Casimir Zeglen (Polish: Kazimierz Żegleń; born in 1869 near Tarnopol; died not before 1927) was a Polish priest who invented a silk bulletproof vest. At the age of 18 he entered the Resurrectionist Order in Lwów (today Lviv, Ukraine). In 1890, he moved to the United States.
In 1893, after the assassination of Carter Harrison Sr., the mayor of Chicago, he worked on an improved silk bulletproof vest. In 1897, he worked on it with Jan Szczepanik. It saved the life of Alfonso XIII, the King of Spain—his carriage was covered with Szczepanik's bulletproof armour when a bomb exploded near it.
He was a Catholic priest of St. Stanislaus Kostka Roman Catholic Church in Chicago, then the largest Polish church in the country, with 40,000 in the parish. In his early twenties, he began experimenting with the cloth, using steel shavings, moss, and hair. In his research, he came upon the work of Dr. George E. Goodfellow, who had written about the bullet-resistive properties of silk. In his mid-thirties he discovered a way to weave the silk, to enable it to capture the bullet, while visiting weaving mills in Vienna, Austria and Aachen, Germany. A 1⁄8 in (3.175 mm) thick, four-ply bulletproof vest produced there was able to protect the wearer from the lower velocity pistol bullets of that era.
- Łotysz, Sławomir (1 October 2014). "Tailored to the Times: The Story of Casimir Zeglen's Silk Bullet-Proof Vest". Arms & Armour. pp. 164–186.
- Edwards, Josh (2 May 1980). "George Goodfellow's Medical Treatment of Stomach Wounds Became Legendary". The Prescott Courier. pp. 3–5.
- "Three Grades of Fabric", Brooklyn Eagle, 9 October 1902
- Łotysz, Sławomir. "Mnich wynalazca" (Monk-inventor). Polonia (Chicago) Vol. 13, No. 1-2 (2007) pp. 68–71, and Vol. 14, No. 3-4 (2007) pp. 64–67.
- Articles in Nowy Dziennik (a Polish Daily News) published in New York City): Kuloodporny ksiądz (Bulletproof priest), 5 May 2006; Polski ksiądz i Polski Edison (A Polish priest and the Polish Edison), 13 May 2006; Od habitu do opony (From a Religious habit to a tire), 20 May 2006.
- The Monk Who Stopped Bullets with Silk: Inventing the Bulletproof Vest, article from Culture.pl
- BULLETPROOF, podcast episode about Żegleń from Stories From The Eastern West