Design of the device
To construct the device computer-aided design (Solidworks, Dassault Systèmes SolidWorks Corporation, Concord, USA) was used. The dimensions were selected according to the desired strength, possible flow distance during injection moulding and ability of the device to fully close. The device consisted of a flexible band, in part perforated, and a case with a locking mechanism where the band could be introduced and pulled through. Details for tissue engaging properties were added to the locking case and the band surface (Figure 1). A protrusion at the connection between locking case and band formed a cog merging with the first hole in the band when the band was fully pulled through. The tissue-engaging ridges at the side of the locking case aligned towards the band surface and connected with tissue caught in the loop.
Design of moulds for injection moulding
The device was manufactured in purpose-built steel moulds (Mecdon, Laxå, Sweden). During development a redesigned second version's mould was made to improve the device. The second version of device had a downsized locking case, thinner walls and rounded-off corners. Consequently it contained less material than the first version (Figure 2). The inlet for the polymer into the second version's mould was adjusted and the thickness of the flexible band increased from 0.55 mm to 0.65 mm which created bands of greater length and strength. Controlled heating was used in the second version's mould for improvement of flow distance during injection moulding.
Injection moulding of polydioxanone
Polydioxanone (Resomer® X, Boehringer Ingelheim Pharma GmbH, D-55216 Ingelheim, Germany) was chosen for the material of the device, a synthetic resorbable polymer which is widely used for suture materials and implants since many years. It is flexible and elastic and its induction of inflammatory reactions is minimal. It is degraded through hydrolysis and may be completely resorbed within 6-12 months [12, 13].
The polymer was heated above melting temperature and injected into the mould using an in-house built injection moulding machine. High pressure was maintained after injection and crystallization was allowed to occur for about one minute before the mould was opened. Polydioxanone of inherent viscosity of 1.9 dL/g was used for injection moulding in the first version's mould. Two batches of polydioxanone with inherent viscosities of 1.9 and 1.3 dL/g respectively were tested for injection moulding in the second version's mould.
Tensile testing
The tensile strength of the flexible band was measured using a 5544 Single Column Testing System (Instron, USA). Samples were prepared by cutting away the locking case of the device and the solid part of the flexible band, leaving approximately 4 cm of the perforated band. The samples were clamped 1 cm from each end and pulled to break at a rate of deformation of 40 mm/min. The tensile strength was determined as the maximum failure load. Eleven samples manufactured from each of the polymer batches of polydioxanone were tested. All tested devices were of the second version. The level of statistical significance was defined as p < 0.05 (Student's t-test, equal variances not assumed).
Animals
One dog weighing 20 kg, which was euthanized prior to the test due to reasons not associated with this study, and six pigs weighing 25-28 kg, which were anaesthetised for other reasons at Uppsala University Hospital, Sweden, were used in the study. The Uppsala Animal Ethics Committee, Sweden, approved this additional test (permission number C172/8).
Test of the devices in tissue
Ovarian pedicle, imaging and histology
An incision was made from approximately 2 cm cranial of the umbilicus along the midline of the abdomen in the dog. The linea alba was incised, the abdomen opened and the uterine horns and ovaries were localized. A hole was made in the broad ligament close to the left ovary. A loop was formed around the ovarian pedicle and the band was pushed into the locking mechanism (first version of the device). The loop was tightened, compressing the tissue inside the loop. The surplus band extending beyond the channel exit in the locking case was cut off.
The echogenicity of the device was examined with ultrasound (Sequoia 512, Acuson, Siemens AG Medical Solutions, Germany) with an 8.0 MHz linear probe. The probe was placed on the abdomen after application of ultrasound gel on the skin. After the investigation the device with adjacent tissue was removed and embedded in resin (Historesin, Leica Microsystems Nussloch GmbH, Germany). After dehydration in increasing concentrations of ethanol, followed by increasing concentrations of water-soluble resin, the sample was embedded in 100% resin. Sections were cut using a microtome (Leica RM 2165, Leica Instruments GmbH, Germany) with glass knives as close as possible to the device. The samples were mounted on glass and stained with hematoxylin and eosin (HE).
Test of haemostasis and tissue grip of renal arteries in six pigs
The abdomen was opened midway along the linea alba and both renal arteries were free-dissected in two (first version) and four pigs (second version), respectively. A loop was formed around the artery with the device and the loop was tightened causing compression of the vessel. When the band was fully pulled through the locking case of the device, the artery was cut between the kidney and the device about 0.5 cm from the device. The renal artery with the attached device was inspected for haemostasis for about 5 minutes. The ability of the device to withstand a ligature slip-off was tested by applying a force of 10 N using a dynamometer attached to the device.
Arterial blood pressure was registered by invasive continuous measurements in the femoral artery of the pigs and was recorded once a minute (SC 9000XL, Siemens Medical Solutions, USA). The least square means of systolic and diastolic blood pressures during ligation were calculated.