Feasibility of Parylene C for encapsulating piezoelectric actuators in active medical implants

Abstract

Parylene C is well-known as an encapsulation material for medical implants. Within the approach of miniaturization and automatization of a bone distractor, piezoelectric actuators were encapsulated with Parylene C. The stretchability of the polymer was investigated with respect to the encapsulation functionality of piezoelectric chips. We determined a linear yield strain of 1% of approximately 12-µm-thick Parylene C foil. Parylene C encapsulation withstands the mechanical stress of a minimum of 5×10⁵ duty cycles by continuous actuation. The experiments demonstrate that elongation of the encapsulation on piezoelectric actuators and thus the elongation of Parylene C up to 0.8 mm are feasible.

Keywords: Parylene C; active implant; encapsulation; piezoelectric actuator; polymer; stretchability.

Figure 1

(A) Piezoelectric chip contacted with enameled copper wires and placed inside a 3D printed holder. (B) Representative stress/strain diagram of Parylene C. (C) Examination of the long-term performance of Parylene C encapsulation of piezoelectric actuators obtained by pulsing a specified amount of load cycles and performing continuous and subsequent measurement of the breakdown strength, respectively.

Figure 2

(A,B) Optical micrographs of the failure of the Parylene C coating at the insulation of the contact lead from two different piezo samples. The coating failure is visible after electroplating with copper through a gray deposit. (C) Suggested failure mechanism of the Parylene C coating on copper wires insulated by a silicone sheath due to movement of the insulation.