Substrate Topography Guides Pore Morphology Evolution in Nanoporous Gold Thin Films.

This paper illustrates the effect of substrate topography on morphology evolution in nanoporous gold (np-Au) thin films. One micron-high silicon ridges with widths varying between 150 nm to 50 µm were fabricated and coated with 500 nm-thick np-Au films obtained by dealloying sputtered gold-silver alloy films. Analysis of scanning electron micrographs of the np-Au films following dealloying and thermal annealing revealed two distinct regimes where the ratio of film thickness to ridge width determines the morphological evolution of np-Au films.

Ultrahigh Resolution Titanium Deep Reactive Ion Etching.

Titanium (Ti) represents a promising new material for microelectromechanical systems (MEMS) because of its unique properties. Recently, this has been made possible with the advent of processes that enable deep reactive ion etching (DRIE) of high-aspect-ratio (HAR) structures in bulk Ti substrates. However, to date, these processes have been limited to minimum feature sizes (MFS) ≥750 nm. Although this is sufficient for many applications, MFS reduction to the deep submicrometer range opens potential for further device miniaturization and an opportunity for endowing devices with unique functionalities that are derived from precisely defined structures within this length scale regime. Herein, we report results from studies seeking to create means for realizing such opportunities through extension of Ti DRIE to the deep submicrometer scale. The effects of key process parameters on etch performance were investigated, and the understanding gained from these studies formed the development of a new ultrahigh resolution (UHR) Ti DRIE process. Using this process, we demonstrate, for the first time, fabrication of HAR structures in bulk Ti substrates with 150 nm MFS, smooth vertical sidewalls (88°), good etch rate (587 nm/min), and mask selectivity (11.1). This represents a fivefold or greater improvement in MFS relative to our previously reported processes and a 29-fold or greater improvement over more recent processes reported by others. As such, the UHR Ti DRIE process extends the state-of-the-art considerably, and it opens important new opportunities for Ti MEMS, particularly in the implantable medical device realm.

Micro- and Nanopatterned Topographical Cues for Regulating Macrophage Cell Shape and Phenotype.

Controlling the interactions between macrophages and biomaterials is critical for modulating the response to implants. While it has long been thought that biomaterial surface chemistry regulates the immune response, recent studies have suggested that material geometry may in fact dominate. Our previous work demonstrated that elongation of macrophages regulates their polarization toward a pro-healing phenotype. In this work, we elucidate how surface topology might be leveraged to alter macrophage cell morphology and polarization state. Using a deep etch technique, we fabricated titanium surfaces containing micro- and nanopatterned grooves, which have been previously shown to promote cell elongation. Morphology, phenotypic markers, and cytokine secretion of murine bone marrow derived macrophages on different groove widths were analyzed. The results suggest that micro- and nanopatterned grooves influenced macrophage elongation, which peaked on substrates with 400-500 nm wide grooves. Surface grooves did not affect inflammatory activation but drove macrophages toward an anti-inflammatory, pro-healing phenotype. While secretion of TNF-alpha remained low in macrophages across all conditions, macrophages secreted significantly higher levels of anti-inflammatory cytokine, IL-10, on intermediate groove widths compared to cells on other Ti surfaces. Our findings highlight the potential of using surface topography to regulate macrophage function, and thus control the wound healing and tissue repair response to biomaterials.

Bone marrow stromal cell adhesion and morphology on micro- and sub-micropatterned titanium.

The objective of this study was to investigate the adhesion and morphology of bone marrow derived stromal cells (BMSCs) on bulk titanium (Ti) substrates with precisely-patterned surfaces consisting of groove-based gratings with groove widths ranging from 50 micro m down to 0.5 micro m (500 nm). Although it is well known that certain surface patterning enhances osteoblast (bone-forming cell) functions, past studies on cell-pattern interactions reported in the literature have heavily relied on surface patterning on materials with limited clinical relevance for orthopedic applications, such as polymeric substrates. The clinical need for improving osseointegration and juxtaposed bone formation around load-bearing Ti implants motivated this in vitro study. BMSCs were selected as model cells due to their important role in bone regeneration. The results showed significantly greater BMSC adhesion density and more favorable cell morphology on sub-micropatterned gratings when compared with larger micropatterned gratings and non-patterned control surfaces after both 24 hr and 72 hr cultures. We observed increasing cellular alignment and elongation with decreasing feature size. We also identified two distinctive cellular morphologies: Type I-Attached and spread cells that elongated along the pattern axes; and Type II-Superficially adhered round cells. Sub-micropatterned gratings demonstrated significantly greater Type I cell density than the non-patterned control, and lower Type II cell density than the larger micropatterned gratings. Collectively, these results suggest potential for rationally designing nano-scale surface topography on Ti implants to improve osseointegration.

