Variable stiffness for robotics is attracting increasing attention from researchers in the field of surgical robots. A surgical robot that can access the human colon or stomach via natural orifices must be flexible enough to pass through tortuous paths and to work in a confined space. Meanwhile, the robot must also be stiff enough to ensure pushability and to hold high payloads during the surgery. Thus, surgical robots with variable stiffness are desirable. This paper presents a new design concept for variable stiffness manipulators using a thermoplastic material - Polyethylene Terephthalate (PET) - and a flexible stainless steel sheath as a heating solution. The stiffness of PET can be flexibly adjusted through temperature. Experiments and validations were carried out at different conditions. The results showed that our proposed design is at least as flexible as a typical commercial endoscope when flexibility is desired and meanwhile at least 9 times stiffer than the endoscope when stiffness is desired (Flexural modulus was compared). A tendon-driven manipulator based on the proposed concept was also developed. Validation tests showed that the manipulator in compliant mode can be significantly bent through cable actuation, and the manipulator in stiff mode is able to maintain its shape against considerably large loads.