Multilayer dental appliances and related methods and systems

An orthodontic system for positioning a patient's teeth, comprising:

• a plurality of incremental position adjustment appliances having teeth receiving cavities shaped to directly receive at least some of the patient's teeth and apply a resilient positioning force to the patient's teeth, wherein the appliances are successively worn by a patient to move teeth from one arrangement to a successive arrangement, and wherein at least one of the appliances comprises:
• a hard inner polymer layer comprising a co-polyester and having a hard polymer layer elastic modulus;
• and
• a first soft outer polymer layer and a second soft outer polymer layer each comprising a thermoplastic polyurethane elastomer and each having a soft polymer elastic modulus less than the hard polymer layer elastic modulus, a flexural modulus of greater than about 35,000 psi, a hardness of about 60A to about 85D, and a thickness in a range from 25 μm to 100 μm,
• wherein the hard inner polymer layer is disposed between the first soft outer polymer layer and the second soft outer polymer layer so as to reduce degradation of the resilient positioning force applied to the teeth when the appliance is worn.

An orthodontic appliance for repositioning a patient's teeth, the appliance comprising:

• a shell having a plurality of teeth receiving cavities shaped to apply a repositioning force to the patient's teeth, wherein the shell is formed from a multilayer material configured to reduce degradation of the repositioning force over time when the shell is worn on the patient's teeth, the multilayer material comprising:
• a first soft polymer layer and a second soft polymer layer each having a compression set greater than 40% after 24 hours at 70° C., wherein the first and second soft polymer layers each comprise a thermoplastic polyurethane elastomer;
• and
• a single hard polymer layer disposed between the first and second soft polymer layers and having a stress relaxation greater than 10% at 24 hour testing between 90% to 100% humidity, wherein the single hard polymer layer comprises a co-polyester.