What is the subject of the sentence?

The sentence "Missing, however, are materials that respond effectively to low-power NIR" seems to offer no clue for what is "Missing."

Context:

Adah Almutairi and colleagues explain that near-infrared (NIR) light (which is just beyond what human can see) penetrates through the skin and almost four inches into the body, with great potential for diagnosing and treating diseases. Low-power NIR does not damage body tissues as it passes. Missing, however, are materials that respond effectively to low-power NIR. Plastics that disintegrate when hit with NIR, for instance, could be filled with anti-cancer medicine, injected into tumors, and release the medicine when hit with NIR. Current NIR-responsive smart materials require high-power NIR light, which could damage cells and tissues. That's why Almutairi's team began research on development of a new smart polymer that responds to low-power NIR light.