Robin A. Chivers

The strength of adhesive-bonded tissue joints

Abstract Bonds have been formed in vitro between pieces of porcine tissue (cartilage, bone and skin) with a range of commercially available surgical adhesives (cyanoacrylate, gelatin-based and fibrin). They were either butt joints or in a lap-shear configuration. These bonds were tested with a standard tensile test procedure and the strengths were measured. Bond strengths showed little dependence on the nature of the tissue, but were strongly dependent on the adhesive type. Orders of magnitude differences were seen, with strengths in the order: cyanoacrylate > gelatin/resorcinol/formaldehyde > gelatin/resorcinol/glyoxal=fibrin. Broad agreement was seen with literature results on similar systems. The appearance of the failures differed: cyanoacrylate was hard and brittle, while the others were softer and rubbery, like sealants. Mussel adhesion protein was also tested, but gave very poor results.

Easy removal of pressure sensitive adhesives for skin applications

Abstract There are two essential requirements of medical pressure sensitive adhesives: that they should stick firmly to a difficult substrate (skin) and that they should be easily and cleanly removed from that substrate when desired. These requirements would seem to be in conflict: a high peel force usually signals the ability to stick firmly, while a low force is needed when removing dressings by peeling. A number of ways have been considered to resolve this conflict. These may be divided into two broad categories: those that make the best of existing pressure sensitive adhesives technology, broadly taking a physical approach, and those that introduce novel chemistry into the process. Physical approaches consider such details as the dependence of peel force on peel angle, peel rate, backing materials, the deformation of the skin during peeling and use of barrier films and solvents. As an alternative to simply making the best of the physics of the peeling process, various workers have devised chemical systems for making the adhesive less strongly adhering at the time of removal. These systems usually consist of introducing a ‘switch’ mechanism into a strongly adhering adhesive so that its adherence may be reduced significantly at the time of removal by operation of the ‘switch’. Means of activating the ‘switch’ include: heat (warming or cooling), application of water via an absorbent backing and exposure to visible light. These may produce physical or chemical changes in the adhesive. While these approaches bring benefits to patients, consideration of the science behind them is leading to an enhanced understanding of the peeling process.

The effect of flexible substrates on pressure-sensitive adhesive performance

Abstract The adhesive fracture energy (fracture toughness) of tapes during globally elastic unpeeling is often calculated from the relation “ G=P/b(1− cos θ) ”. We show that while this expression is correct for elastic peeling from rigid substrates, it gives misleading results when peeling from reversible flexible substrates. A two-dimensional analysis is presented for peeling from non-linear elastic substrates that give consistent fracture energies from experimental data.

Simulation of ultrasound in the knee

A large body of evidence supports the use of low intensity pulsed ultrasound to augment bone healing and similar therapeutic benefits have been observed with damaged joint cartilage. It is an objective of this work to find paths of propagation of ultrasound in the knee and hence the optimal position for the ultrasound device. Determining the above in real tissues is complex and hard to achieve in practical terms; one solution is the use of computer modelling. A number of ultrasound simulation packages are available. In this work, Wave2000Pro (CyberLogic®, Inc) has been used to model ultrasound pulses with a frequency of 1·5 MHz in geometries similar in size and shape to knee. These simulations are two dimensional and require a number of assumptions to be made including linear propagation, homogeneity and isotropy of the materials under investigation. Simulations have been performed using Perspex shapes so that the software package could be validated in the laboratory. The outcome of these simulations matches measurements made in the laboratory with two different diameter needle hydrophones.

MODELLING THE PROPAGATION OF ULTRASOUND IN THE JOINT SPACE OF A HUMAN KNEE

There is strong evidence to support the clinical use of low-intensity pulsed ultrasound (LIPUS) to augment fracture healing. A previous experimental study showed that ultrasound can propagate in the joint space of a single human cadaveric knee. A full experimental investigation of this propagation is not possible due to poor reproducibility, the scarcity of human cadaveric tissues and the practical difficulties in making ultrasound measurements in the knee. The aim of the present work is to investigate whether a computer simulation (Wave2000 Pro®; Cyberlogic Inc., New York, NY, USA) can give a good representation of the experimental model. The simulations provided a good agreement with the experimental data, giving some confidence in the application of this computer simulation method as a means of determining whether ultrasound can propagate through different anatomical regions where bone is present.