Medical implants are often available in fixed size. The inability of such devices to accommodate with normal tissue growth remains a challenge, particularly in case of children. Hence, development of implant devices that could correct themselves in accordance with anatomical deformities and easily accommodate with tissue growth is highly warranted.
Researchers at the Harvard Medical School, USA and University College Dublin, Ireland have developed a growth-accommodating device that consists of a tubular braided sleeve and a biodegradable polymer core (Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825).
Fig. 1 (a) Schematic of a degradable polymer core (dark blue) placed inside a braided sleeve to control sleeve diameter, coupling inner polymer degradation to braided sleeve (and overall device) elongation; (b) A dissolvable spherical sucrose core (red) inside a nitinol biaxial braid acts as a degradable polymer surrogate. Upon immersion in water, the sucrose core gradually dissolves leading to a gradual decrease in the braided sleeve diameter along with a concomitant autonomous elongation; (c) Variation in length and diameter of the braided sleeve during core degradation.
A hydrophobic surface-eroding, biodegradable and biocompatible polymer poly(glycerol sebacate) (PGS) was used as the base material. The rationale behind the choice of PGS was justified based on its minimal swelling in water, ability to offer the requisite mechanical properties to resist compressive forces from the braided sleeve and capability to maintain structural integrity throughout degradation. To minimize the stretching of PGS to less than 5%, it was treated at 155 °C for 86 h in vacuum, which maximizes its cross-linking leading to the formation of extra-stiff PGS (ESPGS). The ESPGS was used as the polymer core while the braided sleeve was made of nitinol alloy.
The concept behind the development involves coupling the degradation of a surface-eroding polymer core to the braid length and overall device elongation. After implantation, once the polymer core starts to degrade, the braided sleeve begins to thin out and elongates in response to surrounding tissue growth (Figs. 1(a) and 1(b)), without the necessity for any additional interventions. Since the sleeve length and diameter of the braid are inversely related, thinning of the sleeve results in its elongation (Fig. 1(c)).
The flexible nature of the braided sleeve and polymer contributes to the durability of the device and no evidence of fatigue failure of either the braided sleeve or the ESPGS core could be observed during in vivo studies using animal models.
By altering the number and thickness of braid fibres, the braid geometry and rate of degradation of the polymer core, it would be possible to modify the device elongation profile to match a wide spectrum of clinical applications.
Variability in the polymer erosion rate is the current limitation of the proposed device and achieving uniform degradation of polymer core will be the focus of future work.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer Eric N. Feins et al., A growth-accommodating implant for paediatric applications, Nature Biomedical Engineering, 1 (2017) 818–825.
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