© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.5.3.1.2 Stretch Forces

Endothelial cells can respond to persistent static overstretching in many ways, up to and including apoptosis. For instance, hypertension caused by hydrostatic edema can induce apoptosis in capillary EC [3820].

Vascular wall cells also respond to lateral stretch forces due to cyclical blood vessel expansion in vivo. For example, in one experiment [3854] bovine aortic endothelial cells were seeded to confluence on a flexible membrane to which cyclic strain was then applied at 1 Hz (0.5 sec strain, 0.5 sec relaxation) for 0-60 min. After 15 minutes of this cyclic stretching, there was an increase in adenylyl cyclase (AC), cAMP, and protein kinase A (PKA) activity of 1.5-2.2 times control levels at 10% average strain, as compared to unstretched cells, but there was no activity increase at 6% strain. Evidently, cyclic strain activates the AC signal transduction pathway in endothelial cells by exceeding a strain threshold, thus stimulating the expression of genes containing cAMP-responsive promoter elements. Stretch-activated cation channels in bovine aortic EC are inhibited by GdCl₃ at 10 µM [3809]. Human umbilical EC subjected to a 3-sec stretch pulse show an intracellular rapid-increase Ca⁺⁺ spike, followed by a (ryanodine-inhibitable) slower prolonged influx, due to biphasic Ca⁺⁺ entry into the cell through stretch-activated channels. Mn⁺⁺ also permeates mechanosensitive channels (but not Ca⁺⁺ channels) and enters the intracellular space immediately after an application of mechanical stretch [3787]. Cyclic strains of 10% at 1 Hz induce intracellular increases in Ca⁺⁺ [3823], diacylglycerol [3821, 3822], inositol trisphosphate [3821-3823] and protein kinase C (PKC) [3821] in peak response times of 10-35 sec, often sustained for up to ~500 sec. These strains also induce transcription factor activation over response times ranging from 0.25-24 hours [3779-3782]. Several endothelial cytokines are elicited by cyclic mechanical stretch [3824], and cyclic mechanical strain modulates tissue factor activity differently in endothelial cells originating from different tissues [3825]. The physical and mission designs of nanorobotic organs containing moving components or of vasculomobile nanorobot aggregates must take these differences properly into account.

Similarly, bovine aortic smooth muscle cells (SMC) seeded on a silastic membrane and subjected to cyclic strains up to 24% enhanced SMC proliferation at any strain level [3826], although SMC under high strain (7-24%) showed more proliferation than SMC at low strain (0-7%) in this experiment. High-strained SMC aligned themselves perpendicular to the strain gradient, whereas low-strained SMC remained aligned randomly. PKA activity and CRE (cAMP response element) binding protein levels increased for highly strained cells, compared to low-strained cells [3826]. Other experiments have found that:

(1) small mechanical strains of 1-4% at 1 Hz applied to human vascular smooth muscle cells can inhibit intracellular PDGF- or TNFalpha-induced synthesis of matrix metalloproteinase (MMP)-1 [3855];

(2) saphenous vein SMC distention by 0.5 atm pressure subsequently elevates cell apoptosis [3827];

(3) cyclic mechanical strain at normal physiological levels decreases the DNA synthesis of vascular smooth muscle cells, holding SMC proliferation to a low level [3828];

(4) 1 Hz, 10% cyclic strain on SMC activate tyrosine phosphorylation and PKC, PKA, and cAMP pathways over response times from 10 sec to 30 min [3826, 3829]; and

(5) vascular SMC exhibited stretch-induced apoptosis when subjected to cyclic 20% elongation stretching at 0.5 Hz for 6 hours [3862].

Hipper and Isenberg [3828] suggest that abnormally low strains can also induce vascular SMC proliferation. If true, then medical nanorobot aggregates which shield the vasculature from normal cyclical strains might elicit excess growth of vascular smooth muscle cells, which growth is normally held in check by the rhythmic stretching from the arterial pulse. On the other hand, intravascular nanorobot aggregates that apply cyclic mechanical strains exceeding a few percent might encourage increased SMC proliferation and activate mechanosensitive and stretch-activated channels in EC, along with cellular realignment and subsequent SMC apoptosis at the highest strain levels. These factors must be taken into consideration during nanorobot mission design so that mechanisms can be incorporated to prevent or to attenuate such effects.

In 2002 it was unknown whether high frequency (>KHz) cyclic mechanical strains likely to be employed by vasculomobile medical nanorobots (Section 9.4.3.5) would have biological effects similar to or different from those described above for low-frequency cyclic strains – excepting certain specialized mechanoreceptor cells such as the cochlear stereocilia [3830], other hair cells [3831-3833], and somatosensory neurons [3834-3836] – since most mechanical cell stimulation experiments have been conducted at low frequencies*. Unrecognized effects that might be triggered by high-frequency cyclic strains cannot be ruled out. However, given the relative safety of procedures involving intravascular ultrasound [3837-3846] with its low complication rate (e.g., only 1.1%, including spasms, vessel dissection and guidewire entrapment [3840]) using frequencies as high as 10-20 MHz [3837-3839], it seems improbable that KHz or MHz acoustic waves of the intensities that might be employed by medical nanorobots for communication (Section 7.2.2) or power supply (Section 6.4.1) will damage the endothelial vascular walls. Interestingly, relatively high-intensity intravascular ultrasound has been used to dissolve occlusive platelet-rich thrombi safely and effectively in myocardial infarctions [3842] and in restenosed stents [3845].

* Specifically, between 0.05-5 Hz (Section 9.4.3.2.1) and more recently at: 0.01 Hz [3847], 0.03 Hz [5085], 0.05 Hz [3848, 3864], 0.1 Hz [3847, 3852], 0.15 Hz [5085], 0.2 Hz [3849], 0.3 Hz [3850-3853, 5085, 5086], 0.4 Hz [3860], 0.5 Hz [3828, 3861-3864, 4767, 5087], 1 Hz [3853-3859, 5088-5094, 5331], 2 Hz [5095], 3 Hz [3853], 4 Hz [3865], 5 Hz [3866, 5094], 6 Hz [3860], 10/20/50 Hz [3865, 5094], and DC-100 Hz [3867].

In the case of intrusive vasculoid-class devices [4609] (Chapter 30), it is likely that the appliance will need to control smooth muscle cell proliferation [4610-4617], in the simplest case releasing specific cytokines into the vasculoid-endothelial space. Such factors may include known SMC proliferation promoters [4618, 4619] such as thrombin (esp. alpha-thrombin), PDGF’s (esp. PDGF-AA), FGF (esp. basic FGF), HBEGF (heparin binding epidermal growth factor), TGFbeta (transforming growth factor-beta) at low concentrations, angiotensin II, thrombospondin-1 (stretch/tension), and known SMC proliferation inhibitors [4620-4626] such as heparin sulfate, TGFbeta (transforming growth factor-beta) at high concentrations, nitric oxide, prostaglandins, calcium antagonists, agonists that activate guanylate and adenylate cyclases, inhibitors of angiotensin-converting enzyme, interferon gamma, 18-beta-estradiol, sodium salicylate, and the topoisomerase I inhibitor topotecan. (Note that these promoters and inhibitors can have multiple effects on other cells, so these effects must be considered prior to use.) Adult arterial walls contain both differentiated and immature SMCs [4627]. Reviewer R. Bradbury notes that further research may be needed regarding how SMCs handle conflict resolution between simultaneous “divide” and “don’t divide” signals they may be receiving. Given the large number of signals that SMCs currently respond to, it seems highly likely that the vasculoid can “manage” them.

Last updated on 30 April 2004