimpanzees recently underwent a series of health checks at the Humane World for Animals’ Second Chance Chimpanzee Refuge in central Liberia, including dental cleanings, X-rays and pedicures, sanctuary director Liliana Pacheco Ricote told The Associated Press (AP).

More than 400 chimpanzees were used by American researchers in Liberia in hepatitis B vaccine experiments conducted from the 1970s through the early 2000s, Ricote said. Many were captured in the wild as infants and brought into laboratories for research.

Now, the animals are being cared for at the sanctuary, where staff monitor their health and provide treatment as needed.

Liberia is home to the second-largest population of chimpanzees in West Africa, after Guinea. But the region’s West African chimpanzee population has declined sharply, and the species is classified as critically endangered.

"They were used in very invasive research ... (and) went through a lot of experiments and pain,” Ricote said.

Humane World for Animals said that in 2015 it assumed permanent responsibility for the 60 chimpanzees remaining at the end of the research program. The chimps now mostly live at the sanctuary on a series of mangrove-covered islands.

The health checks, carried out under anesthesia, gave the chimpanzees head-to-toe examinations to assess their long-term health, detect early signs of disease and tailor care plans as they age, Humane World for Animals said in a statement to the AP.

The tests included dental examinations, heart, lung and nervous system assessments, as well as blood tests. The chimpanzees also received booster vaccinations and fingernail trims, the organization said, adding that plans are in place to conduct the checks on the remaining animals.

"The health checks are a vital part of caring for these chimpanzees who have been through so much,” said Belen Perez Hernando, the sanctuary’s veterinarian.

The extent of the experiments' impact on the chimps, whose average age is 30 to 35, is unclear because findings from the assessments have yet to be finalized. However, "for those who were under the experiments, you could still feel the hurt and the pain that they went through,” Ricote said.

"Seeing how much they hurt about what we’ve done, it gives me a lot of pleasure ... to give them this second chance to live with dignity the life that they deserve,” she added.