Evolution of the Laser-Induced Spallation Technique in Film Adhesion Measurement

Laser-induced spallation is a process in which a stress wave generated from a rapid, high-energy laser pulse initiates the ejection of surface material opposite the surface of laser impingement. Through knowledge of the stress wave amplitude that causes film separation, the adhesion and interfacial properties of a film-on-substrate system are determined. Some advantages of the laser spallation technique are the non-contact loading, development of large stresses (on the order of GPa) and high strain rates, up to 108 /s. The applicability to both relatively thick films, tens of microns, and thin films, tens of nm, make it a unique technique for a wide range of materials and applications. This review combines the available knowledge and experience in laser spallation, as a state-of-the-art measurement tool, in a comprehensive pedagogical publication for the first time. An historical review of adhesion measurement by the laser-induced spallation technique, from its inception in the 1970s through the present day, is provided. An overview of the technique together with the physics governing the laser-induced spallation process, including functions of the absorbing and confining materials, are also discussed. Special attention is given to applications of laser spallation as an adhesion quantification technique in metals, polymers, composites, ceramics, and biological films. A compendium of available experimental parameters is provided that summarizes key laser spallation experiments across these thin film materials. This review concludes with a future outlook for the laser spallation technique, which approaches its semicentennial anniversary.

Biofilm and Cell Adhesion Strength on Dental Implant Surfaces via the Laser Spallation Technique

ObjectivesThe aim of this study is to quantify the adhesion strength differential between an oral bacterial biofilm and an osteoblast-like cell monolayer to a dental implant-simulant surface and develop a metric that quantifies the biocompatible efficacy of implant surfaces.MethodsHigh-amplitude short-duration stress waves generated by laser pulse absorption are used to spall bacteria and cells from titanium substrates. By carefully controlling laser fluence and calibration of laser fluence with applied stress, the adhesion difference between dental carry Streptococcus mutans biofilms and MG 63 osteoblast-like cell monolayers on smooth and rough titanium substrates is obtained. The Adhesion Index consists of a ratio of cell adhesion strength to biofilm adhesion strength obtaining a nondimensionalized parameter for biocompatibility assessments.ResultsAdhesion strength of 145±42 MPa is measured for MG 63 on smooth titanium, which increases to 288±24 MPa on roughened titanium. Adhesion strength for S. mutans on smooth titanium is 315±9 MPa and remained relatively constant at 332±9 MPa on roughened titanium. The Adhesion Index for smooth titanium is 0.46±0.12 which increased to 0.87±0.05 on roughened titanium.SignificanceThe laser spallation technique provides a platform to examine the tradeoffs of adhesion modulators on both biofilm and cell adhesion. This tradeoff is characterized by the Adhesion Index, which is proposed to aid biocompatibility screening and could result in improved implantation outcomes. The Adhesion Index is implemented to determine surface factors that promote favorable adhesion of cells greater than biofilms. Here, an Adhesion Index >> 1 suggests favorable biocompatibility.Graphical AbstractHighlightsBiofilm and cell monolayer adhesion are measured via the laser spallation techniqueSmooth and roughened dental implant-mimicking titanium surfaces are investigatedSurface roughness increases cell adhesion but does not alter the adhesion of biofilmsAn Adhesion Index is developed to directly quantify the adhesive competition between bacteria and cells on an implant surface

Biofilm rupture by laser-induced stress waves increases with loading amplitude, independent of location

Integral to the production of safe and biocompatible medical devices is to determine the interfacial properties that affect or control strong biofilm adhesion. The laser spallation technique has recently emerged as an advantageous method to quantify biofilm adhesion across candidate biomedical surfaces. However, there is a possibility that membrane tension is a factor that contributes to the stress required to separate biofilm and substrate. In that case, the stress amplitude, controlled by laser fluence, that initiates biofilm rupture would vary systematically with location on the biofilm. Film rupture, also known as spallation, occurs when film material is ejected during stress wave loading. In order to determine effects of membrane tension, we present a protocol that measures spall size with increasing laser fluence (variable fluence) and with respect to distance from the biofilm centroid (iso-fluence). Streptococcus mutans biofilms on titanium substrates serves as our model system. A total of 185 biofilm loading locations are analyzed in this study. We demonstrate that biofilm spall size increases monotonically with laser fluence and apply our procedure to failure of non-biological films. In iso-fluence experiments, no correlation is found between biofilm spall size and loading location, thus providing evidence that membrane tension does not play a dominant role in biofilm adhesion measurements. We recommend our procedure as a straightforward method to determine membrane effects in the measurement of adhesion of biological films on substrate surfaces via the laser spallation technique.Graphical Abstract