# Nondestructive Mechanical Characterization of Bioengineered Tissues by Digital Holography

## TL;DR

This paper introduces a nondestructive method using digital holography to assess the mechanical properties of engineered skin tissues during production.

## Contribution

The study presents a noncontact, nondestructive technique for in-line quality control of bioengineered tissues using digital holographic vibrometry.

## Key findings

- Digital holographic vibrometry (DHV) can measure mechanical behavior of tissues through clear packaging in various conditions.
- Vibration patterns observed in air showed the closest match to theoretical predictions.
- FE analysis simulated the effects of tissue geometry and composition on mechanical behavior.

## Abstract

Mechanical
properties of engineered connective tissues
are critical
for their success, yet modern sensors that measure physical qualities
of tissues for quality control are invasive and destructive. The goal
of this work was to develop a noncontact, nondestructive method to
measure mechanical attributes of engineered skin substitutes during
production without disturbing the sterile culture packaging. We optimized
a digital holographic vibrometry (DHV) system to measure the mechanical
behavior of Apligraf living cellular skin substitute through the clear
packaging in multiple conditions: resting on solid agar as when the
tissue is shipped, on liquid media in which it is grown, and freely
suspended in air as occurs when the media is removed for feeding.
We utilized full-field measurement to assess the complete surface
deformation pattern to compare with vibration theory and found the
patterns observed in air showed the closest behavior to theory. To
simulate the effects of the actual culture dish geometry and the trilayer
composition of the tissue on the porous membrane support, we employed
finite element (FE) analysis. To simulate changes in thickness and
stiffness that may occur with manufacturing process variations, we
dried samples over time and observed measurable increases in the fundamental
mode frequency which could be predicted by altering the thickness
of the tissue layers in the FE model. However, quantitative estimates
of the engineered tissue stiffness based on vibration theory are unrealistically
high due to the signal being dominated by the stiff underlying membrane
on which the tissue is cultured. Thus, although DHV is not able to
specifically quantify the thickness or modulus or identify small spot
defects, it has the potential to be used assess the overall properties
of a tissue in-line and noninvasively for quality control.