Robert A. Peattie

Pulsatile Flow in Fusiform Models of Abdominal Aortic Aneurysms: Flow Fields, Velocity Patterns and Flow-Induced Wall Stresses

As one important step in the investigation of the mechanical factors that lead to rupture of abdominal aortic aneurysms, flow fields and flow-induced wall stress distributions have been investigated in model aneurysms under pulsatile flow conditions simulating the in vivo aorta at rest. Vortex pattern emergence and evolution were evaluated, and conditions for flow stability were delineated. Systolic flow was found to be forward-directed throughout the bulge in all the models, regardless of size. Vortices appeared in the bulge initially during deceleration from systole, then expanded during the retrograde flow phase. The complexity of the vortex field depended strongly on bulge diameter In every model, the maximum shear stress occurred at peak systole at the distal bulge end, with the greatest shear stress developing in a model corresponding to a 4.3 cm AAA in vivo. Although the smallest models exhibited stable flow throughout the cycle, flow in the larger models became increasingly unstable as bulge size increased, with strong amplification of instability in the distal half of the bulge. These data suggest that larger aneurysms in vivo may be subject to more frequent and intense turbulence than smaller aneurysms. Concomitantly, increased turbulence may contribute significantly to wall stress magnitude and thereby to risk of rupture.

Stimulation of In Vivo Angiogenesis by In Situ Crosslinked, Dual Growth Factor-loaded, Glycosaminoglycan Hydrogels

As part of a study of elicited angiogenesis, hyaluronan (HA)-based hydrogels crosslinked by polyethylene glycol diacrylate (PEGDA) were loaded with combinations of the cytokine growth factors vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor (KGF) and platelet-derived growth factor (PDGF). GF release in vivo was controlled by covalent incorporation of thiol-modified heparin into thiolated HA hydrogels, which were injected into the ear pinnae of mice and allowed to crosslink in situ. GF release in vivo was controlled by covalent incorporation of thiol-modified heparin in the gels. The ears were harvested at 7 or 14 days post-implantation, and vascularization evaluated via a Neovascularization Index (NI). The study demonstrates that in situ gelling implants produced no gross inflammation, redness or swelling, and an improved tolerance compared to HA-based dry film implants. All treatments showed significantly more vascularization than either contralateral ears or ears receiving a sham surgery. The maximum response was observed after 14 days in the ears receiving 0.3% Hp, gelatin-containing gels loaded with VEGF + KGF (NI = 3.91). The study revealed injected growth factor-loaded HA-based hydrogels can successfully produce localized controllable vascularization, while minimizing tissue necrosis, polymorphonuclear leukocytes and inflammation. The ability to target and controllably release growth factors can prove a useful tool in specific diseased tissue/organ angiogenesis.

Experimental investigation of steady flow in rigid models of abdominal aortic aneurysms

Abdominal aortic aneurysms occur in as much as 2–3% of the population, and their rupture produces a mortality rate of 78–94% (1), causing 15,000 deaths per year in the U.S. alone. As an investigation into the mechanical factors that lead to aneurysm rupture, flow field measurements are presented for steady flow through a range of aneurysm sizes and Reynolds numbers. Seven rigid symmetric models of aneurysms were constructed with uniform lengths of 4d and diameters that ranged from 1.4 to 3.3d, whered is the inner diameter of the undilated entrance tube. Color Doppler flow imaging was used to visualize the flow fields, while laser Doppler velocimetry was used to quantify the flow field velocities and to determine critical Reynolds numbers for the onset of, and complete transition to, turbulent flow. Estimates of mean and peak wall shear stresses were derived from velocity measurements. Flow in these models varied from fully laminar to fully turbulent over the range of Reynolds numbers corresponding toin vivo flows. There was a large range over which the flow was intermittently turbulent. High wall shear occurred in the models when the flow was turbulent, suggesting that turbulence inin vivo aneurysms may contribute significantly to their risk of rupture.

