According to this interview with her, the designs are printed digitally on cotton fabric.
Callahan's quilts are a combination of handcrafts and digital technology. "It begins with me sketching, sketching, sketching," she says. "I try to find something interesting and unique and work out the visuals by hand." After that, Callahan uses a computer to figure out all of the exact measurements and dimensions. She then sends the file to a printer who applies that pattern to a piece of 100-percent cotton fabric. Then, the printed fabric is joined with batting and another layer of material and sewn on a computer-controlled machine—"It's like a CNC mill for sewing," she explains.
There is some hand work but a computer controlled machine is also involved in the sewing process.