He was born in Edo, the eldest son of Ezawa Yōju, a physician serving the Ōgaki domain. He was the eldest of eight children, and the Ezawa family had served for generations as Ōgaki's domain doctors. Growing up in a medical family sparked his deep interest in Western learning and set him on the path to laying the foundations of modern science in Japan.
He was adopted by Udagawa Genshin, a physician of the Tsuyama domain and scholar of Dutch learning (rangaku), and studied Dutch and Western sciences under him. The Udagawa were known as a leading family of Dutch learning (rangaku). Yōan deeply loved his adoptive father, and a story tells that he later studied the effects of hot springs to help Genshin's health.
Around this time, with the rangaku scholar Tsuji Ranshitsu, he copied and translated an album of vivid color drawings of plants from Java. It has been suggested that this may be the oldest surviving modern botanical atlas in Japan. His interest in plants grew ever deeper, leading to his later work introducing Western botany to Japan.
He published the Botanika-kyō, introducing Western botany. "Botanika" rendered the Latin word for botany in Chinese characters, and the unusual book was written in the style of a Buddhist sutra. Around the same time he published books on Western medicines with Genshin. That year he married the daughter of a Satsuma domain doctor.
He became a translator at the shogunate office for translating Dutch books, attached to the astronomy bureau, and is said to have joined the work on the Kōsei Shinpen, a translation of Chomel's encyclopedia. That year he also met the German physician Siebold, who visited Edo, and is known to have become friendly with him.
With his adoptive father Genshin, he began publishing a revised and enlarged book on Western medicines. Drawing on Dutch books to study the properties and uses of drugs, it compiled them so that Japanese doctors could use them. This joint work by father and son helped spread Western medicine in Japan.
His beloved adoptive father Udagawa Genshin died. The Dutch and medical knowledge he had learned under Genshin was the foundation of Yōan's work. Having no children of his own, Yōan later adopted Kōsai, son of the botanist Iinuma Yokusai, passing the Udagawa family's learning on to the next generation.
He published the Shokugaku Keigen, fully introducing Western botany to Japan. Explaining plant structure and classification, he coined new words such as "saibō" (cell) and "zoku" (genus). The book brought a new perspective to Japanese plant study, which had until then centered on medicinal herbs.
He began publishing the Seimi Kaisō, the first book to introduce modern chemistry to Japan. "Seimi" rendered the sound of the Dutch word for chemistry. Rather than simply translating, he added knowledge from many other books and the results of his own experiments. He coined many words still used today, such as those for oxygen, hydrogen, temperature and analysis.
He died in Edo. The Seimi Kaisō was completed the year after his death. He also wrote an essay introducing coffee, and he is sometimes said to have first used the characters now written for coffee in Japanese, though this is disputed. Yōan's work, building whole new fields from their very vocabulary, became a foundation of modern science in Japan.
Philipp Franz von SieboldUtagawa HiroshigeUtagawa KuniyoshiTakashima ShūhanTokugawa NariakiEgawa HidetatsuKawaji ToshiakiraTakano Chōei
People of the Edo period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.