Microvascular maturity elicited in tissue treated with cytokine-loaded hyaluronan-based hydrogels

Hydrogels composed of crosslinked, chemically modified hyaluronic acid (HA), gelatin (Gtn) and heparin (Hp) were preloaded with vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor (KGF) or platelet derived growth factor (PDGF) either individually or in combination with VEGF and implanted into the Balb/c mouse ear pinna. At 7 and 14 days post-surgery, elicited vascular maturity levels were quantified using immunohistochemical (IHC) staining techniques and reported as a vascular maturity index (VMI). At both time points, it was discovered that the dual cytokine combinations elicited greater maturity levels than that of cytokine administered individually. For example, VEGF and KGF-containing HA:Hp implants at day 7 yielded VMI values of -0.1375 and -0.092, respectively, whereas their combination resulted in a VMI of 0.176 (p<0.007). At day 7, only one of the seven HA:Hp experimental cases yielded a positive VMI (VEGF+KGF), whereas four of the seven HA:Hp cases yielded positive VMI values at day 14, indicating a sustained maturity response. The same general trends were found to exist in tissue treated with HA:Hp:Gtn experimental implants. Differences in elicited maturity also existed between tissue treated with HA:Hp and HA-containing hydrogels (VMI=0.176 for HA:Hp-VEGF+KGF vs. -0.064 for HA-VEGF+KGF, p<0.012), and these differences are thought to result from differences in characteristic cytokine release rates. This result also suggests that the presentation of multiple growth factors (GFs) on immobilized Hp may actively contribute to cytokine related signal transduction, a characteristic that may be exploited in the future to improve the efficacy of cytokine-loaded implants towards tissue regeneration therapeutic strategies.

Effect of Gelatin on Heparin Regulation of Cytokine Release from Hyaluronan-Based Hydrogels

The hypothesis that incorporation of small amounts (0.3% w/w) of modified heparin in thiol-modified hyaluronan or HA and gelatin hydrogels would regulate release of cytokine growth factors (GFs) from those gels has been investigated in vitro. In addition, the physiologic response to gel implantation has been evaluated in vivo. Tests were performed with 6 GFs: basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor, platelet-derived growth factor-AA (PDGF), and transforming growth factor-β 1. Release profiles for all 6 over several weeks were well fit by first order exponential kinetics (R2 > 0.9 for all cases). The most remarkable result of the experiment was a dramatic variation in the total mass ultimately released, which varied from as much as 90.2% of the initial load for bFGF to as little as 1.8% for PDGF, a 45-fold difference. Furthermore, gels containing either VEGF of Ang-1 produced twice the vascularization response in vivo as gels not containing a growth factor. Thus, those GFs maintained strong physiologic effectiveness.

Steady flow in models of abdominal aortic aneurysms. Part II: Wall stresses and their implication for in vivo thrombosis and rupture

In a continuing investigation into the mechanical factors that lead to rupture of abdominal aortic aneurysms, wall pressure and shear stress measurements are presented for steady flow through the series aneurysm models described in Part I. These models simulate in vivo aortic aneurysms of diameters from 3.3 to 7.5 cm; the flow rates through the models were dynamically matched to aortic flows under conditions ranging from rest to exercise. For all models, at all flow rates, a pressure maximum was found at the midpoint of the model bulge. This maximum increased with bulge diameter, suggesting that the largest aneurysms in vivo are exposed to the greatest pressures. When the flow was turbulent, the mean wall shear stress at the proximal end of the model bulge had magnitude 2 to 4 dynes/cm2, approximately equal to its value in an undilated tube. However, at the distal end of the model bulge the mean shear stress increased to 5 to 10 dynes/cm2, whereas the peak instantaneous shear stress exceeded the mean by an order of magnitude. When extrapolated to in vivo parameters, the maximum distal wall shear stress reached levels near those capable of disrupting endothelium. This suggests that turbulence in in vivo aneurysms may precipitate thrombus formation. Subsequent decreased luminal diameters in the presence of thrombus would then lessen the likelihood of turbulent flow and reduce the strength of any turbulence that did occur. It would also reduce the pressure in the aneurysmal bulge. Thus, the presence of turbulent flow may significantly affect risk of rupture.

