The two key properties for a racquet material are stiffness and light weight. Graphite remains the most common choice for stiff racquets, and the technology for adding stiffness without adding weight continues to improve. Probably the most famous of the early graphite racquets was the Dunlop Max 200G, used by both John McEnroe and Steffi Graf. Its weight in 1980 was 12.5 ounces. Over the 20 years since, average racquet weights have decreased to around 10.5 ounces, with some racquets as light as 7 ounces. New materials such as ceramics, fiberglass, boron, titanium, Kevlar, and Twaron are constantly being tried, almost always in a mix with graphite.
In 1987, Wilson came up with an idea for increasing racquet stiffness without finding a stiffer material. Wilson's Profile racquet was the first "widebody." In retrospect, it seems strange that no one thought of the idea sooner to increase the thickness of the frame along the direction in which it must resist the impact of the ball. The Profile was a monster of a racquet, with a frame 39 mm wide at the middle of its tapered head, more than twice the width of the classic wooden frame. By the mid 1990's, such extreme widths had fallen out of favor, but the widebody innovation carries forward: most frames sold today are wider than the pre-widebody standard.
The racquet makers have, to some extent, suffered from their own success. Unlike wood racquets, which warped, cracked, and dried out with age, graphite racquets can last for many years without a noticeable loss of performance. A 10-year-old graphite racquet can be so good and so durable that its owner has little motivation to replace it. The racquet companies have met this problem with a stream of innovations, some of which, like the oversized head, wider frame, and lighter weight are evident in almost every racquet made today. Other innovations have been less universal, such as extreme head-heavy balance as seen in the Wilson Hammer racquets, and extra length, first introduced by Dunlop.
What's next? How about an electronic racquet? Head has come out with a racquet that uses piezoelectric technology. Piezoelectric materials convert vibration or motion to and from electrical energy. Head's new racquet takes the vibration resulting from impact with the ball and converts it to electrical energy, which serves to dampen that vibration. A circuit board in the racquet's handle then amplifies that electrical energy and sends it back to the piezoelectric ceramic composites in the frame, causing those materials to stiffen.
The medieval French monks would be impressed.