Bilateral swelling self-inflating tissue expanders
Keywords: hydrogel
bilateral swelling
tissue expander
N-vinylpyrrolidone
die-drawing
anisotropic
Methylmethacrylate

Hydrogel tissue expander (HTE) self-inflates in vivo by gradually absorbing body fluid. It eliminates the regular injections required by the conventional balloon-type tissue expander and provides a more progressive stretching process to the tissue. Previously, the hot-compressed anisotropic HTE (A-HTE) has achieved uniaxial swelling and could potentially match defects of anisotropy. This present work applied die-drawing as the solid-state processing to impart shape memory to the crosslinked copolymer of N-vinyl-pyrrolidone and methyl methacrylate, poly(VP-MMA) and successfully produced a novel self-inflating tissue expander with bilateral swelling properties.
This research is based on the assumption that A-HTE, especially bilateral swelling ones, could be more efficient, beneficial and convenient in the cases of anisotropic tissue expansion. In order to prove the concept, a mathematical model was established to study the tissue generation capacity of uniaxial (1D-Uni), bilateral (2D-Bi), and isotropic (3D-Iso) swelling HTEs. The simple syndactyly case was considered, and the complicated irregular anatomy was simplified into a regular geometry. The insertion mode was specifically designed in accordance with their primary swelling directions to achieve optimal expansion efficiency for each type of HTE.
As the required tissue area for syndactyly reconstruction was fixed in the model, the correspondingly required swelling ratio and swollen volume of HTEs could be obtained. A few assumptions were made to constrain the model to obtain the analytical solutions, among which valid results were screened out using the critical buckling situation and swelling capacity of poly(VP-MMA) (VP: MMA 90:10, 0.2% crosslinker). The result showed that for generally 1D-Uni required the minimum swollen volume, however, 2D-Bi HTE provided the most comprehensive application possibility and generally required the minimum swelling ratio and the smallest incision size at implantation, compared to other HTEs.
With the proven advantages of 2D-Bi, the research was then focused on the development of the 2D-Bi poly(VP-MMA) HTE. The influence of processing parameters, including die angle, drawing temperature, deformation ratio and drawing rates were examined using single-factor methods. It was found that poly(VP-MMA) rods (VP: MMA 90:10, 0.2% crosslinker) could be drawn through a converging conical die with a 5o to 10o semi-angle around its glass transition temperature of 145 ~ 180℃℃ at drawing rates of 0.1 to 6 mm/s. A maximum 3.7 times of elongation was achieved. The swelling rate and ratio of the die-drawn sample were significantly increased in the bilateral directions and greatly inhibited in the longitudinal (axial) direction. The extent of this anisotropy was tunable through the die-drawing deformation ratio. The preliminary study on the swelling rate control of die-drawn samples by polydimethylsiloxane showed an effective control in the swelling rate but simultaneously reduced the volume expansion prominently.
The characterization of die-drawn samples showed induced orientation but no evidence of crystallization. DSC analysis reflected an increased glass transition temperature of 168.8℃ compared to 163.1℃ of the isotropic sample. The optical microscope observed the aligned structure and elongated voids formation in the die-drawing direction on the lateral side of the die-drawn sample.