Silk–hyaluronan-based composite hydrogels: A novel, securable vehicle for drug delivery

A new, biocompatible hyaluronic acid (HA)–silk hydrogel composite was fabricated and tested for use as a securable drug delivery vehicle. The composite consisted of a hydrogel formed by cross-linking thiol–modified HA with poly(ethylene glycol)-diacrylate, within which was embedded a reinforcing mat composed of electrospun silk fibroin protein. Both HA and silk are biocompatible, selectively degradable biomaterials with independently controllable material properties. Mechanical characterization showed the composite tensile strength as fabricated to be 4.43 ± 2.87 kPa, two orders of magnitude above estimated tensions found around potential target organs. In the presence of hyaluronidase (HAse) in vitro, the rate of gel degradation increased with enzyme concentration although the reinforcing silk mesh was not digested. Composite gels demonstrated the ability to store and sustainably deliver therapeutic agents. Time constants for in vitro release of selected representative antibacterial and anti-inflammatory drugs varied from 46.7 min for cortisone to 418 min for hydrocortisone. This biocomposite showed promising mechanical characteristics for direct fastening to tissue and organs, as well as controllable degradation properties suitable for storage and release of therapeutically relevant drugs.

Steady flow in models of abdominal aortic aneurysms. Part I: Investigation of the velocity patterns.

As an investigation into the mechanical factors that lead to rupture of abdominal aortic aneurysms, both color Doppler flow imaging and laser Doppler velocimetry measurements of steady flow through a series of aneurysm models are presented. The flow pattern in each model consisted of a core of relatively fast‐moving fluid traveling through the center of the dilation, surrounded by an outer annulus of slowly recirculating fluid. At flow rates below a Reynolds number of 1750 +/‐ 150, the flow was smooth, steady, and laminar. At higher flow rates (Reynolds number above 2250 +/‐ 250), the flow was always irregular and turbulent. Between these fully laminar and fully turbulent regimes the flow was intermittently turbulent. Larger models showed a tendency to become turbulent at lower Reynolds numbers than smaller models. In addition, turbulence was amplified in the distal half of the model dilation, with the largest models producing velocity fluctuations as great as 35% of the time‐average centerline velocities. These data suggest that larger aneurysms in vivo may be subject to more frequent and intense turbulence than smaller aneurysms. Concomitantly, increased turbulence may contribute significantly to risk of rupture, as is discussed in Part II.

DEVELOPMENT OF TURBULENCE IN STEADY FLOW THROUGH MODELS OF ABDOMINAL AORTIC ANEURYSMS

As part of a general examination of the effects of blood flow patterns on the risk of rupture of abdominal aortic aneurysms, we have investigated steady flow through a series of polyvinyl chloride models of fusiform aneurysms. A series of steady flow rates between 460 ml/min and 2.0 l/min have been delivered through the models, and the resulting flow patterns have been evaluated by color Doppler flow imaging. At low volume flow rates, the pattern of flow through the model is smooth and laminar. At higher flow rates, however, this is replaced by an irregular, fluctuating, and apparently turbulent motion. This finding is consistent with previous clinical observations. Turbulence appears initially at the distal end of the model aneurysm and then spreads proximally to fill the whole model only if the flow rate continues to increase. The onset of irregular, turbulent flow is found to be governed by both the overall flow rate through the model and the model diameter. Critical conditions for the appearance of turbulence are presented and discussed, and the critical value of the Reynolds number for both the initial distal transition and the final full transition is shown to decrease as the size of the model aneurysm increases.

A Qualitative Investigation of Turbulence in Flow Through a Model Abdominal Aortic Aneurysm

RATIONALE AND OBJECTIVES.As part of a general examination of the effect of blood flow patterns on the growth and risk of rupture of abdominal aortic aneurysms, the authors investigated the flow through an experimental model of a fusiform aneurysm. MATERIALS AND METHODS.Aneurysms were simulated in polyvinyl chloride tubing. A steady flow of 450 mL/minute to 2 L/minute was investigated by color Doppler flow imaging. RESULTS.Two types of flow patterns existed in the model aneurysm, depending on the overall flow rate through the model. At low velocity flow, this pattern is smooth, steady, and laminar, whereas at higher flows it is irregular and turbulent. CONCLUSION.This finding corresponds to recent clinical observations. The onset of unsteadiness and irregularity in the flow through the model is related to flow velocity.