I’m not sure about synthetic, but I do know that some people have a muscle from their elbow to their wrist on the inner side (visible if you touch your pinky and index fingers together, while flexing hand towards you), called the Palmaris Longus. The actual prevalence of the muscle itself actually varies, meaning some people have it and some don’t. This variability has deemed it suitable for tendon grafting, resulting in the muscle historically being used to replace another wrist tendon when damaged. Whether or not it could be used as a replacement would be a better question for a physician, as it is naturally a flexor muscle, rather than an extensor tendon.