Biomedical Engineering Reference
In-Depth Information
The
in vitro
study showed evidence of effective bone cell mechanical stimulation,
and the concept was further explored
in vivo
. The
in vivo
implantation of piezoelectric
actuators for tissue mechanical stimulation is innovative and a potential use in the
development of smart implants.
Piezoelectric materials for Bone mechanical stimulation-the
in vivo
study
The actuator device was developed, composed of a micro-board containing a ultra-low
power 16-bit microcontroller (eZ430-RF2500, Texas Instruments, USA), powered by
lithium battery and encapsulated in polymethylmetacrilate (PMMA) and a set of six
actuators composed of PVDF and silver electrodes, electrically insulated by dip-coat-
ing as previously described. A similar, but static, control device was also developed,
sterilized and implanted.
The sterilization of the device posed a challenge in itself. The devices included a
16-bit processor, which corrupted its memory when submitted, in a Co-60 source, to
25 kGy at a dose rate of 2 kGy/hr. On the other hand, it was not possible to sterilize by
moist or dry heat since the PVDF actuators depolarize at temperatures equal or above
60ºC. An alternative sterilization method, which ensured absence of toxic residues,
was developed. The methodology of its development and validation was based on ISO
11737-1 and ISO 14937, as described elsewhere (Reis et al., 2010).
The actuator device was implanted in the left hind limb and the control static de-
vice was implanted in the right hind limb of a 4 year old merino ewe, with 45 kg
body-weigh, under general inhalatory anaesthesia. Two osteotomies were made on
the medial surface of the tibial proximal physis using an especially metal designed
guide to make two regular and well orientated osteotomies using an oscillating saw.
The bone was continuously irrigated with a sterile saline solution during the process
of low speed drilling and cutting. The same procedure was followed with a different
design guide for the distal femoral physis, where four osteotomies were done. The
portion of the devices containing the microprocessor and the power supply were left
in the subcutaneous space.
One week after implantation calcein (Sigma, USA) was injected subcutaneously
(15 mg/kg) and 1 week prior to sacrifice the same procedure was done with alizarin
complex one (25 mg/kg) (Sigma). Thirty days after implantation the ewe was sacri-
ficed by intravenous sodium pentobarbital injection. The present study was authorized
by competent national authorities and conducted accordingly to FELASA's guidelines
for animal care. Proper analgesia procedures began before the surgery and were main-
tained through a week.
Both hind limbs were dissected, the implanted materials and surrounding tissue
removed and fixed in 4% paraformaldehyde for 2 weeks. Bone samples were cut trans-
versally to the long axis of the bone, each including a piezoelectric film and the sur-
rounding bone
Specimens were dehydrated through an ascending ethanol series. Soft tissues (lo-
cal lymph nodes and samples of the fibrous capsule surrounding the implants) were
routinely processed and embedded in paraffin. Undecalcified bone samples of each
of the implants were included in resin (Technovit® 9100, Heraeus Kulzer, Germany)
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