Comparative endothelial cell response on topographically patterned titanium and silicon substrates with micrometer to sub-micrometer feature sizes.

In this work, we evaluate the in vitro response of endothelial cells (EC) to variation in precisely-defined, micrometer to sub-micrometer scale topography on two different substrate materials, titanium (Ti) and silicon (Si). Both substrates possess identically-patterned surfaces composed of microfabricated, groove-based gratings with groove widths ranging from 0.5 to 50 µm, grating pitch twice the groove width, and groove depth of 1.3 µm. These specific materials are chosen due to their relevance for implantable microdevice applications, while grating-based patterns are chosen for the potential they afford for inducing elongated and aligned cellular morphologies reminiscent of the native endothelium. Using EA926 cells, a human EC variant, we show significant improvement in cellular adhesion, proliferation, morphology, and function with decreasing feature size on patterned Ti substrates. Moreover, we show similar trending on patterned Si substrates, albeit to a lesser extent than on comparably patterned Ti substrates. Collectively, these results suggest promise for sub-micrometer topographic patterning in general, and sub-micrometer patterning of Ti specifically, as a means for enhancing endothelialization and neovascularisation for novel implantable microdevice applications.

Vascular stents with rationally-designed surface patterning.

Herein, we discuss our recent progress towards realization of next-generation vascular stents that seek to mitigate adverse physiological responses to stenting via rational design of stent surface topography at the nanoscale. Specifically, we will discuss advances in patterning of deep sub-micrometer scale features in titanium (Ti) substrates, creation of cylindrical stents from micromachined planar Ti substrates, and integration of these processes to produce devices that will eventually allow evaluation of rationally-designed nanopatterning in physiologically-relevant contexts. We will also discuss results from mechanical testing and finite element modeling of these devices to assess their mechanical performance. These efforts represent key steps towards our long-term goal of developing a new paradigm for stents in which rationally-designed surface nanopatterning provides a physical means for complementing, or replacing, current pharmacological interventions.

Improved bone marrow stromal cell adhesion on micropatterned titanium surfaces.

Implant longevity is desired for all bone replacements and fixatives. Titanium (Ti) implants fail due to lack of juxtaposed bone formation, resulting in implant loosening. Implant surface modifications have shown to affect the interactions between the implant and bone. In clinical applications, it is crucial to improve osseointegration and implant fixation at the implant and bone interface. Moreover, bone marrow derived cells play a significant role for implant and tissue integration. Therefore, the objective of this study is to investigate how surface micropatterning on Ti influences its interactions with bone marrow derived cells containing mesenchymal and hematopoietic stem cells. Bone marrow derived mesenchymal stem cells (BMSC) have the capability of differentiating into osteoblasts that contribute to bone growth, and therefore implant/bone integration. Hematopoietic stem cell derivatives are precursor cells that contribute to inflammatory response. By using all three cells naturally contained within bone marrow, we mimic the physiological environment to which an implant is exposed. Primary rat bone marrow derived cells were seeded onto Ti with surfaces composed of arrays of grooves of equal width and spacing ranging from 0.5 to 50 µm, fabricated using a novel plasma-based dry etching technique. Results demonstrated enhanced total cell adhesion on smaller micrometer-scale Ti patterns compared with larger micrometer-scale Ti patterns, after 24-hr culture. Further studies are needed to determine bone marrow derived cell proliferation and osteogenic differentiation potential on micropatterned Ti, and eventually nanopatterned Ti.